2338 lines
100 KiB
Plaintext
2338 lines
100 KiB
Plaintext
<!-- doc/src/sgml/high-availability.sgml -->
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<chapter id="high-availability">
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<title>High Availability, Load Balancing, and Replication</title>
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<indexterm><primary>high availability</primary></indexterm>
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<indexterm><primary>failover</primary></indexterm>
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<indexterm><primary>replication</primary></indexterm>
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<indexterm><primary>load balancing</primary></indexterm>
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<indexterm><primary>clustering</primary></indexterm>
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<indexterm><primary>data partitioning</primary></indexterm>
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<para>
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Database servers can work together to allow a second server to
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take over quickly if the primary server fails (high
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availability), or to allow several computers to serve the same
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data (load balancing). Ideally, database servers could work
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together seamlessly. Web servers serving static web pages can
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be combined quite easily by merely load-balancing web requests
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to multiple machines. In fact, read-only database servers can
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be combined relatively easily too. Unfortunately, most database
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servers have a read/write mix of requests, and read/write servers
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are much harder to combine. This is because though read-only
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data needs to be placed on each server only once, a write to any
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server has to be propagated to all servers so that future read
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requests to those servers return consistent results.
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</para>
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<para>
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This synchronization problem is the fundamental difficulty for
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servers working together. Because there is no single solution
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that eliminates the impact of the sync problem for all use cases,
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there are multiple solutions. Each solution addresses this
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problem in a different way, and minimizes its impact for a specific
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workload.
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</para>
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<para>
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Some solutions deal with synchronization by allowing only one
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server to modify the data. Servers that can modify data are
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called read/write, <firstterm>master</firstterm> or <firstterm>primary</firstterm> servers.
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Servers that track changes in the primary are called <firstterm>standby</firstterm>
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or <firstterm>secondary</firstterm> servers. A standby server that cannot be connected
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to until it is promoted to a primary server is called a <firstterm>warm
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standby</firstterm> server, and one that can accept connections and serves read-only
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queries is called a <firstterm>hot standby</firstterm> server.
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</para>
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<para>
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Some solutions are synchronous,
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meaning that a data-modifying transaction is not considered
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committed until all servers have committed the transaction. This
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guarantees that a failover will not lose any data and that all
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load-balanced servers will return consistent results no matter
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which server is queried. In contrast, asynchronous solutions allow some
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delay between the time of a commit and its propagation to the other servers,
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opening the possibility that some transactions might be lost in
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the switch to a backup server, and that load balanced servers
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might return slightly stale results. Asynchronous communication
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is used when synchronous would be too slow.
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</para>
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<para>
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Solutions can also be categorized by their granularity. Some solutions
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can deal only with an entire database server, while others allow control
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at the per-table or per-database level.
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</para>
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<para>
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Performance must be considered in any choice. There is usually a
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trade-off between functionality and
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performance. For example, a fully synchronous solution over a slow
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network might cut performance by more than half, while an asynchronous
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one might have a minimal performance impact.
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</para>
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<para>
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The remainder of this section outlines various failover, replication,
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and load balancing solutions.
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</para>
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<sect1 id="different-replication-solutions">
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<title>Comparison of Different Solutions</title>
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<variablelist>
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<varlistentry>
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<term>Shared Disk Failover</term>
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<listitem>
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<para>
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Shared disk failover avoids synchronization overhead by having only one
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copy of the database. It uses a single disk array that is shared by
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multiple servers. If the main database server fails, the standby server
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is able to mount and start the database as though it were recovering from
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a database crash. This allows rapid failover with no data loss.
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</para>
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<para>
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Shared hardware functionality is common in network storage devices.
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Using a network file system is also possible, though care must be
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taken that the file system has full <acronym>POSIX</acronym> behavior (see <xref
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linkend="creating-cluster-nfs"/>). One significant limitation of this
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method is that if the shared disk array fails or becomes corrupt, the
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primary and standby servers are both nonfunctional. Another issue is
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that the standby server should never access the shared storage while
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the primary server is running.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>File System (Block Device) Replication</term>
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<listitem>
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<para>
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A modified version of shared hardware functionality is file system
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replication, where all changes to a file system are mirrored to a file
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system residing on another computer. The only restriction is that
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the mirroring must be done in a way that ensures the standby server
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has a consistent copy of the file system — specifically, writes
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to the standby must be done in the same order as those on the primary.
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<productname>DRBD</productname> is a popular file system replication solution
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for Linux.
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</para>
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<!--
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https://forge.continuent.org/pipermail/sequoia/2006-November/004070.html
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Oracle RAC is a shared disk approach and just send cache invalidations
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to other nodes but not actual data. As the disk is shared, data is
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only committed once to disk and there is a distributed locking
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protocol to make nodes agree on a serializable transactional order.
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-->
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>Write-Ahead Log Shipping</term>
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<listitem>
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<para>
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Warm and hot standby servers can be kept current by reading a
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stream of write-ahead log (<acronym>WAL</acronym>)
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records. If the main server fails, the standby contains
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almost all of the data of the main server, and can be quickly
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made the new primary database server. This can be synchronous or
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asynchronous and can only be done for the entire database server.
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</para>
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<para>
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A standby server can be implemented using file-based log shipping
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(<xref linkend="warm-standby"/>) or streaming replication (see
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<xref linkend="streaming-replication"/>), or a combination of both. For
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information on hot standby, see <xref linkend="hot-standby"/>.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>Logical Replication</term>
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<listitem>
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<para>
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Logical replication allows a database server to send a stream of data
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modifications to another server. <productname>PostgreSQL</productname>
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logical replication constructs a stream of logical data modifications
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from the WAL. Logical replication allows replication of data changes on
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a per-table basis. In addition, a server that is publishing its own
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changes can also subscribe to changes from another server, allowing data
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to flow in multiple directions. For more information on logical
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replication, see <xref linkend="logical-replication"/>. Through the
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logical decoding interface (<xref linkend="logicaldecoding"/>),
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third-party extensions can also provide similar functionality.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>Trigger-Based Primary-Standby Replication</term>
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<listitem>
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<para>
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A trigger-based replication setup typically funnels data modification
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queries to a designated primary server. Operating on a per-table basis,
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the primary server sends data changes (typically) asynchronously to the
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standby servers. Standby servers can answer queries while the primary is
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running, and may allow some local data changes or write activity. This
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form of replication is often used for offloading large analytical or data
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warehouse queries.
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</para>
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<para>
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<productname>Slony-I</productname> is an example of this type of
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replication, with per-table granularity, and support for multiple standby
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servers. Because it updates the standby server asynchronously (in
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batches), there is possible data loss during fail over.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>SQL-Based Replication Middleware</term>
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<listitem>
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<para>
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With SQL-based replication middleware, a program intercepts
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every SQL query and sends it to one or all servers. Each server
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operates independently. Read-write queries must be sent to all servers,
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so that every server receives any changes. But read-only queries can be
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sent to just one server, allowing the read workload to be distributed
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among them.
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</para>
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<para>
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If queries are simply broadcast unmodified, functions like
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<function>random()</function>, <function>CURRENT_TIMESTAMP</function>, and
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sequences can have different values on different servers.
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This is because each server operates independently, and because
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SQL queries are broadcast rather than actual data changes. If
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this is unacceptable, either the middleware or the application
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must determine such values from a single source and then use those
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values in write queries. Care must also be taken that all
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transactions either commit or abort on all servers, perhaps
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using two-phase commit (<xref linkend="sql-prepare-transaction"/>
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and <xref linkend="sql-commit-prepared"/>).
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<productname>Pgpool-II</productname> and <productname>Continuent Tungsten</productname>
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are examples of this type of replication.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>Asynchronous Multimaster Replication</term>
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<listitem>
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<para>
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For servers that are not regularly connected or have slow
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communication links, like laptops or
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remote servers, keeping data consistent among servers is a
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challenge. Using asynchronous multimaster replication, each
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server works independently, and periodically communicates with
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the other servers to identify conflicting transactions. The
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conflicts can be resolved by users or conflict resolution rules.
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Bucardo is an example of this type of replication.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>Synchronous Multimaster Replication</term>
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<listitem>
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<para>
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In synchronous multimaster replication, each server can accept
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write requests, and modified data is transmitted from the
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original server to every other server before each transaction
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commits. Heavy write activity can cause excessive locking and
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commit delays, leading to poor performance. Read requests can
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be sent to any server. Some implementations use shared disk
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to reduce the communication overhead. Synchronous multimaster
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replication is best for mostly read workloads, though its big
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advantage is that any server can accept write requests —
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there is no need to partition workloads between primary and
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standby servers, and because the data changes are sent from one
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server to another, there is no problem with non-deterministic
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functions like <function>random()</function>.
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</para>
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<para>
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<productname>PostgreSQL</productname> does not offer this type of replication,
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though <productname>PostgreSQL</productname> two-phase commit (<xref
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linkend="sql-prepare-transaction"/> and <xref
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linkend="sql-commit-prepared"/>)
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can be used to implement this in application code or middleware.
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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<para>
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<xref linkend="high-availability-matrix"/> summarizes
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the capabilities of the various solutions listed above.
