postgresql/src/backend/replication
Alvaro Herrera a8aaa0c786
Morph pg_replication_slots.min_safe_lsn to safe_wal_size
The previous definition of the column was almost universally disliked,
so provide this updated definition which is more useful for monitoring
purposes: a large positive value is good, while zero or a negative value
means danger.  This should be operationally more convenient.

Backpatch to 13, where the new column to pg_replication_slots (and the
feature it represents) were added.

Author: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Author: Álvaro Herrera <alvherre@alvh.no-ip.org>
Reported-by: Fujii Masao <masao.fujii@oss.nttdata.com>
Discussion: https://postgr.es/m/9ddfbf8c-2f67-904d-44ed-cf8bc5916228@oss.nttdata.com
2020-07-07 13:08:00 -04:00
..
libpqwalreceiver walreceiver uses a temporary replication slot by default 2020-01-14 14:40:41 +01:00
logical Remove extra whitespace in comments atop ReorderBufferCheckMemoryLimit. 2020-07-06 08:49:09 +05:30
pgoutput Initial pgindent and pgperltidy run for v13. 2020-05-14 13:06:50 -04:00
.gitignore Support multiple synchronous standby servers. 2016-04-06 17:18:25 +09:00
Makefile Move the server's backup manifest code to a separate file. 2020-04-20 14:38:15 -04:00
README Move description of libpqwalreceiver hooks out of the replication's README 2020-07-02 13:57:03 +09:00
backup_manifest.c Fix buffile.c error handling. 2020-06-16 16:59:07 +12:00
basebackup.c Improve server code to read files as part of a base backup. 2020-06-17 11:39:17 -04:00
repl_gram.y Generate backup manifests for base backups, and validate them. 2020-04-03 15:05:59 -04:00
repl_scanner.l Generate backup manifests for base backups, and validate them. 2020-04-03 15:05:59 -04:00
slot.c Persist slot invalidation correctly 2020-06-26 20:41:29 -04:00
slotfuncs.c Morph pg_replication_slots.min_safe_lsn to safe_wal_size 2020-07-07 13:08:00 -04:00
syncrep.c Fix race conditions in synchronous standby management. 2020-04-18 14:02:44 -04:00
syncrep_gram.y Update copyrights for 2020 2020-01-01 12:21:45 -05:00
syncrep_scanner.l Update copyrights for 2020 2020-01-01 12:21:45 -05:00
walreceiver.c Make pg_stat_wal_receiver consistent with the WAL receiver's shmem info 2020-05-17 09:22:07 +09:00
walreceiverfuncs.c Initial pgindent and pgperltidy run for v13. 2020-05-14 13:06:50 -04:00
walsender.c Fix crash in WAL sender when starting physical replication 2020-06-08 10:12:24 +09:00

README

src/backend/replication/README

Walreceiver - libpqwalreceiver API
----------------------------------

The transport-specific part of walreceiver, responsible for connecting to
the primary server, receiving WAL files and sending messages, is loaded
dynamically to avoid having to link the main server binary with libpq.
The dynamically loaded module is in libpqwalreceiver subdirectory.

The dynamically loaded module implements a set of functions with details
about each one of them provided in src/include/replication/walreceiver.h.

This API should be considered internal at the moment, but we could open it
up for 3rd party replacements of libpqwalreceiver in the future, allowing
pluggable methods for receiving WAL.

Walreceiver IPC
---------------

When the WAL replay in startup process has reached the end of archived WAL,
restorable using restore_command, it starts up the walreceiver process
to fetch more WAL (if streaming replication is configured).

Walreceiver is a postmaster subprocess, so the startup process can't fork it
directly. Instead, it sends a signal to postmaster, asking postmaster to launch
it. Before that, however, startup process fills in WalRcvData->conninfo
and WalRcvData->slotname, and initializes the starting point in
WalRcvData->receiveStart.

As walreceiver receives WAL from the master server, and writes and flushes
it to disk (in pg_wal), it updates WalRcvData->flushedUpto and signals
the startup process to know how far WAL replay can advance.

Walreceiver sends information about replication progress to the master server
whenever it either writes or flushes new WAL, or the specified interval elapses.
This is used for reporting purpose.

Walsender IPC
-------------

At shutdown, postmaster handles walsender processes differently from regular
backends. It waits for regular backends to die before writing the
shutdown checkpoint and terminating pgarch and other auxiliary processes, but
that's not desirable for walsenders, because we want the standby servers to
receive all the WAL, including the shutdown checkpoint, before the master
is shut down. Therefore postmaster treats walsenders like the pgarch process,
and instructs them to terminate at PM_SHUTDOWN_2 phase, after all regular
backends have died and checkpointer has issued the shutdown checkpoint.

When postmaster accepts a connection, it immediately forks a new process
to handle the handshake and authentication, and the process initializes to
become a backend. Postmaster doesn't know if the process becomes a regular
backend or a walsender process at that time - that's indicated in the
connection handshake - so we need some extra signaling to let postmaster
identify walsender processes.

When walsender process starts up, it marks itself as a walsender process in
the PMSignal array. That way postmaster can tell it apart from regular
backends.

Note that no big harm is done if postmaster thinks that a walsender is a
regular backend; it will just terminate the walsender earlier in the shutdown
phase. A walsender will look like a regular backend until it's done with the
initialization and has marked itself in PMSignal array, and at process
termination, after unmarking the PMSignal slot.

Each walsender allocates an entry from the WalSndCtl array, and tracks
information about replication progress. User can monitor them via
statistics views.


Walsender - walreceiver protocol
--------------------------------

See manual.