mirror of
https://github.com/postgres/postgres.git
synced 2025-11-21 00:42:43 +03:00
7d6d2c4bbd
The type argument wasn't actually really necessary. It was a remnant of converting the API of the gist strategy translation from using opclass to using opfamily+opcintype (commitsc09e5a6a01,622f678c10). For looking up the gist translation function, we used the convention "amproclefttype = amprocrighttype = opclass's opcintype" (see pg_amproc.h). But each operator family should only have one translation function, and getting the right type for the lookup is sometimes cumbersome and fragile, so this is all unnecessarily complicated. To simplify this, change the gist stategy support procedure to take "any", "any" as argument. (This is arbitrary but seems intuitive. The alternative of using InvalidOid as argument(s) upsets various DDL commands, so it's not practical.) Then we don't need opcintype for the lookup, and we can remove it from all the API layers introduced by commitc09e5a6a01. This also adds some more documentation about the correct signature of the gist support function and adds more checks in gistvalidate(). This was previously underspecified. (It relied implicitly on convention mentioned above.) Discussion: https://www.postgresql.org/message-id/flat/E72EAA49-354D-4C2E-8EB9-255197F55330@enterprisedb.com
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 the PM_WAIT_XLOG_ARCHIVAL 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.