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</para>
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<table id="high-availability-matrix">
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<title>High Availability, Load Balancing, and Replication Feature Matrix</title>
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<tgroup cols="9">
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<colspec colname="col1" colwidth="1.1*"/>
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<colspec colname="col2" colwidth="1*"/>
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<colspec colname="col3" colwidth="1*"/>
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<colspec colname="col4" colwidth="1*"/>
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<colspec colname="col5" colwidth="1*"/>
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<colspec colname="col6" colwidth="1*"/>
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<colspec colname="col7" colwidth="1*"/>
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<colspec colname="col8" colwidth="1*"/>
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<colspec colname="col9" colwidth="1*"/>
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<thead>
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<row>
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<entry>Feature</entry>
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<entry>Shared Disk</entry>
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<entry>File System Repl.</entry>
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<entry>Write-Ahead Log Shipping</entry>
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<entry>Logical Repl.</entry>
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<entry>Trigger-&zwsp;Based Repl.</entry>
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<entry>SQL Repl. Middle-ware</entry>
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<entry>Async. MM Repl.</entry>
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<entry>Sync. MM Repl.</entry>
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</row>
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</thead>
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<tbody>
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<row>
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<entry>Popular examples</entry>
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<entry align="center">NAS</entry>
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<entry align="center">DRBD</entry>
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<entry align="center">built-in streaming repl.</entry>
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<entry align="center">built-in logical repl., pglogical</entry>
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<entry align="center">Londiste, Slony</entry>
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<entry align="center">pgpool-II</entry>
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<entry align="center">Bucardo</entry>
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<entry align="center"></entry>
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</row>
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<row>
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<entry>Comm. method</entry>
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<entry align="center">shared disk</entry>
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<entry align="center">disk blocks</entry>
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<entry align="center">WAL</entry>
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<entry align="center">logical decoding</entry>
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<entry align="center">table rows</entry>
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<entry align="center">SQL</entry>
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<entry align="center">table rows</entry>
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<entry align="center">table rows and row locks</entry>
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</row>
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<row>
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<entry>No special hardware required</entry>
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<entry align="center"></entry>
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<entry align="center">•</entry>
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<entry align="center">•</entry>
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<entry align="center">•</entry>
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<entry align="center">•</entry>
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<entry align="center">•</entry>
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<entry align="center">•</entry>
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<entry align="center">•</entry>
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</row>
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<row>
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<entry>Allows multiple primary servers</entry>
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<entry align="center"></entry>
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<entry align="center"></entry>
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<entry align="center"></entry>
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<entry align="center">•</entry>
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<entry align="center"></entry>
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<entry align="center">•</entry>
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<entry align="center">•</entry>
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<entry align="center">•</entry>
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</row>
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<row>
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<entry>No overhead on primary</entry>
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<entry align="center">•</entry>
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<entry align="center"></entry>
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<entry align="center">•</entry>
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<entry align="center">•</entry>
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<entry align="center"></entry>
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<entry align="center">•</entry>
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<entry align="center"></entry>
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<entry align="center"></entry>
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</row>
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<row>
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<entry>No waiting for multiple servers</entry>
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<entry align="center">•</entry>
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<entry align="center"></entry>
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<entry align="center">with sync off</entry>
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<entry align="center">with sync off</entry>
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<entry align="center">•</entry>
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<entry align="center"></entry>
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<entry align="center">•</entry>
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<entry align="center"></entry>
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</row>
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<row>
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<entry>Primary failure will never lose data</entry>
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<entry align="center">•</entry>
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<entry align="center">•</entry>
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<entry align="center">with sync on</entry>
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<entry align="center">with sync on</entry>
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<entry align="center"></entry>
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<entry align="center">•</entry>
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<entry align="center"></entry>
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<entry align="center">•</entry>
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</row>
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<row>
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<entry>Replicas accept read-only queries</entry>
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<entry align="center"></entry>
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<entry align="center"></entry>
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<entry align="center">with hot standby</entry>
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<entry align="center">•</entry>
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<entry align="center">•</entry>
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<entry align="center">•</entry>
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<entry align="center">•</entry>
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<entry align="center">•</entry>
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</row>
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<row>
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<entry>Per-table granularity</entry>
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<entry align="center"></entry>
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<entry align="center"></entry>
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<entry align="center"></entry>
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<entry align="center">•</entry>
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<entry align="center">•</entry>
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<entry align="center"></entry>
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<entry align="center">•</entry>
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<entry align="center">•</entry>
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</row>
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<row>
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<entry>No conflict resolution necessary</entry>
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<entry align="center">•</entry>
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<entry align="center">•</entry>
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<entry align="center">•</entry>
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<entry align="center"></entry>
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<entry align="center">•</entry>
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<entry align="center">•</entry>
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<entry align="center"></entry>
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<entry align="center">•</entry>
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</row>
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</tbody>
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</tgroup>
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</table>
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<para>
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There are a few solutions that do not fit into the above categories:
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</para>
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<variablelist>
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<varlistentry>
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<term>Data Partitioning</term>
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<listitem>
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<para>
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Data partitioning splits tables into data sets. Each set can
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be modified by only one server. For example, data can be
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partitioned by offices, e.g., London and Paris, with a server
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in each office. If queries combining London and Paris data
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are necessary, an application can query both servers, or
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primary/standby replication can be used to keep a read-only copy
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of the other office's data on each server.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>Multiple-Server Parallel Query Execution</term>
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<listitem>
|
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<para>
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Many of the above solutions allow multiple servers to handle multiple
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queries, but none allow a single query to use multiple servers to
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complete faster. This solution allows multiple servers to work
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concurrently on a single query. It is usually accomplished by
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splitting the data among servers and having each server execute its
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part of the query and return results to a central server where they
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are combined and returned to the user. This can be implemented using the
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<productname>PL/Proxy</productname> tool set.
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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|
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<para>
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It should also be noted that because <productname>PostgreSQL</productname>
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is open source and easily extended, a number of companies have
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taken <productname>PostgreSQL</productname> and created commercial
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closed-source solutions with unique failover, replication, and load
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balancing capabilities. These are not discussed here.
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</para>
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|
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</sect1>
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|
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<sect1 id="warm-standby">
|
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<title>Log-Shipping Standby Servers</title>
|
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|
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|
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<para>
|
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Continuous archiving can be used to create a <firstterm>high
|
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availability</firstterm> (HA) cluster configuration with one or more
|
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<firstterm>standby servers</firstterm> ready to take over operations if the
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primary server fails. This capability is widely referred to as
|
|
<firstterm>warm standby</firstterm> or <firstterm>log shipping</firstterm>.
|
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</para>
|
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|
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<para>
|
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The primary and standby server work together to provide this capability,
|
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though the servers are only loosely coupled. The primary server operates
|
|
in continuous archiving mode, while each standby server operates in
|
|
continuous recovery mode, reading the WAL files from the primary. No
|
|
changes to the database tables are required to enable this capability,
|
|
so it offers low administration overhead compared to some other
|
|
replication solutions. This configuration also has relatively low
|
|
performance impact on the primary server.
|
|
</para>
|
|
|
|
<para>
|
|
Directly moving WAL records from one database server to another
|
|
is typically described as log shipping. <productname>PostgreSQL</productname>
|
|
implements file-based log shipping by transferring WAL records
|
|
one file (WAL segment) at a time. WAL files (16MB) can be
|
|
shipped easily and cheaply over any distance, whether it be to an
|
|
adjacent system, another system at the same site, or another system on
|
|
the far side of the globe. The bandwidth required for this technique
|
|
varies according to the transaction rate of the primary server.
|
|
Record-based log shipping is more granular and streams WAL changes
|
|
incrementally over a network connection (see <xref
|
|
linkend="streaming-replication"/>).
|
|
</para>
|
|
|
|
<para>
|
|
It should be noted that log shipping is asynchronous, i.e., the WAL
|
|
records are shipped after transaction commit. As a result, there is a
|
|
window for data loss should the primary server suffer a catastrophic
|
|
failure; transactions not yet shipped will be lost. The size of the
|
|
data loss window in file-based log shipping can be limited by use of the
|
|
<varname>archive_timeout</varname> parameter, which can be set as low
|
|
as a few seconds. However such a low setting will
|
|
substantially increase the bandwidth required for file shipping.
|
|
Streaming replication (see <xref linkend="streaming-replication"/>)
|
|
allows a much smaller window of data loss.
|
|
</para>
|
|
|
|
<para>
|
|
Recovery performance is sufficiently good that the standby will
|
|
typically be only moments away from full
|
|
availability once it has been activated. As a result, this is called
|
|
a warm standby configuration which offers high
|
|
availability. Restoring a server from an archived base backup and
|
|
rollforward will take considerably longer, so that technique only
|
|
offers a solution for disaster recovery, not high availability.
|
|
A standby server can also be used for read-only queries, in which case
|
|
it is called a <firstterm>hot standby</firstterm> server. See
|
|
<xref linkend="hot-standby"/> for more information.
|
|
</para>
|
|
|
|
<indexterm zone="high-availability">
|
|
<primary>warm standby</primary>
|
|
</indexterm>
|
|
|
|
<indexterm zone="high-availability">
|
|
<primary>PITR standby</primary>
|
|
</indexterm>
|
|
|
|
<indexterm zone="high-availability">
|
|
<primary>standby server</primary>
|
|
</indexterm>
|
|
|
|
<indexterm zone="high-availability">
|
|
<primary>log shipping</primary>
|
|
</indexterm>
|
|
|
|
<indexterm zone="high-availability">
|
|
<primary>witness server</primary>
|
|
</indexterm>
|
|
|
|
<indexterm zone="high-availability">
|
|
<primary>STONITH</primary>
|
|
</indexterm>
|
|
|
|
<sect2 id="standby-planning">
|
|
<title>Planning</title>
|
|
|
|
<para>
|
|
It is usually wise to create the primary and standby servers
|
|
so that they are as similar as possible, at least from the
|
|
perspective of the database server. In particular, the path names
|
|
associated with tablespaces will be passed across unmodified, so both
|
|
primary and standby servers must have the same mount paths for
|
|
tablespaces if that feature is used. Keep in mind that if
|
|
<xref linkend="sql-createtablespace"/>
|
|
is executed on the primary, any new mount point needed for it must
|
|
be created on the primary and all standby servers before the command
|
|
is executed. Hardware need not be exactly the same, but experience shows
|
|
that maintaining two identical systems is easier than maintaining two
|
|
dissimilar ones over the lifetime of the application and system.
|
|
In any case the hardware architecture must be the same — shipping
|
|
from, say, a 32-bit to a 64-bit system will not work.
|
|
</para>
|
|
|
|
<para>
|
|
In general, log shipping between servers running different major
|
|
<productname>PostgreSQL</productname> release
|
|
levels is not possible. It is the policy of the PostgreSQL Global
|
|
Development Group not to make changes to disk formats during minor release
|
|
upgrades, so it is likely that running different minor release levels
|
|
on primary and standby servers will work successfully. However, no
|
|
formal support for that is offered and you are advised to keep primary
|
|
and standby servers at the same release level as much as possible.
|
|
When updating to a new minor release, the safest policy is to update
|
|
the standby servers first — a new minor release is more likely
|
|
to be able to read WAL files from a previous minor release than vice
|
|
versa.
|
|
</para>
|
|
|
|
</sect2>
|
|
|
|
<sect2 id="standby-server-operation" xreflabel="Standby Server Operation">
|
|
<title>Standby Server Operation</title>
|
|
|
|
<para>
|
|
A server enters standby mode if a
|
|
<anchor id="file-standby-signal" xreflabel="standby.signal"/>
|
|
<filename>standby.signal</filename>
|
|
<indexterm><primary><filename>standby.signal</filename></primary></indexterm>
|
|
file exists in the data directory when the server is started.
|
|
</para>
|
|
|
|
<para>
|
|
In standby mode, the server continuously applies WAL received from the
|
|
primary server. The standby server can read WAL from a WAL archive
|
|
(see <xref linkend="guc-restore-command"/>) or directly from the primary
|
|
over a TCP connection (streaming replication). The standby server will
|
|
also attempt to restore any WAL found in the standby cluster's
|
|
<filename>pg_wal</filename> directory. That typically happens after a server
|
|
restart, when the standby replays again WAL that was streamed from the
|
|
primary before the restart, but you can also manually copy files to
|
|
<filename>pg_wal</filename> at any time to have them replayed.
|
|
</para>
|
|
|
|
<para>
|
|
At startup, the standby begins by restoring all WAL available in the
|
|
archive location, calling <varname>restore_command</varname>. Once it
|
|
reaches the end of WAL available there and <varname>restore_command</varname>
|
|
fails, it tries to restore any WAL available in the <filename>pg_wal</filename> directory.
|
|
If that fails, and streaming replication has been configured, the
|
|
standby tries to connect to the primary server and start streaming WAL
|
|
from the last valid record found in archive or <filename>pg_wal</filename>. If that fails
|
|
or streaming replication is not configured, or if the connection is
|
|
later disconnected, the standby goes back to step 1 and tries to
|
|
restore the file from the archive again. This loop of retries from the
|
|
archive, <filename>pg_wal</filename>, and via streaming replication goes on until the server
|
|
is stopped or is promoted.
|
|
</para>
|
|
|
|
<para>
|
|
Standby mode is exited and the server switches to normal operation
|
|
when <command>pg_ctl promote</command> is run, or
|
|
<function>pg_promote()</function> is called. Before failover,
|
|
any WAL immediately available in the archive or in <filename>pg_wal</filename>
|
|
will be restored, but no attempt is made to connect to the primary.
|
|
</para>
|
|
</sect2>
|
|
|
|
<sect2 id="preparing-primary-for-standby">
|
|
<title>Preparing the Primary for Standby Servers</title>
|
|
|
|
<para>
|
|
Set up continuous archiving on the primary to an archive directory
|
|
accessible from the standby, as described
|
|
in <xref linkend="continuous-archiving"/>. The archive location should be
|
|
accessible from the standby even when the primary is down, i.e., it should
|
|
reside on the standby server itself or another trusted server, not on
|
|
the primary server.
