From 293913568e6a7a86fd1479e1cff8e2ecb58d6568 Mon Sep 17 00:00:00 2001 From: Daniel Baumann Date: Sat, 13 Apr 2024 15:44:03 +0200 Subject: Adding upstream version 16.2. Signed-off-by: Daniel Baumann --- src/backend/replication/README | 76 ++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 76 insertions(+) create mode 100644 src/backend/replication/README (limited to 'src/backend/replication/README') diff --git a/src/backend/replication/README b/src/backend/replication/README new file mode 100644 index 0000000..8fcd78d --- /dev/null +++ b/src/backend/replication/README @@ -0,0 +1,76 @@ +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 primary 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 primary 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 primary +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. -- cgit v1.2.3