Logical Replication
Logical replication is a method of replicating data objects and their
changes, based upon their replication identity (usually a primary key). We
use the term logical in contrast to physical replication, which uses exact
block addresses and byte-by-byte replication. PostgreSQL supports both
mechanisms concurrently, see . Logical
replication allows fine-grained control over both data replication and
security.
Logical replication uses a publish
and subscribe model with one or
more subscribers subscribing to one or more
publications on a publisher
node. Subscribers pull data from the publications they subscribe to and may
subsequently re-publish data to allow cascading replication or more complex
configurations.
Logical replication of a table typically starts with taking a snapshot
of the data on the publisher database and copying that to the subscriber.
Once that is done, the changes on the publisher are sent to the subscriber
as they occur in real-time. The subscriber applies the data in the same
order as the publisher so that transactional consistency is guaranteed for
publications within a single subscription. This method of data replication
is sometimes referred to as transactional replication.
The typical use-cases for logical replication are:
Sending incremental changes in a single database or a subset of a
database to subscribers as they occur.
Firing triggers for individual changes as they arrive on the
subscriber.
Consolidating multiple databases into a single one (for example for
analytical purposes).
Replicating between different major versions of PostgreSQL.
Replicating between PostgreSQL instances on different platforms (for
example Linux to Windows)
Giving access to replicated data to different groups of users.
Sharing a subset of the database between multiple databases.
The subscriber database behaves in the same way as any other PostgreSQL
instance and can be used as a publisher for other databases by defining its
own publications. When the subscriber is treated as read-only by
application, there will be no conflicts from a single subscription. On the
other hand, if there are other writes done either by an application or by other
subscribers to the same set of tables, conflicts can arise.
Publication
A publication can be defined on any physical
replication primary. The node where a publication is defined is referred to
as publisher. A publication is a set of changes
generated from a table or a group of tables, and might also be described as
a change set or replication set. Each publication exists in only one database.
Publications are different from schemas and do not affect how the table is
accessed. Each table can be added to multiple publications if needed.
Publications may currently only contain tables. Objects must be added
explicitly, except when a publication is created for ALL
TABLES.
Publications can choose to limit the changes they produce to
any combination of INSERT, UPDATE,
DELETE, and TRUNCATE, similar to how triggers are fired by
particular event types. By default, all operation types are replicated.
A published table must have a replica identity configured in
order to be able to replicate UPDATE
and DELETE operations, so that appropriate rows to
update or delete can be identified on the subscriber side. By default,
this is the primary key, if there is one. Another unique index (with
certain additional requirements) can also be set to be the replica
identity. If the table does not have any suitable key, then it can be set
to replica identity full, which means the entire row becomes
the key. This, however, is very inefficient and should only be used as a
fallback if no other solution is possible. If a replica identity other
than full is set on the publisher side, a replica identity
comprising the same or fewer columns must also be set on the subscriber
side. See for details on
how to set the replica identity. If a table without a replica identity is
added to a publication that replicates UPDATE
or DELETE operations then
subsequent UPDATE or DELETE
operations will cause an error on the publisher. INSERT
operations can proceed regardless of any replica identity.
Every publication can have multiple subscribers.
A publication is created using the CREATE PUBLICATION
command and may later be altered or dropped using corresponding commands.
The individual tables can be added and removed dynamically using
ALTER PUBLICATION. Both the ADD
TABLE and DROP TABLE operations are
transactional; so the table will start or stop replicating at the correct
snapshot once the transaction has committed.
Subscription
A subscription is the downstream side of logical
replication. The node where a subscription is defined is referred to as
the subscriber. A subscription defines the connection
to another database and set of publications (one or more) to which it wants
to subscribe.
The subscriber database behaves in the same way as any other PostgreSQL
instance and can be used as a publisher for other databases by defining its
own publications.
A subscriber node may have multiple subscriptions if desired. It is
possible to define multiple subscriptions between a single
publisher-subscriber pair, in which case care must be taken to ensure
that the subscribed publication objects don't overlap.
Each subscription will receive changes via one replication slot (see
). Additional replication
slots may be required for the initial data synchronization of
pre-existing table data and those will be dropped at the end of data
synchronization.
A logical replication subscription can be a standby for synchronous
replication (see ). The standby
name is by default the subscription name. An alternative name can be
specified as application_name in the connection
information of the subscription.
Subscriptions are dumped by pg_dump if the current user
is a superuser. Otherwise a warning is written and subscriptions are
skipped, because non-superusers cannot read all subscription information
from the pg_subscription catalog.
The subscription is added using CREATE SUBSCRIPTION and
can be stopped/resumed at any time using the
ALTER SUBSCRIPTION command and removed using
DROP SUBSCRIPTION.
When a subscription is dropped and recreated, the synchronization
information is lost. This means that the data has to be resynchronized
afterwards.
The schema definitions are not replicated, and the published tables must
exist on the subscriber. Only regular tables may be
the target of replication. For example, you can't replicate to a view.
The tables are matched between the publisher and the subscriber using the
fully qualified table name. Replication to differently-named tables on the
subscriber is not supported.