|
|
</para>
|
|
|
|
<para>
|
|
If you want to use streaming replication, set up authentication on the
|
|
primary server to allow replication connections from the standby
|
|
server(s); that is, create a role and provide a suitable entry or
|
|
entries in <filename>pg_hba.conf</filename> with the database field set to
|
|
<literal>replication</literal>. Also ensure <varname>max_wal_senders</varname> is set
|
|
to a sufficiently large value in the configuration file of the primary
|
|
server. If replication slots will be used,
|
|
ensure that <varname>max_replication_slots</varname> is set sufficiently
|
|
high as well.
|
|
</para>
|
|
|
|
<para>
|
|
Take a base backup as described in <xref linkend="backup-base-backup"/>
|
|
to bootstrap the standby server.
|
|
</para>
|
|
</sect2>
|
|
|
|
<sect2 id="standby-server-setup">
|
|
<title>Setting Up a Standby Server</title>
|
|
|
|
<para>
|
|
To set up the standby server, restore the base backup taken from primary
|
|
server (see <xref linkend="backup-pitr-recovery"/>). Create a file
|
|
<link linkend="file-standby-signal"><filename>standby.signal</filename></link><indexterm><primary>standby.signal</primary></indexterm>
|
|
in the standby's cluster data
|
|
directory. Set <xref linkend="guc-restore-command"/> to a simple command to copy files from
|
|
the WAL archive. If you plan to have multiple standby servers for high
|
|
availability purposes, make sure that <varname>recovery_target_timeline</varname> is set to
|
|
<literal>latest</literal> (the default), to make the standby server follow the timeline change
|
|
that occurs at failover to another standby.
|
|
</para>
|
|
|
|
<note>
|
|
<para>
|
|
<xref linkend="guc-restore-command"/> should return immediately
|
|
if the file does not exist; the server will retry the command again if
|
|
necessary.
|
|
</para>
|
|
</note>
|
|
|
|
<para>
|
|
If you want to use streaming replication, fill in
|
|
<xref linkend="guc-primary-conninfo"/> with a libpq connection string, including
|
|
the host name (or IP address) and any additional details needed to
|
|
connect to the primary server. If the primary needs a password for
|
|
authentication, the password needs to be specified in
|
|
<xref linkend="guc-primary-conninfo"/> as well.
|
|
</para>
|
|
|
|
<para>
|
|
If you're setting up the standby server for high availability purposes,
|
|
set up WAL archiving, connections and authentication like the primary
|
|
server, because the standby server will work as a primary server after
|
|
failover.
|
|
</para>
|
|
|
|
<para>
|
|
If you're using a WAL archive, its size can be minimized using the <xref
|
|
linkend="guc-archive-cleanup-command"/> parameter to remove files that are no
|
|
longer required by the standby server.
|
|
The <application>pg_archivecleanup</application> utility is designed specifically to
|
|
be used with <varname>archive_cleanup_command</varname> in typical single-standby
|
|
configurations, see <xref linkend="pgarchivecleanup"/>.
|
|
Note however, that if you're using the archive for backup purposes, you
|
|
need to retain files needed to recover from at least the latest base
|
|
backup, even if they're no longer needed by the standby.
|
|
</para>
|
|
|
|
<para>
|
|
A simple example of configuration is:
|
|
<programlisting>
|
|
primary_conninfo = 'host=192.168.1.50 port=5432 user=foo password=foopass options=''-c wal_sender_timeout=5000'''
|
|
restore_command = 'cp /path/to/archive/%f %p'
|
|
archive_cleanup_command = 'pg_archivecleanup /path/to/archive %r'
|
|
</programlisting>
|
|
</para>
|
|
|
|
<para>
|
|
You can have any number of standby servers, but if you use streaming
|
|
replication, make sure you set <varname>max_wal_senders</varname> high enough in
|
|
the primary to allow them to be connected simultaneously.
|
|
</para>
|
|
|
|
</sect2>
|
|
|
|
<sect2 id="streaming-replication">
|
|
<title>Streaming Replication</title>
|
|
|
|
<indexterm zone="high-availability">
|
|
<primary>Streaming Replication</primary>
|
|
</indexterm>
|
|
|
|
<para>
|
|
Streaming replication allows a standby server to stay more up-to-date
|
|
than is possible with file-based log shipping. The standby connects
|
|
to the primary, which streams WAL records to the standby as they're
|
|
generated, without waiting for the WAL file to be filled.
|
|
</para>
|
|
|
|
<para>
|
|
Streaming replication is asynchronous by default
|
|
(see <xref linkend="synchronous-replication"/>), in which case there is
|
|
a small delay between committing a transaction in the primary and the
|
|
changes becoming visible in the standby. This delay is however much
|
|
smaller than with file-based log shipping, typically under one second
|
|
assuming the standby is powerful enough to keep up with the load. With
|
|
streaming replication, <varname>archive_timeout</varname> is not required to
|
|
reduce the data loss window.
|
|
</para>
|
|
|
|
<para>
|
|
If you use streaming replication without file-based continuous
|
|
archiving, the server might recycle old WAL segments before the standby
|
|
has received them. If this occurs, the standby will need to be
|
|
reinitialized from a new base backup. You can avoid this by setting
|
|
<varname>wal_keep_size</varname> to a value large enough to ensure that
|
|
WAL segments are not recycled too early, or by configuring a replication
|
|
slot for the standby. If you set up a WAL archive that's accessible from
|
|
the standby, these solutions are not required, since the standby can
|
|
always use the archive to catch up provided it retains enough segments.
|
|
</para>
|
|
|
|
<para>
|
|
To use streaming replication, set up a file-based log-shipping standby
|
|
server as described in <xref linkend="warm-standby"/>. The step that
|
|
turns a file-based log-shipping standby into streaming replication
|
|
standby is setting the <varname>primary_conninfo</varname> setting
|
|
to point to the primary server. Set
|
|
<xref linkend="guc-listen-addresses"/> and authentication options
|
|
(see <filename>pg_hba.conf</filename>) on the primary so that the standby server
|
|
can connect to the <literal>replication</literal> pseudo-database on the primary
|
|
server (see <xref linkend="streaming-replication-authentication"/>).
|
|
</para>
|
|
|
|
<para>
|
|
On systems that support the keepalive socket option, setting
|
|
<xref linkend="guc-tcp-keepalives-idle"/>,
|
|
<xref linkend="guc-tcp-keepalives-interval"/> and
|
|
<xref linkend="guc-tcp-keepalives-count"/> helps the primary promptly
|
|
notice a broken connection.
|
|
</para>
|
|
|
|
<para>
|
|
Set the maximum number of concurrent connections from the standby servers
|
|
(see <xref linkend="guc-max-wal-senders"/> for details).
|
|
</para>
|
|
|
|
<para>
|
|
When the standby is started and <varname>primary_conninfo</varname> is set
|
|
correctly, the standby will connect to the primary after replaying all
|
|
WAL files available in the archive. If the connection is established
|
|
successfully, you will see a <literal>walreceiver</literal> in the standby, and
|
|
a corresponding <literal>walsender</literal> process in the primary.
|
|
</para>
|
|
|
|
<sect3 id="streaming-replication-authentication">
|
|
<title>Authentication</title>
|
|
<para>
|
|
It is very important that the access privileges for replication be set up
|
|
so that only trusted users can read the WAL stream, because it is
|
|
easy to extract privileged information from it. Standby servers must
|
|
authenticate to the primary as an account that has the
|
|
<literal>REPLICATION</literal> privilege or a superuser. It is
|
|
recommended to create a dedicated user account with
|
|
<literal>REPLICATION</literal> and <literal>LOGIN</literal>
|
|
privileges for replication. While <literal>REPLICATION</literal>
|
|
privilege gives very high permissions, it does not allow the user to
|
|
modify any data on the primary system, which the
|
|
<literal>SUPERUSER</literal> privilege does.
|
|
</para>
|
|
|
|
<para>
|
|
Client authentication for replication is controlled by a
|
|
<filename>pg_hba.conf</filename> record specifying <literal>replication</literal> in the
|
|
<replaceable>database</replaceable> field. For example, if the standby is running on
|
|
host IP <literal>192.168.1.100</literal> and the account name for replication
|
|
is <literal>foo</literal>, the administrator can add the following line to the
|
|
<filename>pg_hba.conf</filename> file on the primary:
|
|
|
|
<programlisting>
|
|
# Allow the user "foo" from host 192.168.1.100 to connect to the primary
|
|
# as a replication standby if the user's password is correctly supplied.
|
|
#
|
|
# TYPE DATABASE USER ADDRESS METHOD
|
|
host replication foo 192.168.1.100/32 md5
|
|
</programlisting>
|
|
</para>
|
|
<para>
|
|
The host name and port number of the primary, connection user name,
|
|
and password are specified in the <xref linkend="guc-primary-conninfo"/>.
|
|
The password can also be set in the <filename>~/.pgpass</filename> file on the
|
|
standby (specify <literal>replication</literal> in the <replaceable>database</replaceable>
|
|
field).
|
|
For example, if the primary is running on host IP <literal>192.168.1.50</literal>,
|
|
port <literal>5432</literal>, the account name for replication is
|
|
<literal>foo</literal>, and the password is <literal>foopass</literal>, the administrator
|
|
can add the following line to the <filename>postgresql.conf</filename> file on the
|
|
standby:
|
|
|
|
<programlisting>
|
|
# The standby connects to the primary that is running on host 192.168.1.50
|
|
# and port 5432 as the user "foo" whose password is "foopass".
|
|
primary_conninfo = 'host=192.168.1.50 port=5432 user=foo password=foopass'
|
|
</programlisting>
|
|
</para>
|
|
</sect3>
|
|
|
|
<sect3 id="streaming-replication-monitoring">
|
|
<title>Monitoring</title>
|
|
<para>
|
|
An important health indicator of streaming replication is the amount
|
|
of WAL records generated in the primary, but not yet applied in the
|
|
standby. You can calculate this lag by comparing the current WAL write
|
|
location on the primary with the last WAL location received by the
|
|
standby. These locations can be retrieved using
|
|
<function>pg_current_wal_lsn</function> on the primary and
|
|
<function>pg_last_wal_receive_lsn</function> on the standby,
|
|
respectively (see <xref linkend="functions-admin-backup-table"/> and
|
|
<xref linkend="functions-recovery-info-table"/> for details).
|
|
The last WAL receive location in the standby is also displayed in the
|
|
process status of the WAL receiver process, displayed using the
|
|
<command>ps</command> command (see <xref linkend="monitoring-ps"/> for details).
|
|
</para>
|
|
<para>
|
|
You can retrieve a list of WAL sender processes via the
|
|
<link linkend="monitoring-pg-stat-replication-view"><structname>
|
|
pg_stat_replication</structname></link> view. Large differences between
|
|
<function>pg_current_wal_lsn</function> and the view's <literal>sent_lsn</literal> field
|
|
might indicate that the primary server is under heavy load, while
|
|
differences between <literal>sent_lsn</literal> and
|
|
<function>pg_last_wal_receive_lsn</function> on the standby might indicate
|
|
network delay, or that the standby is under heavy load.
|
|
</para>
|
|
<para>
|
|
On a hot standby, the status of the WAL receiver process can be retrieved
|
|
via the <link linkend="monitoring-pg-stat-wal-receiver-view">
|
|
<structname>pg_stat_wal_receiver</structname></link> view. A large
|
|
difference between <function>pg_last_wal_replay_lsn</function> and the
|
|
view's <literal>flushed_lsn</literal> indicates that WAL is being
|
|
received faster than it can be replayed.
