PostgreSQL 各个后台进程关系的理解
启动PostgreSQL 进程后，可以看到：
[root@localhost ~]# ps -ef | grep post
root 2991 2925 0 10:42 pts/1 00:00:00 su - postgres
postgres 2992 2991 0 10:42 pts/1 00:00:00 -bash
postgres 3029 2992 0 10:42 pts/1 00:00:00 /usr/local/pgsql/bin/postgres -D /usr/local/pgsql/data
postgres 3031 3029 0 10:42 ? 00:00:00 postgres: checkpointer process
postgres 3032 3029 0 10:42 ? 00:00:00 postgres: writer process
postgres 3033 3029 0 10:42 ? 00:00:00 postgres: wal writer process
postgres 3034 3029 0 10:42 ? 00:00:00 postgres: autovacuum launcher process
postgres 3035 3029 0 10:42 ? 00:00:00 postgres: stats collector process
root 3061 3039 0 10:43 pts/2 00:00:00 grep post
[root@localhost ~]#
不难发现，
checkpointer process,
writer process,
wal writer process
autovacuum launcher process
stats collector process
都是 postgres 进程的子进程。
那么是如何做到的呢？各个子进程名称不同，而且还被同一个父进程来生成，用来完成不同的工作。
有点 龙生九子各不同的感觉。
看代码(Postmaster.c)：
#define StartupDataBase() StartChildProcess(StartupProcess) #define StartBackgroundWriter() StartChildProcess(BgWriterProcess) #define StartCheckpointer() StartChildProcess(CheckpointerProcess) #define StartWalWriter() StartChildProcess(WalWriterProcess) #define StartWalReceiver() StartChildProcess(WalReceiverProcess) "/* * Main idle loop of postmaster */ " static int ServerLoop(void) { … for (;;) { …… /* * If no background writer process is running, and we are not in a * state that prevents it, start one. It doesn't matter if this * fails, we'll just try again later. Likewise for the checkpointer. */ if (pmState == PM_RUN || pmState == PM_RECOVERY || pmState == PM_HOT_STANDBY) { if (CheckpointerPID == 0) CheckpointerPID = StartCheckpointer(); if (BgWriterPID == 0) BgWriterPID = StartBackgroundWriter(); } … } … } * StartChildProcess -- start an auxiliary process for the postmaster * * xlop determines what kind of child will be started. All child types * initially go to AuxiliaryProcessMain, which will handle common setup. * * Return value of StartChildProcess is subprocess' PID, or 0 if failed * to start subprocess. */ static pid_t StartChildProcess(AuxProcType type) { pid_t pid; char *av[10]; int ac = 0; char typebuf[32]; /* * Set up command-line arguments for subprocess */ av[ac++] = "postgres"; #ifdef EXEC_BACKEND av[ac++] = "--forkboot"; av[ac++] = NULL; /* filled in by postmaster_forkexec */ #endif snprintf(typebuf, sizeof(typebuf), "-x%d", type); av[ac++] = typebuf; av[ac] = NULL; Assert(ac < lengthof(av)); #ifdef EXEC_BACKEND pid = postmaster_forkexec(ac, av); #else /* !EXEC_BACKEND */ pid = fork_process(); if (pid == 0) /* child */ { IsUnderPostmaster = true; /* we are a postmaster subprocess now */ /* Close the postmaster's sockets */ ClosePostmasterPorts(false); /* Lose the postmaster's on-exit routines and port connections */ on_exit_reset(); /* Release postmaster's working memory context */ MemoryContextSwitchTo(TopMemoryContext); MemoryContextDelete(PostmasterContext); PostmasterContext = NULL; AuxiliaryProcessMain(ac, av); ExitPostmaster(0); } #endif /* EXEC_BACKEND */ if (pid < 0) { /* in parent, fork failed */ int save_errno = errno; errno = save_errno; switch (type) { case StartupProcess: ereport(LOG, (errmsg("could not fork startup process: %m"))); break; case BgWriterProcess: ereport(LOG, (errmsg("could not fork background writer process: %m"))); break; case CheckpointerProcess: ereport(LOG, (errmsg("could not fork checkpointer process: %m"))); break; case WalWriterProcess: ereport(LOG, (errmsg("could not fork WAL writer process: %m"))); break; case WalReceiverProcess: ereport(LOG, (errmsg("could not fork WAL receiver process: %m"))); break; default: ereport(LOG, (errmsg("could not fork process: %m"))); break; } /* * fork failure is fatal during startup, but there's no need to choke * immediately if starting other child types fails. */ if (type == StartupProcess) ExitPostmaster(1); return 0; } /* * in parent, successful fork */ return pid; }
可以看到 ，Postmaster.c 的 主循环中，安排了如下部分：
if (BgWriterPID == 0) BgWriterPID = StartBackgroundWriter();
而根据宏定义， StartBackgroundWriter() 就是 StartChildProcess(BgWriterProcess)
在 通用程序 StartChildProcess 中，根据参数的不同，来完成对不同的程序的 fork动作：
av[ac++] = "postgres";
snprintf(typebuf, sizeof(typebuf), "-x%d", type);
两句，就是我们ps 的时候能够看到 postgres: writer process 信息的原因。
AuxiliaryProcessMain(ac, av); 是各个子进程真正各自为战的入口点。
而关于 EXEC_BACKEND ,可以看如下这段(I put the following just for memo)：
http://postgresql.1045698.n5.nabble.com/where-EXEC-BACKEND-td2008305.html
> hi,
>
> actually i try to execute postgres step by step (on paper)
> i don't retreive where EXEC_BACKEND is initialized
> can any one help me?
> it is very important for me
Nowhere. If you want it, you have to define it manually in
pg_config_manual.h.
EXEC_BACKEND is a source code hack that allows the Unix build (which
normally uses only fork() without exec()) to follow the same startup
code as the Windows version (which uses CreateProcess(), equivalent to
both fork() and exec()), allowing for better debuggability for those of
us that do not use Windows.
If you want to follow postmaster initialization on a POSIX platform,
it's easier if you just assume that EXEC_BACKEND is not defined.