Columns of a table are also matched by name. The order of columns in the
subscriber table does not need to match that of the publisher. The data
types of the columns do not need to match, as long as the text
representation of the data can be converted to the target type. For
example, you can replicate from a column of type integer to a
column of type bigint. The target table can also have
additional columns not provided by the published table. Any such columns
will be filled with the default value as specified in the definition of the
target table.
Replication Slot Management
As mentioned earlier, each (active) subscription receives changes from a
replication slot on the remote (publishing) side.
Additional table synchronization slots are normally transient, created
internally to perform initial table synchronization and dropped
automatically when they are no longer needed. These table synchronization
slots have generated names: pg_%u_sync_%u_%llu
(parameters: Subscription oid,
Table relid, system identifier sysid)
Normally, the remote replication slot is created automatically when the
subscription is created using CREATE SUBSCRIPTION and it
is dropped automatically when the subscription is dropped using
DROP SUBSCRIPTION. In some situations, however, it can
be useful or necessary to manipulate the subscription and the underlying
replication slot separately. Here are some scenarios:
When creating a subscription, the replication slot already exists. In
that case, the subscription can be created using
the create_slot = false option to associate with the
existing slot.
When creating a subscription, the remote host is not reachable or in an
unclear state. In that case, the subscription can be created using
the connect = false option. The remote host will then not
be contacted at all. This is what pg_dump
uses. The remote replication slot will then have to be created
manually before the subscription can be activated.
When dropping a subscription, the replication slot should be kept.
This could be useful when the subscriber database is being moved to a
different host and will be activated from there. In that case,
disassociate the slot from the subscription using ALTER
SUBSCRIPTION before attempting to drop the subscription.
When dropping a subscription, the remote host is not reachable. In
that case, disassociate the slot from the subscription
using ALTER SUBSCRIPTION before attempting to drop
the subscription. If the remote database instance no longer exists, no
further action is then necessary. If, however, the remote database
instance is just unreachable, the replication slot (and any still
remaining table synchronization slots) should then be
dropped manually; otherwise it/they would continue to reserve WAL and might
eventually cause the disk to fill up. Such cases should be carefully
investigated.
Conflicts
Logical replication behaves similarly to normal DML operations in that
the data will be updated even if it was changed locally on the subscriber
node. If incoming data violates any constraints the replication will
stop. This is referred to as a conflict. When
replicating UPDATE or DELETE
operations, missing data will not produce a conflict and such operations
will simply be skipped.
A conflict will produce an error and will stop the replication; it must be
resolved manually by the user. Details about the conflict can be found in
the subscriber's server log.
The resolution can be done either by changing data on the subscriber so
that it does not conflict with the incoming change or by skipping the
transaction that conflicts with the existing data. The transaction can be
skipped by calling the
pg_replication_origin_advance() function with
a node_name corresponding to the subscription name,
and a position. The current position of origins can be seen in the
pg_replication_origin_status system view.
Restrictions
Logical replication currently has the following restrictions or missing
functionality. These might be addressed in future releases.
The database schema and DDL commands are not replicated. The initial
schema can be copied by hand using pg_dump
--schema-only. Subsequent schema changes would need to be kept
in sync manually. (Note, however, that there is no need for the schemas
to be absolutely the same on both sides.) Logical replication is robust
when schema definitions change in a live database: When the schema is
changed on the publisher and replicated data starts arriving at the
subscriber but does not fit into the table schema, replication will error
until the schema is updated. In many cases, intermittent errors can be
avoided by applying additive schema changes to the subscriber first.
Sequence data is not replicated. The data in serial or identity columns
backed by sequences will of course be replicated as part of the table,
but the sequence itself would still show the start value on the
subscriber. If the subscriber is used as a read-only database, then this
should typically not be a problem. If, however, some kind of switchover
or failover to the subscriber database is intended, then the sequences
would need to be updated to the latest values, either by copying the
current data from the publisher (perhaps
using pg_dump) or by determining a sufficiently high
value from the tables themselves.
Replication of TRUNCATE commands is supported, but
some care must be taken when truncating groups of tables connected by
foreign keys. When replicating a truncate action, the subscriber will
truncate the same group of tables that was truncated on the publisher,
either explicitly specified or implicitly collected via
CASCADE, minus tables that are not part of the
subscription. This will work correctly if all affected tables are part
of the same subscription. But if some tables to be truncated on the
subscriber have foreign-key links to tables that are not part of the same
(or any) subscription, then the application of the truncate action on the
subscriber will fail.
Large objects (see ) are not replicated.
There is no workaround for that, other than storing data in normal
tables.
Replication is only supported by tables, including partitioned tables.
Attempts to replicate other types of relations, such as views, materialized
views, or foreign tables, will result in an error.
When replicating between partitioned tables, the actual replication
originates, by default, from the leaf partitions on the publisher, so
partitions on the publisher must also exist on the subscriber as valid
target tables. (They could either be leaf partitions themselves, or they
could be further subpartitioned, or they could even be independent
tables.) Publications can also specify that changes are to be replicated
using the identity and schema of the partitioned root table instead of
that of the individual leaf partitions in which the changes actually
originate (see CREATE PUBLICATION).