|
|
</para>
|
|
</sect3>
|
|
</sect2>
|
|
|
|
<sect2 id="streaming-replication-slots">
|
|
<title>Replication Slots</title>
|
|
<indexterm>
|
|
<primary>replication slot</primary>
|
|
<secondary>streaming replication</secondary>
|
|
</indexterm>
|
|
<para>
|
|
Replication slots provide an automated way to ensure that the
|
|
primary server does
|
|
not remove WAL segments until they have been received by all standbys,
|
|
and that the primary does not remove rows which could cause a
|
|
<link linkend="hot-standby-conflict">recovery conflict</link> even when the
|
|
standby is disconnected.
|
|
</para>
|
|
<para>
|
|
In lieu of using replication slots, it is possible to prevent the removal
|
|
of old WAL segments using <xref linkend="guc-wal-keep-size"/>, or by
|
|
storing the segments in an archive using
|
|
<xref linkend="guc-archive-command"/> or <xref linkend="guc-archive-library"/>.
|
|
A disadvantage of these methods is that they
|
|
often result in retaining more WAL segments than
|
|
required, whereas replication slots retain only the number of segments
|
|
known to be needed.
|
|
</para>
|
|
<para>
|
|
Similarly, <xref linkend="guc-hot-standby-feedback"/> on its own, without
|
|
also using a replication slot, provides protection against relevant rows
|
|
being removed by vacuum, but provides no protection during any time period
|
|
when the standby is not connected.
|
|
</para>
|
|
|
|
<caution>
|
|
<para>
|
|
Beware that replication slots can cause the server to retain so
|
|
many WAL segments that they fill up the space allocated for
|
|
<literal>pg_wal</literal>.
|
|
<xref linkend="guc-max-slot-wal-keep-size"/> can be used to limit the size
|
|
of WAL files retained by replication slots.
|
|
</para>
|
|
</caution>
|
|
|
|
<sect3 id="streaming-replication-slots-manipulation">
|
|
<title>Querying and Manipulating Replication Slots</title>
|
|
<para>
|
|
Each replication slot has a name, which can contain lower-case letters,
|
|
numbers, and the underscore character.
|
|
</para>
|
|
<para>
|
|
Existing replication slots and their state can be seen in the
|
|
<link linkend="view-pg-replication-slots"><structname>pg_replication_slots</structname></link>
|
|
view.
|
|
</para>
|
|
<para>
|
|
Slots can be created and dropped either via the streaming replication
|
|
protocol (see <xref linkend="protocol-replication"/>) or via SQL
|
|
functions (see <xref linkend="functions-replication"/>).
|
|
</para>
|
|
</sect3>
|
|
<sect3 id="streaming-replication-slots-config">
|
|
<title>Configuration Example</title>
|
|
<para>
|
|
You can create a replication slot like this:
|
|
<programlisting>
|
|
postgres=# SELECT * FROM pg_create_physical_replication_slot('node_a_slot');
|
|
slot_name | lsn
|
|
-------------+-----
|
|
node_a_slot |
|
|
|
|
postgres=# SELECT slot_name, slot_type, active FROM pg_replication_slots;
|
|
slot_name | slot_type | active
|
|
-------------+-----------+--------
|
|
node_a_slot | physical | f
|
|
(1 row)
|
|
</programlisting>
|
|
To configure the standby to use this slot, <varname>primary_slot_name</varname>
|
|
should be configured on the standby. Here is a simple example:
|
|
<programlisting>
|
|
primary_conninfo = 'host=192.168.1.50 port=5432 user=foo password=foopass'
|
|
primary_slot_name = 'node_a_slot'
|
|
</programlisting>
|
|
</para>
|
|
</sect3>
|
|
</sect2>
|
|
|
|
<sect2 id="cascading-replication">
|
|
<title>Cascading Replication</title>
|
|
|
|
<indexterm zone="high-availability">
|
|
<primary>Cascading Replication</primary>
|
|
</indexterm>
|
|
|
|
<para>
|
|
The cascading replication feature allows a standby server to accept replication
|
|
connections and stream WAL records to other standbys, acting as a relay.
|
|
This can be used to reduce the number of direct connections to the primary
|
|
and also to minimize inter-site bandwidth overheads.
|
|
</para>
|
|
|
|
<para>
|
|
A standby acting as both a receiver and a sender is known as a cascading
|
|
standby. Standbys that are more directly connected to the primary are known
|
|
as upstream servers, while those standby servers further away are downstream
|
|
servers. Cascading replication does not place limits on the number or
|
|
arrangement of downstream servers, though each standby connects to only
|
|
one upstream server which eventually links to a single primary server.
|
|
</para>
|
|
|
|
<para>
|
|
A cascading standby sends not only WAL records received from the
|
|
primary but also those restored from the archive. So even if the replication
|
|
connection in some upstream connection is terminated, streaming replication
|
|
continues downstream for as long as new WAL records are available.
|
|
</para>
|
|
|
|
<para>
|
|
Cascading replication is currently asynchronous. Synchronous replication
|
|
(see <xref linkend="synchronous-replication"/>) settings have no effect on
|
|
cascading replication at present.
|
|
</para>
|
|
|
|
<para>
|
|
Hot standby feedback propagates upstream, whatever the cascaded arrangement.
|
|
</para>
|
|
|
|
<para>
|
|
If an upstream standby server is promoted to become the new primary, downstream
|
|
servers will continue to stream from the new primary if
|
|
<varname>recovery_target_timeline</varname> is set to <literal>'latest'</literal> (the default).
|
|
</para>
|
|
|
|
<para>
|
|
To use cascading replication, set up the cascading standby so that it can
|
|
accept replication connections (that is, set
|
|
<xref linkend="guc-max-wal-senders"/> and <xref linkend="guc-hot-standby"/>,
|
|
and configure
|
|
<link linkend="auth-pg-hba-conf">host-based authentication</link>).
|
|
You will also need to set <varname>primary_conninfo</varname> in the downstream
|
|
standby to point to the cascading standby.
|
|
</para>
|
|
</sect2>
|
|
|
|
<sect2 id="synchronous-replication">
|
|
<title>Synchronous Replication</title>
|
|
|
|
<indexterm zone="high-availability">
|
|
<primary>Synchronous Replication</primary>
|
|
</indexterm>
|
|
|
|
<para>
|
|
<productname>PostgreSQL</productname> streaming replication is asynchronous by
|
|
default. If the primary server
|
|
crashes then some transactions that were committed may not have been
|
|
replicated to the standby server, causing data loss. The amount
|
|
of data loss is proportional to the replication delay at the time of
|
|
failover.
|
|
</para>
|
|
|
|
<para>
|
|
Synchronous replication offers the ability to confirm that all changes
|
|
made by a transaction have been transferred to one or more synchronous
|
|
standby servers. This extends that standard level of durability
|
|
offered by a transaction commit. This level of protection is referred
|
|
to as 2-safe replication in computer science theory, and group-1-safe
|
|
(group-safe and 1-safe) when <varname>synchronous_commit</varname> is set to
|
|
<literal>remote_write</literal>.
|
|
</para>
|
|
|
|
<para>
|
|
When requesting synchronous replication, each commit of a
|
|
write transaction will wait until confirmation is
|
|
received that the commit has been written to the write-ahead log on disk
|
|
of both the primary and standby server. The only possibility that data
|
|
can be lost is if both the primary and the standby suffer crashes at the
|
|
same time. This can provide a much higher level of durability, though only
|
|
if the sysadmin is cautious about the placement and management of the two
|
|
servers. Waiting for confirmation increases the user's confidence that the
|
|
changes will not be lost in the event of server crashes but it also
|
|
necessarily increases the response time for the requesting transaction.
|
|
The minimum wait time is the round-trip time between primary and standby.
|
|
</para>
|
|
|
|
<para>
|
|
Read-only transactions and transaction rollbacks need not wait for
|
|
replies from standby servers. Subtransaction commits do not wait for
|
|
responses from standby servers, only top-level commits. Long
|
|
running actions such as data loading or index building do not wait
|
|
until the very final commit message. All two-phase commit actions
|
|
require commit waits, including both prepare and commit.
|
|
</para>
|
|
|
|
<para>
|
|
A synchronous standby can be a physical replication standby or a logical
|
|
replication subscriber. It can also be any other physical or logical WAL
|
|
replication stream consumer that knows how to send the appropriate
|
|
feedback messages. Besides the built-in physical and logical replication
|
|
systems, this includes special programs such
|
|
as <command>pg_receivewal</command> and <command>pg_recvlogical</command>
|
|
as well as some third-party replication systems and custom programs.
|
|
Check the respective documentation for details on synchronous replication
|
|
support.
|
|
</para>
|
|
|
|
<sect3 id="synchronous-replication-config">
|
|
<title>Basic Configuration</title>
|
|
|
|
<para>
|
|
Once streaming replication has been configured, configuring synchronous
|
|
replication requires only one additional configuration step:
|
|
<xref linkend="guc-synchronous-standby-names"/> must be set to
|
|
a non-empty value. <varname>synchronous_commit</varname> must also be set to
|
|
<literal>on</literal>, but since this is the default value, typically no change is
|
|
required. (See <xref linkend="runtime-config-wal-settings"/> and
|
|
<xref linkend="runtime-config-replication-primary"/>.)
|
|
This configuration will cause each commit to wait for
|
|
confirmation that the standby has written the commit record to durable
|
|
storage.
|
|
<varname>synchronous_commit</varname> can be set by individual
|
|
users, so it can be configured in the configuration file, for particular
|
|
users or databases, or dynamically by applications, in order to control
|
|
the durability guarantee on a per-transaction basis.
|
|
</para>
|
|
|
|
<para>
|
|
After a commit record has been written to disk on the primary, the
|
|
WAL record is then sent to the standby. The standby sends reply
|
|
messages each time a new batch of WAL data is written to disk, unless
|
|
<varname>wal_receiver_status_interval</varname> is set to zero on the standby.
|
|
In the case that <varname>synchronous_commit</varname> is set to
|
|
<literal>remote_apply</literal>, the standby sends reply messages when the commit
|
|
record is replayed, making the transaction visible.
|
|
If the standby is chosen as a synchronous standby, according to the setting
|
|
of <varname>synchronous_standby_names</varname> on the primary, the reply
|
|
messages from that standby will be considered along with those from other
|
|
synchronous standbys to decide when to release transactions waiting for
|
|
confirmation that the commit record has been received. These parameters
|
|
allow the administrator to specify which standby servers should be
|
|
synchronous standbys. Note that the configuration of synchronous
|
|
replication is mainly on the primary. Named standbys must be directly
|
|
connected to the primary; the primary knows nothing about downstream
|
|
standby servers using cascaded replication.
|
|
</para>
|
|
|
|
<para>
|
|
Setting <varname>synchronous_commit</varname> to <literal>remote_write</literal> will
|
|
cause each commit to wait for confirmation that the standby has received
|
|
the commit record and written it out to its own operating system, but not
|
|
for the data to be flushed to disk on the standby. This
|
|
setting provides a weaker guarantee of durability than <literal>on</literal>
|
|
does: the standby could lose the data in the event of an operating system
|
|
crash, though not a <productname>PostgreSQL</productname> crash.
|
|
However, it's a useful setting in practice
|
|
because it can decrease the response time for the transaction.
|
|
Data loss could only occur if both the primary and the standby crash and
|
|
the database of the primary gets corrupted at the same time.
|
|
</para>
|
|
|
|
<para>
|
|
Setting <varname>synchronous_commit</varname> to <literal>remote_apply</literal> will
|
|
cause each commit to wait until the current synchronous standbys report
|
|
that they have replayed the transaction, making it visible to user
|
|
queries. In simple cases, this allows for load balancing with causal
|
|
consistency.
|
|
</para>
|
|
|
|
<para>
|
|
Users will stop waiting if a fast shutdown is requested. However, as
|
|
when using asynchronous replication, the server will not fully
|
|
shutdown until all outstanding WAL records are transferred to the currently
|
|
connected standby servers.
