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f82ec32ac3
"xlog" is not a particularly clear abbreviation for "write-ahead log", and it sometimes confuses users into believe that the contents of the "pg_xlog" directory are not critical data, leading to unpleasant consequences. So, rename the directory to "pg_wal". This patch modifies pg_upgrade and pg_basebackup to understand both the old and new directory layouts; the former is necessary given the purpose of the tool, while the latter merely avoids an unnecessary backward-compatibility break. We may wish to consider renaming other programs, switches, and functions which still use the old "xlog" naming to also refer to "wal". However, that's still under discussion, so let's do just this much for now. Discussion: CAB7nPqTeC-8+zux8_-4ZD46V7YPwooeFxgndfsq5Rg8ibLVm1A@mail.gmail.com Michael Paquier
102 lines
4.1 KiB
Plaintext
102 lines
4.1 KiB
Plaintext
src/backend/replication/README
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Walreceiver - libpqwalreceiver API
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----------------------------------
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The transport-specific part of walreceiver, responsible for connecting to
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the primary server, receiving WAL files and sending messages, is loaded
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dynamically to avoid having to link the main server binary with libpq.
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The dynamically loaded module is in libpqwalreceiver subdirectory.
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The dynamically loaded module implements four functions:
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bool walrcv_connect(char *conninfo, XLogRecPtr startpoint)
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Establish connection to the primary, and starts streaming from 'startpoint'.
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Returns true on success.
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int walrcv_receive(char **buffer, pgsocket *wait_fd)
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Retrieve any message available without blocking through the
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connection. If a message was successfully read, returns its
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length. If the connection is closed, returns -1. Otherwise returns 0
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to indicate that no data is available, and sets *wait_fd to a socket
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descriptor which can be waited on before trying again. On success, a
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pointer to the message payload is stored in *buffer. The returned
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buffer is valid until the next call to walrcv_* functions, and the
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caller should not attempt to free it.
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void walrcv_send(const char *buffer, int nbytes)
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Send a message to XLOG stream.
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void walrcv_disconnect(void);
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Disconnect.
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This API should be considered internal at the moment, but we could open it
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up for 3rd party replacements of libpqwalreceiver in the future, allowing
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pluggable methods for receiving WAL.
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Walreceiver IPC
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---------------
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When the WAL replay in startup process has reached the end of archived WAL,
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restorable using restore_command, it starts up the walreceiver process
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to fetch more WAL (if streaming replication is configured).
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Walreceiver is a postmaster subprocess, so the startup process can't fork it
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directly. Instead, it sends a signal to postmaster, asking postmaster to launch
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it. Before that, however, startup process fills in WalRcvData->conninfo
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and WalRcvData->slotname, and initializes the starting point in
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WalRcvData->receiveStart.
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As walreceiver receives WAL from the master server, and writes and flushes
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it to disk (in pg_wal), it updates WalRcvData->receivedUpto and signals
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the startup process to know how far WAL replay can advance.
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Walreceiver sends information about replication progress to the master server
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whenever it either writes or flushes new WAL, or the specified interval elapses.
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This is used for reporting purpose.
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Walsender IPC
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-------------
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At shutdown, postmaster handles walsender processes differently from regular
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backends. It waits for regular backends to die before writing the
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shutdown checkpoint and terminating pgarch and other auxiliary processes, but
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that's not desirable for walsenders, because we want the standby servers to
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receive all the WAL, including the shutdown checkpoint, before the master
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is shut down. Therefore postmaster treats walsenders like the pgarch process,
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and instructs them to terminate at PM_SHUTDOWN_2 phase, after all regular
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backends have died and checkpointer has issued the shutdown checkpoint.
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When postmaster accepts a connection, it immediately forks a new process
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to handle the handshake and authentication, and the process initializes to
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become a backend. Postmaster doesn't know if the process becomes a regular
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backend or a walsender process at that time - that's indicated in the
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connection handshake - so we need some extra signaling to let postmaster
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identify walsender processes.
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When walsender process starts up, it marks itself as a walsender process in
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the PMSignal array. That way postmaster can tell it apart from regular
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backends.
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Note that no big harm is done if postmaster thinks that a walsender is a
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regular backend; it will just terminate the walsender earlier in the shutdown
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phase. A walsender will look like a regular backend until it's done with the
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initialization and has marked itself in PMSignal array, and at process
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termination, after unmarking the PMSignal slot.
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Each walsender allocates an entry from the WalSndCtl array, and tracks
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information about replication progress. User can monitor them via
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statistics views.
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Walsender - walreceiver protocol
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--------------------------------
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See manual.
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