Architecture
Logical replication starts by copying a snapshot of the data on the
publisher database. Once that is done, changes on the publisher are sent
to the subscriber as they occur in real time. The subscriber applies data
in the order in which commits were made on the publisher so that
transactional consistency is guaranteed for the publications within any
single subscription.
Logical replication is built with an architecture similar to physical
streaming replication (see ). It is
implemented by walsender and apply
processes. The walsender process starts logical decoding (described
in ) of the WAL and loads the standard
logical decoding plugin (pgoutput). The plugin transforms the changes read
from WAL to the logical replication protocol
(see ) and filters the data
according to the publication specification. The data is then continuously
transferred using the streaming replication protocol to the apply worker,
which maps the data to local tables and applies the individual changes as
they are received, in correct transactional order.
The apply process on the subscriber database always runs with
session_replication_role set
to replica, which produces the usual effects on triggers
and constraints.
The logical replication apply process currently only fires row triggers,
not statement triggers. The initial table synchronization, however, is
implemented like a COPY command and thus fires both row
and statement triggers for INSERT.
Initial Snapshot
The initial data in existing subscribed tables are snapshotted and
copied in a parallel instance of a special kind of apply process.
This process will create its own replication slot and copy the existing
data. As soon as the copy is finished the table contents will become
visible to other backends. Once existing data is copied, the worker
enters synchronization mode, which ensures that the table is brought
up to a synchronized state with the main apply process by streaming
any changes that happened during the initial data copy using standard
logical replication. During this synchronization phase, the changes
are applied and committed in the same order as they happened on the
publisher. Once the synchronization is done, the control of the
replication of the table is given back to the main apply process where
the replication continues as normal.
Monitoring
Because logical replication is based on a similar architecture as
physical streaming replication,
the monitoring on a publication node is similar to monitoring of a
physical replication primary
(see ).
The monitoring information about subscription is visible in
pg_stat_subscription.
This view contains one row for every subscription worker. A subscription
can have zero or more active subscription workers depending on its state.
Normally, there is a single apply process running for an enabled
subscription. A disabled subscription or a crashed subscription will have
zero rows in this view. If the initial data synchronization of any
table is in progress, there will be additional workers for the tables
being synchronized.
Security
A user able to modify the schema of subscriber-side tables can execute
arbitrary code as a superuser. Limit ownership
and TRIGGER privilege on such tables to roles that
superusers trust. Moreover, if untrusted users can create tables, use only
publications that list tables explicitly. That is to say, create a
subscription FOR ALL TABLES only when superusers trust
every user permitted to create a non-temp table on the publisher or the
subscriber.
The role used for the replication connection must have
the REPLICATION attribute (or be a superuser). If the
role lacks SUPERUSER and BYPASSRLS,
publisher row security policies can execute. If the role does not trust
all table owners, include options=-crow_security=off in
the connection string; if a table owner then adds a row security policy,
that setting will cause replication to halt rather than execute the policy.
Access for the role must be configured in pg_hba.conf
and it must have the LOGIN attribute.
In order to be able to copy the initial table data, the role used for the
replication connection must have the SELECT privilege on
a published table (or be a superuser).
To create a publication, the user must have the CREATE
privilege in the database.
To add tables to a publication, the user must have ownership rights on the
table. To create a publication that publishes all tables automatically,
the user must be a superuser.
To create a subscription, the user must be a superuser.
The subscription apply process will run in the local database with the
privileges of a superuser.
Privileges are only checked once at the start of a replication connection.
They are not re-checked as each change record is read from the publisher,
nor are they re-checked for each change when applied.
Configuration Settings
Logical replication requires several configuration options to be set.
On the publisher side, wal_level must be set to
logical, and max_replication_slots
must be set to at least the number of subscriptions expected to connect,
plus some reserve for table synchronization. And
max_wal_senders should be set to at least the same as
max_replication_slots plus the number of physical
replicas that are connected at the same time.
The subscriber also requires the max_replication_slots
to be set. In this case it should be set to at least the number of
subscriptions that will be added to the subscriber, plus some reserve for
table synchronization. max_logical_replication_workers
must be set to at least the number of subscriptions, again plus some reserve
for the table synchronization. Additionally the max_worker_processes
may need to be adjusted to accommodate for replication workers, at least
(max_logical_replication_workers
+ 1). Note that some extensions and parallel queries
also take worker slots from max_worker_processes.
Quick Setup
First set the configuration options in postgresql.conf:
wal_level = logical
The other required settings have default values that are sufficient for a
basic setup.
pg_hba.conf needs to be adjusted to allow replication
(the values here depend on your actual network configuration and user you
want to use for connecting):
host all repuser 0.0.0.0/0 md5
Then on the publisher database:
CREATE PUBLICATION mypub FOR TABLE users, departments;
And on the subscriber database:
CREATE SUBSCRIPTION mysub CONNECTION 'dbname=foo host=bar user=repuser' PUBLICATION mypub;
The above will start the replication process, which synchronizes the
initial table contents of the tables users and
departments and then starts replicating
incremental changes to those tables.