|
|
</para>
|
|
|
|
</sect3>
|
|
|
|
<sect3 id="synchronous-replication-multiple-standbys">
|
|
<title>Multiple Synchronous Standbys</title>
|
|
|
|
<para>
|
|
Synchronous replication supports one or more synchronous standby servers;
|
|
transactions will wait until all the standby servers which are considered
|
|
as synchronous confirm receipt of their data. The number of synchronous
|
|
standbys that transactions must wait for replies from is specified in
|
|
<varname>synchronous_standby_names</varname>. This parameter also specifies
|
|
a list of standby names and the method (<literal>FIRST</literal> and
|
|
<literal>ANY</literal>) to choose synchronous standbys from the listed ones.
|
|
</para>
|
|
<para>
|
|
The method <literal>FIRST</literal> specifies a priority-based synchronous
|
|
replication and makes transaction commits wait until their WAL records are
|
|
replicated to the requested number of synchronous standbys chosen based on
|
|
their priorities. The standbys whose names appear earlier in the list are
|
|
given higher priority and will be considered as synchronous. Other standby
|
|
servers appearing later in this list represent potential synchronous
|
|
standbys. If any of the current synchronous standbys disconnects for
|
|
whatever reason, it will be replaced immediately with the
|
|
next-highest-priority standby.
|
|
</para>
|
|
<para>
|
|
An example of <varname>synchronous_standby_names</varname> for
|
|
a priority-based multiple synchronous standbys is:
|
|
<programlisting>
|
|
synchronous_standby_names = 'FIRST 2 (s1, s2, s3)'
|
|
</programlisting>
|
|
In this example, if four standby servers <literal>s1</literal>, <literal>s2</literal>,
|
|
<literal>s3</literal> and <literal>s4</literal> are running, the two standbys
|
|
<literal>s1</literal> and <literal>s2</literal> will be chosen as synchronous standbys
|
|
because their names appear early in the list of standby names.
|
|
<literal>s3</literal> is a potential synchronous standby and will take over
|
|
the role of synchronous standby when either of <literal>s1</literal> or
|
|
<literal>s2</literal> fails. <literal>s4</literal> is an asynchronous standby since
|
|
its name is not in the list.
|
|
</para>
|
|
<para>
|
|
The method <literal>ANY</literal> specifies a quorum-based synchronous
|
|
replication and makes transaction commits wait until their WAL records
|
|
are replicated to <emphasis>at least</emphasis> the requested number of
|
|
synchronous standbys in the list.
|
|
</para>
|
|
<para>
|
|
An example of <varname>synchronous_standby_names</varname> for
|
|
a quorum-based multiple synchronous standbys is:
|
|
<programlisting>
|
|
synchronous_standby_names = 'ANY 2 (s1, s2, s3)'
|
|
</programlisting>
|
|
In this example, if four standby servers <literal>s1</literal>, <literal>s2</literal>,
|
|
<literal>s3</literal> and <literal>s4</literal> are running, transaction commits will
|
|
wait for replies from at least any two standbys of <literal>s1</literal>,
|
|
<literal>s2</literal> and <literal>s3</literal>. <literal>s4</literal> is an asynchronous
|
|
standby since its name is not in the list.
|
|
</para>
|
|
<para>
|
|
The synchronous states of standby servers can be viewed using
|
|
the <structname>pg_stat_replication</structname> view.
|
|
</para>
|
|
</sect3>
|
|
|
|
<sect3 id="synchronous-replication-performance">
|
|
<title>Planning for Performance</title>
|
|
|
|
<para>
|
|
Synchronous replication usually requires carefully planned and placed
|
|
standby servers to ensure applications perform acceptably. Waiting
|
|
doesn't utilize system resources, but transaction locks continue to be
|
|
held until the transfer is confirmed. As a result, incautious use of
|
|
synchronous replication will reduce performance for database
|
|
applications because of increased response times and higher contention.
|
|
</para>
|
|
|
|
<para>
|
|
<productname>PostgreSQL</productname> allows the application developer
|
|
to specify the durability level required via replication. This can be
|
|
specified for the system overall, though it can also be specified for
|
|
specific users or connections, or even individual transactions.
|
|
</para>
|
|
|
|
<para>
|
|
For example, an application workload might consist of:
|
|
10% of changes are important customer details, while
|
|
90% of changes are less important data that the business can more
|
|
easily survive if it is lost, such as chat messages between users.
|
|
</para>
|
|
|
|
<para>
|
|
With synchronous replication options specified at the application level
|
|
(on the primary) we can offer synchronous replication for the most
|
|
important changes, without slowing down the bulk of the total workload.
|
|
Application level options are an important and practical tool for allowing
|
|
the benefits of synchronous replication for high performance applications.
|
|
</para>
|
|
|
|
<para>
|
|
You should consider that the network bandwidth must be higher than
|
|
the rate of generation of WAL data.
|
|
</para>
|
|
|
|
</sect3>
|
|
|
|
<sect3 id="synchronous-replication-ha">
|
|
<title>Planning for High Availability</title>
|
|
|
|
<para>
|
|
<varname>synchronous_standby_names</varname> specifies the number and
|
|
names of synchronous standbys that transaction commits made when
|
|
<varname>synchronous_commit</varname> is set to <literal>on</literal>,
|
|
<literal>remote_apply</literal> or <literal>remote_write</literal> will wait for
|
|
responses from. Such transaction commits may never be completed
|
|
if any one of the synchronous standbys should crash.
|
|
</para>
|
|
|
|
<para>
|
|
The best solution for high availability is to ensure you keep as many
|
|
synchronous standbys as requested. This can be achieved by naming multiple
|
|
potential synchronous standbys using <varname>synchronous_standby_names</varname>.
|
|
</para>
|
|
|
|
<para>
|
|
In a priority-based synchronous replication, the standbys whose names
|
|
appear earlier in the list will be used as synchronous standbys.
|
|
Standbys listed after these will take over the role of synchronous standby
|
|
if one of current ones should fail.
|
|
</para>
|
|
|
|
<para>
|
|
In a quorum-based synchronous replication, all the standbys appearing
|
|
in the list will be used as candidates for synchronous standbys.
|
|
Even if one of them should fail, the other standbys will keep performing
|
|
the role of candidates of synchronous standby.
|
|
</para>
|
|
|
|
<para>
|
|
When a standby first attaches to the primary, it will not yet be properly
|
|
synchronized. This is described as <literal>catchup</literal> mode. Once
|
|
the lag between standby and primary reaches zero for the first time
|
|
we move to real-time <literal>streaming</literal> state.
|
|
The catch-up duration may be long immediately after the standby has
|
|
been created. If the standby is shut down, then the catch-up period
|
|
will increase according to the length of time the standby has been down.
|
|
The standby is only able to become a synchronous standby
|
|
once it has reached <literal>streaming</literal> state.
|
|
This state can be viewed using
|
|
the <structname>pg_stat_replication</structname> view.
|
|
</para>
|
|
|
|
<para>
|
|
If primary restarts while commits are waiting for acknowledgment, those
|
|
waiting transactions will be marked fully committed once the primary
|
|
database recovers.
|
|
There is no way to be certain that all standbys have received all
|
|
outstanding WAL data at time of the crash of the primary. Some
|
|
transactions may not show as committed on the standby, even though
|
|
they show as committed on the primary. The guarantee we offer is that
|
|
the application will not receive explicit acknowledgment of the
|
|
successful commit of a transaction until the WAL data is known to be
|
|
safely received by all the synchronous standbys.
|
|
</para>
|
|
|
|
<para>
|
|
If you really cannot keep as many synchronous standbys as requested
|
|
then you should decrease the number of synchronous standbys that
|
|
transaction commits must wait for responses from
|
|
in <varname>synchronous_standby_names</varname> (or disable it) and
|
|
reload the configuration file on the primary server.
|
|
</para>
|
|
|
|
<para>
|
|
If the primary is isolated from remaining standby servers you should
|
|
fail over to the best candidate of those other remaining standby servers.
|
|
</para>
|
|
|
|
<para>
|
|
If you need to re-create a standby server while transactions are
|
|
waiting, make sure that the functions <function>pg_backup_start()</function>
|
|
and <function>pg_backup_stop()</function> are run in a session with
|
|
<varname>synchronous_commit</varname> = <literal>off</literal>, otherwise those
|
|
requests will wait forever for the standby to appear.
|
|
</para>
|
|
|
|
</sect3>
|
|
</sect2>
|
|
|
|
<sect2 id="continuous-archiving-in-standby">
|
|
<title>Continuous Archiving in Standby</title>
|
|
|
|
<indexterm>
|
|
<primary>continuous archiving</primary>
|
|
<secondary>in standby</secondary>
|
|
</indexterm>
|
|
|
|
<para>
|
|
When continuous WAL archiving is used in a standby, there are two
|
|
different scenarios: the WAL archive can be shared between the primary
|
|
and the standby, or the standby can have its own WAL archive. When
|
|
the standby has its own WAL archive, set <varname>archive_mode</varname>
|
|
to <literal>always</literal>, and the standby will call the archive
|
|
command for every WAL segment it receives, whether it's by restoring
|
|
from the archive or by streaming replication. The shared archive can
|
|
be handled similarly, but the <varname>archive_command</varname> or <varname>archive_library</varname> must
|
|
test if the file being archived exists already, and if the existing file
|
|
has identical contents. This requires more care in the
|
|
<varname>archive_command</varname> or <varname>archive_library</varname>, as it must
|
|
be careful to not overwrite an existing file with different contents,
|
|
but return success if the exactly same file is archived twice. And
|
|
all that must be done free of race conditions, if two servers attempt
|
|
to archive the same file at the same time.
|
|
</para>
|
|
|
|
<para>
|
|
If <varname>archive_mode</varname> is set to <literal>on</literal>, the
|
|
archiver is not enabled during recovery or standby mode. If the standby
|
|
server is promoted, it will start archiving after the promotion, but
|
|
will not archive any WAL or timeline history files that
|
|
it did not generate itself. To get a complete
|
|
series of WAL files in the archive, you must ensure that all WAL is
|
|
archived, before it reaches the standby. This is inherently true with
|
|
file-based log shipping, as the standby can only restore files that
|
|
are found in the archive, but not if streaming replication is enabled.
|
|
When a server is not in recovery mode, there is no difference between
|
|
<literal>on</literal> and <literal>always</literal> modes.
|
|
</para>
|
|
</sect2>
|
|
</sect1>
|
|
|
|
<sect1 id="warm-standby-failover">
|
|
<title>Failover</title>
|
|
|
|
<para>
|
|
If the primary server fails then the standby server should begin
|
|
failover procedures.
|
|
</para>
|
|
|
|
<para>
|
|
If the standby server fails then no failover need take place. If the
|
|
standby server can be restarted, even some time later, then the recovery
|
|
process can also be restarted immediately, taking advantage of
|
|
restartable recovery. If the standby server cannot be restarted, then a
|
|
full new standby server instance should be created.
|
|
</para>
|
|
|
|
<para>
|
|
If the primary server fails and the standby server becomes the
|
|
new primary, and then the old primary restarts, you must have
|
|
a mechanism for informing the old primary that it is no longer the primary. This is
|
|
sometimes known as <acronym>STONITH</acronym> (Shoot The Other Node In The Head), which is
|
|
necessary to avoid situations where both systems think they are the
|
|
primary, which will lead to confusion and ultimately data loss.
|
|
</para>
|
|
|
|
<para>
|
|
Many failover systems use just two systems, the primary and the standby,
|
|
connected by some kind of heartbeat mechanism to continually verify the
|
|
connectivity between the two and the viability of the primary. It is
|
|
also possible to use a third system (called a witness server) to prevent
|
|
some cases of inappropriate failover, but the additional complexity
|
|
might not be worthwhile unless it is set up with sufficient care and
|
|
rigorous testing.
|
|
</para>
|
|
|
|
<para>
|
|
<productname>PostgreSQL</productname> does not provide the system
|
|
software required to identify a failure on the primary and notify
|
|
the standby database server. Many such tools exist and are well
|
|
integrated with the operating system facilities required for
|
|
successful failover, such as IP address migration.
|
|
</para>
|
|
|
|
<para>
|
|
Once failover to the standby occurs, there is only a
|
|
single server in operation. This is known as a degenerate state.
|
|
The former standby is now the primary, but the former primary is down
|
|
and might stay down. To return to normal operation, a standby server
|
|
must be recreated,
|
|
either on the former primary system when it comes up, or on a third,
|
|
possibly new, system. The <xref linkend="app-pgrewind"/> utility can be
|
|
used to speed up this process on large clusters.
|
|
Once complete, the primary and standby can be
|
|
considered to have switched roles. Some people choose to use a third
|
|
server to provide backup for the new primary until the new standby
|
|
server is recreated,
|
|
though clearly this complicates the system configuration and
|
|
operational processes.
|
|
</para>
|
|
|
|
<para>
|
|
So, switching from primary to standby server can be fast but requires
|
|
some time to re-prepare the failover cluster. Regular switching from
|
|
primary to standby is useful, since it allows regular downtime on
|
|
each system for maintenance. This also serves as a test of the
|
|
failover mechanism to ensure that it will really work when you need it.
|
|
Written administration procedures are advised.
|
|
</para>
|
|
|
|
<para>
|
|
To trigger failover of a log-shipping standby server, run
|
|
<command>pg_ctl promote</command> or call <function>pg_promote()</function>.
|
|
If you're setting up reporting servers that are only used to offload
|
|
read-only queries from the primary, not for high availability purposes,
|
|
you don't need to promote.
|
|
</para>
|
|
</sect1>
|
|
|
|
<sect1 id="hot-standby">
|
|
<title>Hot Standby</title>
|
|
|
|
<indexterm zone="high-availability">
|
|
<primary>hot standby</primary>
|
|
</indexterm>
|
|
|
|
<para>
|
|
Hot standby is the term used to describe the ability to connect to
|
|
the server and run read-only queries while the server is in archive
|
|
recovery or standby mode. This
|
|
is useful both for replication purposes and for restoring a backup
|
|
to a desired state with great precision.
|
|
The term hot standby also refers to the ability of the server to move
|
|
from recovery through to normal operation while users continue running
|
|
queries and/or keep their connections open.
|
|
</para>
|
|
|
|
<para>
|
|
Running queries in hot standby mode is similar to normal query operation,
|
|
though there are several usage and administrative differences
|
|
explained below.
|
|
</para>
|
|
|
|
<sect2 id="hot-standby-users">
|
|
<title>User's Overview</title>
|
|
|
|
<para>
|
|
When the <xref linkend="guc-hot-standby"/> parameter is set to true on a
|
|
standby server, it will begin accepting connections once the recovery has
|
|
brought the system to a consistent state. All such connections are
|
|
strictly read-only; not even temporary tables may be written.
|
|
</para>
|
|
|
|
<para>
|
|
The data on the standby takes some time to arrive from the primary server
|
|
so there will be a measurable delay between primary and standby. Running the
|
|
same query nearly simultaneously on both primary and standby might therefore
|
|
return differing results. We say that data on the standby is
|
|
<firstterm>eventually consistent</firstterm> with the primary. Once the
|
|
commit record for a transaction is replayed on the standby, the changes
|
|
made by that transaction will be visible to any new snapshots taken on
|
|
the standby. Snapshots may be taken at the start of each query or at the
|
|
start of each transaction, depending on the current transaction isolation
|
|
level. For more details, see <xref linkend="transaction-iso"/>.
|
|
</para>
|
|
|
|
<para>
|
|
Transactions started during hot standby may issue the following commands:
|
|
|
|
<itemizedlist>
|
|
<listitem>
|
|
<para>
|
|
Query access: <command>SELECT</command>, <command>COPY TO</command>
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
Cursor commands: <command>DECLARE</command>, <command>FETCH</command>, <command>CLOSE</command>
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
Settings: <command>SHOW</command>, <command>SET</command>, <command>RESET</command>
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
Transaction management commands:
|
|
<itemizedlist>
|
|
<listitem>
|
|
<para>
|
|
<command>BEGIN</command>, <command>END</command>, <command>ABORT</command>, <command>START TRANSACTION</command>
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
<command>SAVEPOINT</command>, <command>RELEASE</command>, <command>ROLLBACK TO SAVEPOINT</command>
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
<command>EXCEPTION</command> blocks and other internal subtransactions
|
|
</para>
|
|
</listitem>
|
|
</itemizedlist>
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
<command>LOCK TABLE</command>, though only when explicitly in one of these modes:
|
|
<literal>ACCESS SHARE</literal>, <literal>ROW SHARE</literal> or <literal>ROW EXCLUSIVE</literal>.
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
Plans and resources: <command>PREPARE</command>, <command>EXECUTE</command>,
|
|
<command>DEALLOCATE</command>, <command>DISCARD</command>
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
Plugins and extensions: <command>LOAD</command>
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
<command>UNLISTEN</command>
|
|
</para>
|
|
</listitem>
|
|
</itemizedlist>
|
|
</para>
|
|
|
|
<para>
|
|
Transactions started during hot standby will never be assigned a
|
|
transaction ID and cannot write to the system write-ahead log.
|
|
Therefore, the following actions will produce error messages:
|
|
|
|
<itemizedlist>
|
|
<listitem>
|
|
<para>
|
|
Data Manipulation Language (DML): <command>INSERT</command>,
|
|
<command>UPDATE</command>, <command>DELETE</command>,
|
|
<command>MERGE</command>, <command>COPY FROM</command>,
|
|
<command>TRUNCATE</command>.
|
|
Note that there are no allowed actions that result in a trigger
|
|
being executed during recovery. This restriction applies even to
|
|
temporary tables, because table rows cannot be read or written without
|
|
assigning a transaction ID, which is currently not possible in a
|
|
hot standby environment.
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
Data Definition Language (DDL): <command>CREATE</command>,
|
|
<command>DROP</command>, <command>ALTER</command>, <command>COMMENT</command>.
|
|
This restriction applies even to temporary tables, because carrying
|
|
out these operations would require updating the system catalog tables.
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
<command>SELECT ... FOR SHARE | UPDATE</command>, because row locks cannot be
|
|
taken without updating the underlying data files.
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
Rules on <command>SELECT</command> statements that generate DML commands.
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
<command>LOCK</command> that explicitly requests a mode higher than <literal>ROW EXCLUSIVE MODE</literal>.
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
<command>LOCK</command> in short default form, since it requests <literal>ACCESS EXCLUSIVE MODE</literal>.
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
Transaction management commands that explicitly set non-read-only state:
|
|
<itemizedlist>
|
|
<listitem>
|
|
<para>
|
|
<command>BEGIN READ WRITE</command>,
|
|
<command>START TRANSACTION READ WRITE</command>
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
<command>SET TRANSACTION READ WRITE</command>,
|
|
<command>SET SESSION CHARACTERISTICS AS TRANSACTION READ WRITE</command>
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
<command>SET transaction_read_only = off</command>
|
|
</para>
|
|
</listitem>
|
|
</itemizedlist>
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
Two-phase commit commands: <command>PREPARE TRANSACTION</command>,
|
|
<command>COMMIT PREPARED</command>, <command>ROLLBACK PREPARED</command>
|
|
because even read-only transactions need to write WAL in the
|
|
prepare phase (the first phase of two phase commit).
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
Sequence updates: <function>nextval()</function>, <function>setval()</function>
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
<command>LISTEN</command>, <command>NOTIFY</command>
|
|
</para>
|
|
</listitem>
|
|
</itemizedlist>
|
|
</para>
|
|
|
|
<para>
|
|
In normal operation, <quote>read-only</quote> transactions are allowed to
|
|
use <command>LISTEN</command> and <command>NOTIFY</command>,
|
|
so hot standby sessions operate under slightly tighter
|
|
restrictions than ordinary read-only sessions. It is possible that some
|
|
of these restrictions might be loosened in a future release.
|
|
</para>
|
|
|
|
<para>
|
|
During hot standby, the parameter <varname>transaction_read_only</varname> is always
|
|
true and may not be changed. But as long as no attempt is made to modify
|
|
the database, connections during hot standby will act much like any other
|
|
database connection. If failover or switchover occurs, the database will
|
|
switch to normal processing mode. Sessions will remain connected while the
|
|
server changes mode. Once hot standby finishes, it will be possible to
|
|
initiate read-write transactions (even from a session begun during
|
|
hot standby).
|
|
</para>
|
|
|
|
<para>
|
|
Users can determine whether hot standby is currently active for their
|
|
session by issuing <command>SHOW in_hot_standby</command>.
|
|
(In server versions before 14, the <varname>in_hot_standby</varname>
|
|
parameter did not exist; a workable substitute method for older servers
|
|
is <command>SHOW transaction_read_only</command>.) In addition, a set of
|
|
functions (<xref linkend="functions-recovery-info-table"/>) allow users to
|
|
access information about the standby server. These allow you to write
|
|
programs that are aware of the current state of the database. These
|
|
can be used to monitor the progress of recovery, or to allow you to
|
|
write complex programs that restore the database to particular states.
|
|
</para>
|
|
</sect2>
|
|
|
|
<sect2 id="hot-standby-conflict">
|
|
<title>Handling Query Conflicts</title>
|
|
|
|
<para>
|
|
The primary and standby servers are in many ways loosely connected. Actions
|
|
on the primary will have an effect on the standby. As a result, there is
|
|
potential for negative interactions or conflicts between them. The easiest
|
|
conflict to understand is performance: if a huge data load is taking place
|
|
on the primary then this will generate a similar stream of WAL records on the
|
|
standby, so standby queries may contend for system resources, such as I/O.
|
|
</para>
|
|
|
|
<para>
|
|
There are also additional types of conflict that can occur with hot standby.
|
|
These conflicts are <emphasis>hard conflicts</emphasis> in the sense that queries
|
|
might need to be canceled and, in some cases, sessions disconnected to resolve them.
|
|
The user is provided with several ways to handle these
|
|
conflicts. Conflict cases include:
|
|
|
|
<itemizedlist>
|
|
<listitem>
|
|
<para>
|
|
Access Exclusive locks taken on the primary server, including both
|
|
explicit <command>LOCK</command> commands and various <acronym>DDL</acronym>
|
|
actions, conflict with table accesses in standby queries.
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
Dropping a tablespace on the primary conflicts with standby queries
|
|
using that tablespace for temporary work files.
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
Dropping a database on the primary conflicts with sessions connected
|
|
to that database on the standby.
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
Application of a vacuum cleanup record from WAL conflicts with
|
|
standby transactions whose snapshots can still <quote>see</quote> any of
|
|
the rows to be removed.
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
Application of a vacuum cleanup record from WAL conflicts with
|
|
queries accessing the target page on the standby, whether or not
|
|
the data to be removed is visible.
|
|
</para>
|
|
</listitem>
|
|
</itemizedlist>
|
|
</para>
|
|
|
|
<para>
|
|
On the primary server, these cases simply result in waiting; and the
|
|
user might choose to cancel either of the conflicting actions. However,
|
|
on the standby there is no choice: the WAL-logged action already occurred
|
|
on the primary so the standby must not fail to apply it. Furthermore,
|
|
allowing WAL application to wait indefinitely may be very undesirable,
|
|
because the standby's state will become increasingly far behind the
|
|
primary's. Therefore, a mechanism is provided to forcibly cancel standby
|
|
queries that conflict with to-be-applied WAL records.
|
|
</para>
|
|
|
|
<para>
|
|
An example of the problem situation is an administrator on the primary
|
|
server running <command>DROP TABLE</command> on a table that is currently being
|
|
queried on the standby server. Clearly the standby query cannot continue
|
|
if the <command>DROP TABLE</command> is applied on the standby. If this situation
|
|
occurred on the primary, the <command>DROP TABLE</command> would wait until the
|
|
other query had finished. But when <command>DROP TABLE</command> is run on the
|
|
primary, the primary doesn't have information about what queries are
|
|
running on the standby, so it will not wait for any such standby
|
|
queries. The WAL change records come through to the standby while the
|
|
standby query is still running, causing a conflict. The standby server
|
|
must either delay application of the WAL records (and everything after
|
|
them, too) or else cancel the conflicting query so that the <command>DROP
|
|
TABLE</command> can be applied.
|
|
</para>
|
|
|
|
<para>
|
|
When a conflicting query is short, it's typically desirable to allow it to
|
|
complete by delaying WAL application for a little bit; but a long delay in
|
|
WAL application is usually not desirable. So the cancel mechanism has
|
|
parameters, <xref linkend="guc-max-standby-archive-delay"/> and <xref
|
|
linkend="guc-max-standby-streaming-delay"/>, that define the maximum
|
|
allowed delay in WAL application. Conflicting queries will be canceled
|
|
once it has taken longer than the relevant delay setting to apply any
|
|
newly-received WAL data. There are two parameters so that different delay
|
|
values can be specified for the case of reading WAL data from an archive
|
|
(i.e., initial recovery from a base backup or <quote>catching up</quote> a
|
|
standby server that has fallen far behind) versus reading WAL data via
|
|
streaming replication.
|
|
</para>
|
|
|
|
<para>
|
|
In a standby server that exists primarily for high availability, it's
|
|
best to set the delay parameters relatively short, so that the server
|
|
cannot fall far behind the primary due to delays caused by standby
|
|
queries. However, if the standby server is meant for executing
|
|
long-running queries, then a high or even infinite delay value may be
|
|
preferable. Keep in mind however that a long-running query could
|
|
cause other sessions on the standby server to not see recent changes
|
|
on the primary, if it delays application of WAL records.
|
|
</para>
|
|
|
|
<para>
|
|
Once the delay specified by <varname>max_standby_archive_delay</varname> or
|
|
<varname>max_standby_streaming_delay</varname> has been exceeded, conflicting
|
|
queries will be canceled. This usually results just in a cancellation
|
|
error, although in the case of replaying a <command>DROP DATABASE</command>
|
|
the entire conflicting session will be terminated. Also, if the conflict
|
|
is over a lock held by an idle transaction, the conflicting session is
|
|
terminated (this behavior might change in the future).
|
|
</para>
|
|
|
|
<para>
|
|
Canceled queries may be retried immediately (after beginning a new
|
|
transaction, of course). Since query cancellation depends on
|
|
the nature of the WAL records being replayed, a query that was
|
|
canceled may well succeed if it is executed again.
|
|
</para>
|
|
|
|
<para>
|
|
Keep in mind that the delay parameters are compared to the elapsed time
|
|
since the WAL data was received by the standby server. Thus, the grace
|
|
period allowed to any one query on the standby is never more than the
|
|
delay parameter, and could be considerably less if the standby has already
|
|
fallen behind as a result of waiting for previous queries to complete, or
|
|
as a result of being unable to keep up with a heavy update load.
|
|
</para>
|
|
|
|
<para>
|
|
The most common reason for conflict between standby queries and WAL replay
|
|
is <quote>early cleanup</quote>. Normally, <productname>PostgreSQL</productname> allows
|
|
cleanup of old row versions when there are no transactions that need to
|
|
see them to ensure correct visibility of data according to MVCC rules.
|
|
However, this rule can only be applied for transactions executing on the
|
|
primary. So it is possible that cleanup on the primary will remove row
|
|
versions that are still visible to a transaction on the standby.
|
|
</para>
|
|
|
|
<para>
|
|
Row version cleanup isn't the only potential cause of conflicts with
|
|
standby queries. All index-only scans (including those that run on
|
|
standbys) must use an <acronym>MVCC</acronym> snapshot that
|
|
<quote>agrees</quote> with the visibility map. Conflicts are therefore
|
|
required whenever <command>VACUUM</command> <link
|
|
linkend="vacuum-for-visibility-map">sets a page as all-visible in the
|
|
visibility map</link> containing one or more rows
|
|
<emphasis>not</emphasis> visible to all standby queries. So even running
|
|
<command>VACUUM</command> against a table with no updated or deleted rows
|
|
requiring cleanup might lead to conflicts.
|
|
</para>
|
|
|
|
<para>
|
|
Users should be clear that tables that are regularly and heavily updated
|
|
on the primary server will quickly cause cancellation of longer running
|
|
queries on the standby. In such cases the setting of a finite value for
|
|
<varname>max_standby_archive_delay</varname> or
|
|
<varname>max_standby_streaming_delay</varname> can be considered similar to
|
|
setting <varname>statement_timeout</varname>.
|
|
</para>
|
|
|
|
<para>
|
|
Remedial possibilities exist if the number of standby-query cancellations
|
|
is found to be unacceptable. The first option is to set the parameter
|
|
<varname>hot_standby_feedback</varname>, which prevents <command>VACUUM</command> from
|
|
removing recently-dead rows and so cleanup conflicts do not occur.
|
|
If you do this, you
|
|
should note that this will delay cleanup of dead rows on the primary,
|
|
which may result in undesirable table bloat. However, the cleanup
|
|
situation will be no worse than if the standby queries were running
|
|
directly on the primary server, and you are still getting the benefit of
|
|
off-loading execution onto the standby.
|
|
If standby servers connect and disconnect frequently, you
|
|
might want to make adjustments to handle the period when
|
|
<varname>hot_standby_feedback</varname> feedback is not being provided.
|
|
For example, consider increasing <varname>max_standby_archive_delay</varname>
|
|
so that queries are not rapidly canceled by conflicts in WAL archive
|
|
files during disconnected periods. You should also consider increasing
|
|
<varname>max_standby_streaming_delay</varname> to avoid rapid cancellations
|
|
by newly-arrived streaming WAL entries after reconnection.
|
|
</para>
|
|
|
|
<para>
|
|
The number of query cancels and the reason for them can be viewed using
|
|
the <structname>pg_stat_database_conflicts</structname> system view on the standby
|
|
server. The <structname>pg_stat_database</structname> system view also contains
|
|
summary information.
|
|
</para>
|
|
|
|
<para>
|
|
Users can control whether a log message is produced when WAL replay is waiting
|
|
longer than <varname>deadlock_timeout</varname> for conflicts. This
|
|
is controlled by the <xref linkend="guc-log-recovery-conflict-waits"/> parameter.
|
|
</para>
|
|
</sect2>
|
|
|
|
<sect2 id="hot-standby-admin">
|
|
<title>Administrator's Overview</title>
|
|
|
|
<para>
|
|
If <varname>hot_standby</varname> is <literal>on</literal> in <filename>postgresql.conf</filename>
|
|
(the default value) and there is a
|
|
<link linkend="file-standby-signal"><filename>standby.signal</filename></link><indexterm><primary>standby.signal</primary><secondary>for hot standby</secondary></indexterm>
|
|
file present, the server will run in hot standby mode.
|
|
However, it may take some time for hot standby connections to be allowed,
|
|
because the server will not accept connections until it has completed
|
|
sufficient recovery to provide a consistent state against which queries
|
|
can run. During this period,
|
|
clients that attempt to connect will be refused with an error message.
|
|
To confirm the server has come up, either loop trying to connect from
|
|
the application, or look for these messages in the server logs:
|
|
|
|
<programlisting>
|
|
LOG: entering standby mode
|
|
|
|
... then some time later ...
|
|
|
|
LOG: consistent recovery state reached
|
|
LOG: database system is ready to accept read-only connections
|
|
</programlisting>
|
|
|
|
Consistency information is recorded once per checkpoint on the primary.
|
|
It is not possible to enable hot standby when reading WAL
|
|
written during a period when <varname>wal_level</varname> was not set to
|
|
<literal>replica</literal> or <literal>logical</literal> on the primary. Reaching
|
|
a consistent state can also be delayed in the presence of both of these
|
|
conditions:
|
|
|
|
<itemizedlist>
|
|
<listitem>
|
|
<para>
|
|
A write transaction has more than 64 subtransactions
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
Very long-lived write transactions
|
|
</para>
|
|
</listitem>
|
|
</itemizedlist>
|
|
|
|
If you are running file-based log shipping ("warm standby"), you might need
|
|
to wait until the next WAL file arrives, which could be as long as the
|
|
<varname>archive_timeout</varname> setting on the primary.
|
|
</para>
|
|
|
|
<para>
|
|
The settings of some parameters determine the size of shared memory for
|
|
tracking transaction IDs, locks, and prepared transactions. These shared
|
|
memory structures must be no smaller on a standby than on the primary in
|
|
order to ensure that the standby does not run out of shared memory during
|
|
recovery. For example, if the primary had used a prepared transaction but
|
|
the standby had not allocated any shared memory for tracking prepared
|
|
transactions, then recovery could not continue until the standby's
|
|
configuration is changed. The parameters affected are:
|
|
|
|
<itemizedlist>
|
|
<listitem>
|
|
<para>
|
|
<varname>max_connections</varname>
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
<varname>max_prepared_transactions</varname>
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
<varname>max_locks_per_transaction</varname>
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
<varname>max_wal_senders</varname>
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
<varname>max_worker_processes</varname>
|
|
</para>
|
|
</listitem>
|
|
</itemizedlist>
|
|
|
|
The easiest way to ensure this does not become a problem is to have these
|
|
parameters set on the standbys to values equal to or greater than on the
|
|
primary. Therefore, if you want to increase these values, you should do
|
|
so on all standby servers first, before applying the changes to the
|
|
primary server. Conversely, if you want to decrease these values, you
|
|
should do so on the primary server first, before applying the changes to
|
|
all standby servers. Keep in mind that when a standby is promoted, it
|
|
becomes the new reference for the required parameter settings for the
|
|
standbys that follow it. Therefore, to avoid this becoming a problem
|
|
during a switchover or failover, it is recommended to keep these settings
|
|
the same on all standby servers.
|
|
</para>
|
|
|
|
<para>
|
|
The WAL tracks changes to these parameters on the
|
|
primary. If a hot standby processes WAL that indicates that the current
|
|
value on the primary is higher than its own value, it will log a warning
|
|
and pause recovery, for example:
|
|
<screen>
|
|
WARNING: hot standby is not possible because of insufficient parameter settings
|
|
DETAIL: max_connections = 80 is a lower setting than on the primary server, where its value was 100.
|
|
LOG: recovery has paused
|
|
DETAIL: If recovery is unpaused, the server will shut down.
|
|
HINT: You can then restart the server after making the necessary configuration changes.
|
|
</screen>
|
|
At that point, the settings on the standby need to be updated and the
|
|
instance restarted before recovery can continue. If the standby is not a
|
|
hot standby, then when it encounters the incompatible parameter change, it
|
|
will shut down immediately without pausing, since there is then no value
|
|
in keeping it up.
|
|
</para>
|
|
|
|
<para>
|
|
It is important that the administrator select appropriate settings for
|
|
<xref linkend="guc-max-standby-archive-delay"/> and <xref
|
|
linkend="guc-max-standby-streaming-delay"/>. The best choices vary
|
|
depending on business priorities. For example if the server is primarily
|
|
tasked as a High Availability server, then you will want low delay
|
|
settings, perhaps even zero, though that is a very aggressive setting. If
|
|
the standby server is tasked as an additional server for decision support
|
|
queries then it might be acceptable to set the maximum delay values to
|
|
many hours, or even -1 which means wait forever for queries to complete.
|
|
</para>
|
|
|
|
<para>
|
|
Transaction status "hint bits" written on the primary are not WAL-logged,
|
|
so data on the standby will likely re-write the hints again on the standby.
|
|
Thus, the standby server will still perform disk writes even though
|
|
all users are read-only; no changes occur to the data values
|
|
themselves. Users will still write large sort temporary files and
|
|
re-generate relcache info files, so no part of the database
|
|
is truly read-only during hot standby mode.
|
|
Note also that writes to remote databases using
|
|
<application>dblink</application> module, and other operations outside the
|
|
database using PL functions will still be possible, even though the
|
|
transaction is read-only locally.
|
|
</para>
|
|
|
|
<para>
|
|
The following types of administration commands are not accepted
|
|
during recovery mode:
|
|
|
|
<itemizedlist>
|
|
<listitem>
|
|
<para>
|
|
Data Definition Language (DDL): e.g., <command>CREATE INDEX</command>
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
Privilege and Ownership: <command>GRANT</command>, <command>REVOKE</command>,
|
|
<command>REASSIGN</command>
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
Maintenance commands: <command>ANALYZE</command>, <command>VACUUM</command>,
|
|
<command>CLUSTER</command>, <command>REINDEX</command>
|
|
</para>
|
|
</listitem>
|
|
</itemizedlist>
|
|
</para>
|
|
|
|
<para>
|
|
Again, note that some of these commands are actually allowed during
|
|
"read only" mode transactions on the primary.
|
|
</para>
|
|
|
|
<para>
|
|
As a result, you cannot create additional indexes that exist solely
|
|
on the standby, nor statistics that exist solely on the standby.
|
|
If these administration commands are needed, they should be executed
|
|
on the primary, and eventually those changes will propagate to the
|
|
standby.
|
|
</para>
|
|
|
|
<para>
|
|
<function>pg_cancel_backend()</function>
|
|
and <function>pg_terminate_backend()</function> will work on user backends,
|
|
but not the startup process, which performs
|
|
recovery. <structname>pg_stat_activity</structname> does not show
|
|
recovering transactions as active. As a result,
|
|
<structname>pg_prepared_xacts</structname> is always empty during
|
|
recovery. If you wish to resolve in-doubt prepared transactions, view
|
|
<literal>pg_prepared_xacts</literal> on the primary and issue commands to
|
|
resolve transactions there or resolve them after the end of recovery.
|
|
</para>
|
|
|
|
<para>
|
|
<structname>pg_locks</structname> will show locks held by backends,
|
|
as normal. <structname>pg_locks</structname> also shows
|
|
a virtual transaction managed by the startup process that owns all
|
|
<literal>AccessExclusiveLocks</literal> held by transactions being replayed by recovery.
|
|
Note that the startup process does not acquire locks to
|
|
make database changes, and thus locks other than <literal>AccessExclusiveLocks</literal>
|
|
do not show in <structname>pg_locks</structname> for the Startup
|
|
process; they are just presumed to exist.
|
|
</para>
|
|
|
|
<para>
|
|
The <productname>Nagios</productname> plugin <productname>check_pgsql</productname> will
|
|
work, because the simple information it checks for exists.
|
|
The <productname>check_postgres</productname> monitoring script will also work,
|
|
though some reported values could give different or confusing results.
|
|
For example, last vacuum time will not be maintained, since no
|
|
vacuum occurs on the standby. Vacuums running on the primary
|
|
do still send their changes to the standby.
|
|
</para>
|
|
|
|
<para>
|
|
WAL file control commands will not work during recovery,
|
|
e.g., <function>pg_backup_start</function>, <function>pg_switch_wal</function> etc.
|
|
</para>
|
|
|
|
<para>
|
|
Dynamically loadable modules work, including <structname>pg_stat_statements</structname>.
|
|
</para>
|
|
|
|
<para>
|
|
Advisory locks work normally in recovery, including deadlock detection.
|
|
Note that advisory locks are never WAL logged, so it is impossible for
|
|
an advisory lock on either the primary or the standby to conflict with WAL
|
|
replay. Nor is it possible to acquire an advisory lock on the primary
|
|
and have it initiate a similar advisory lock on the standby. Advisory
|
|
locks relate only to the server on which they are acquired.
|
|
</para>
|
|
|
|
<para>
|
|
Trigger-based replication systems such as <productname>Slony</productname>,
|
|
<productname>Londiste</productname> and <productname>Bucardo</productname> won't run on the
|
|
standby at all, though they will run happily on the primary server as
|
|
long as the changes are not sent to standby servers to be applied.
|
|
WAL replay is not trigger-based so you cannot relay from the
|
|
standby to any system that requires additional database writes or
|
|
relies on the use of triggers.
|
|
</para>
|
|
|
|
<para>
|
|
New OIDs cannot be assigned, though some <acronym>UUID</acronym> generators may still
|
|
work as long as they do not rely on writing new status to the database.
|
|
</para>
|
|
|
|
<para>
|
|
Currently, temporary table creation is not allowed during read-only
|
|
transactions, so in some cases existing scripts will not run correctly.
|
|
This restriction might be relaxed in a later release. This is
|
|
both an SQL standard compliance issue and a technical issue.
|
|
</para>
|
|
|
|
<para>
|
|
<command>DROP TABLESPACE</command> can only succeed if the tablespace is empty.
|
|
Some standby users may be actively using the tablespace via their
|
|
<varname>temp_tablespaces</varname> parameter. If there are temporary files in the
|
|
tablespace, all active queries are canceled to ensure that temporary
|
|
files are removed, so the tablespace can be removed and WAL replay
|
|
can continue.
|
|
</para>
|
|
|
|
<para>
|
|
Running <command>DROP DATABASE</command> or <command>ALTER DATABASE ... SET
|
|
TABLESPACE</command> on the primary
|
|
will generate a WAL entry that will cause all users connected to that
|
|
database on the standby to be forcibly disconnected. This action occurs
|
|
immediately, whatever the setting of
|
|
<varname>max_standby_streaming_delay</varname>. Note that
|
|
<command>ALTER DATABASE ... RENAME</command> does not disconnect users, which
|
|
in most cases will go unnoticed, though might in some cases cause a
|
|
program confusion if it depends in some way upon database name.
|
|
</para>
|
|
|
|
<para>
|
|
In normal (non-recovery) mode, if you issue <command>DROP USER</command> or <command>DROP ROLE</command>
|
|
for a role with login capability while that user is still connected then
|
|
nothing happens to the connected user — they remain connected. The user cannot
|
|
reconnect however. This behavior applies in recovery also, so a
|
|
<command>DROP USER</command> on the primary does not disconnect that user on the standby.
|
|
</para>
|
|
|
|
<para>
|
|
The cumulative statistics system is active during recovery. All scans,
|
|
reads, blocks, index usage, etc., will be recorded normally on the
|
|
standby. However, WAL replay will not increment relation and database
|
|
specific counters. I.e. replay will not increment
|
|
<structname>pg_stat_all_tables</structname> columns (like <structfield>n_tup_ins</structfield>),
|
|
nor will reads or writes performed by the startup process be tracked in the
|
|
<structname>pg_statio_</structname> views, nor will associated
|
|
<structname>pg_stat_database</structname> columns be incremented.
|
|
</para>
|
|
|
|
<para>
|
|
Autovacuum is not active during recovery. It will start normally at the
|
|
end of recovery.
|
|
</para>
|
|
|
|
<para>
|
|
The checkpointer process and the background writer process are active during
|
|
recovery. The checkpointer process will perform restartpoints (similar to
|
|
checkpoints on the primary) and the background writer process will perform
|
|
normal block cleaning activities. This can include updates of the hint bit
|
|
information stored on the standby server.
|
|
The <command>CHECKPOINT</command> command is accepted during recovery,
|
|
though it performs a restartpoint rather than a new checkpoint.
|
|
</para>
|
|
</sect2>
|
|
|
|
<sect2 id="hot-standby-parameters">
|
|
<title>Hot Standby Parameter Reference</title>
|
|
|
|
<para>
|
|
Various parameters have been mentioned above in
|
|
<xref linkend="hot-standby-conflict"/> and
|
|
<xref linkend="hot-standby-admin"/>.
|
|
</para>
|
|
|
|
<para>
|
|
On the primary, the <xref linkend="guc-wal-level"/> parameter can be used.
|
|
<xref linkend="guc-max-standby-archive-delay"/> and
|
|
<xref linkend="guc-max-standby-streaming-delay"/> have no effect if set on
|
|
the primary.
|
|
</para>
|
|
|
|
<para>
|
|
On the standby, parameters <xref linkend="guc-hot-standby"/>,
|
|
<xref linkend="guc-max-standby-archive-delay"/> and
|
|
<xref linkend="guc-max-standby-streaming-delay"/> can be used.
|
|
</para>
|
|
</sect2>
|
|
|
|
<sect2 id="hot-standby-caveats">
|
|
<title>Caveats</title>
|
|
|
|
<para>
|
|
There are several limitations of hot standby.
|
|
These can and probably will be fixed in future releases:
|
|
|
|
<itemizedlist>
|
|
<listitem>
|
|
<para>
|
|
Full knowledge of running transactions is required before snapshots
|
|
can be taken. Transactions that use large numbers of subtransactions
|
|
(currently greater than 64) will delay the start of read-only
|
|
connections until the completion of the longest running write transaction.
|
|
If this situation occurs, explanatory messages will be sent to the server log.
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
Valid starting points for standby queries are generated at each
|
|
checkpoint on the primary. If the standby is shut down while the primary
|
|
is in a shutdown state, it might not be possible to re-enter hot standby
|
|
until the primary is started up, so that it generates further starting
|
|
points in the WAL logs. This situation isn't a problem in the most
|
|
common situations where it might happen. Generally, if the primary is
|
|
shut down and not available anymore, that's likely due to a serious
|
|
failure that requires the standby being converted to operate as
|
|
the new primary anyway. And in situations where the primary is
|
|
being intentionally taken down, coordinating to make sure the standby
|
|
becomes the new primary smoothly is also standard procedure.
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
At the end of recovery, <literal>AccessExclusiveLocks</literal> held by prepared transactions
|
|
will require twice the normal number of lock table entries. If you plan
|
|
on running either a large number of concurrent prepared transactions
|
|
that normally take <literal>AccessExclusiveLocks</literal>, or you plan on having one
|
|
large transaction that takes many <literal>AccessExclusiveLocks</literal>, you are
|
|
advised to select a larger value of <varname>max_locks_per_transaction</varname>,
|
|
perhaps as much as twice the value of the parameter on
|
|
the primary server. You need not consider this at all if
|
|
your setting of <varname>max_prepared_transactions</varname> is 0.
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
The Serializable transaction isolation level is not yet available in hot
|
|
standby. (See <xref linkend="xact-serializable"/> and
|
|
<xref linkend="serializable-consistency"/> for details.)
|
|
An attempt to set a transaction to the serializable isolation level in
|
|
hot standby mode will generate an error.
|
|
</para>
|
|
</listitem>
|
|
</itemizedlist>
|
|
|
|
</para>
|
|
</sect2>
|
|
|
|
</sect1>
|
|
|
|
</chapter>
|