pg主从复制（二）——备库激活过程分析

一、前言

PostgreSQL standby 可以通过两种方法来激活成为主库：

trigger file：配置在recovery.conf中。
pg_ctl promote：发送SIGUSR1信号给postmaster进程。

同时，PostgreSQL支持快速激活（fast promote）和非快速激活(fallback promote)：

fast promote：开启数据库读写前不需要做检查点，而是推到开启读写之后执行一个CHECKPOINT_FORCE检查点。
fallback_promote：在开启数据库读写前需要先做一个检查点，现在这个模式已经不对用户开放，只是用作调试。

二、实现分析

激活过程，根据fast_promote变量判断是否需要先做检查点，再激活。

src/backend/access/transam/xlog.cif (InRecovery) // 当前为Recovery模式{if (bgwriterLaunched){if (fast_promote)  // 如果是快速promote，在打开数据库读写前，不需要创建检查点。只需要创建一个recovery结束标记{checkPointLoc = ControlFile->prevCheckPoint;record = ReadCheckpointRecord(xlogreader, checkPointLoc, 1, false);if (record != NULL){fast_promoted = true;CreateEndOfRecoveryRecord();}}if (!fast_promoted)  //  如果是fallback_promote模式，须先创建一个检查点，再开启读写模式。RequestCheckpoint(CHECKPOINT_END_OF_RECOVERY |CHECKPOINT_IMMEDIATE |CHECKPOINT_WAIT);}
...InRecovery = false;    //  开启读写模式,允许接收用户写请求.LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);ControlFile->state = DB_IN_PRODUCTION;   //  改写控制文件的数据库状态ControlFile->time = (pg_time_t) time(NULL);UpdateControlFile();   //  更新控制文件LWLockRelease(ControlFileLock);...if (fast_promoted)   //  如果是快速promote，在允许用户写请求后，在这里执行一个检查点。所以提高了数据库的可用时间。RequestCheckpoint(CHECKPOINT_FORCE);
......

通过pg_ctl命令行工具，向postmaster发SIGUSR1信号，通知它激活数据库。首先会写一个promote文件，告诉postmaster，是fast_promote模式。

src/bin/pg_ctl/pg_ctl.c
/** promote*/static void
do_promote(void)
{FILE       *prmfile;pgpid_t         pid;struct stat statbuf;pid = get_pgpid(false);......sig = SIGUSR1;if (kill((pid_t) pid, sig) != 0)  // 发送SIGUSR1信号，通知postmaster激活数据库。{write_stderr(_("%s: could not send promote signal (PID: %ld): %s\n"),progname, pid, strerror(errno));if (unlink(promote_file) != 0)write_stderr(_("%s: could not remove promote signal file \"%s\": %s\n"),progname, promote_file, strerror(errno));exit(1);}print_msg(_("server promoting\n"));
}

数据恢复时，检查standby是否收到promote请求或是否存在trigger文件。如果是promote请求，则检查有没有promote文件，或者fallback_promote文件。如果有promote文件，则是fast_promote请求；如果有fallback_promote文件，则不是fast_promote请求（实际上根本不可能检测到fallback_promote文件，因为没有写这个文件的操作）。所以通过pg_ctl promote来激活，一定是fast promote的，即不需要先做检查点再激活。如果检查到trigger文件，同样也是fast promote激活模式。

src/backend/access/transam/xlog.c
#define PROMOTE_SIGNAL_FILE             "promote"
#define FALLBACK_PROMOTE_SIGNAL_FILE "fallback_promote"/** Check to see whether the user-specified trigger file exists and whether a* promote request has arrived.  If either condition holds, return true.*/
static bool
CheckForStandbyTrigger(void)
{struct stat stat_buf;static bool triggered = false;if (triggered)return true;if (IsPromoteTriggered())   // 检查是否收到pg_ctl promote信号{
......if (stat(PROMOTE_SIGNAL_FILE, &stat_buf) == 0)    // 先检查promote文件是否存在{unlink(PROMOTE_SIGNAL_FILE);unlink(FALLBACK_PROMOTE_SIGNAL_FILE);fast_promote = true;    //  快速promote}else if (stat(FALLBACK_PROMOTE_SIGNAL_FILE, &stat_buf) == 0)  // 否则再检查fallback_promote文件是否存在{unlink(FALLBACK_PROMOTE_SIGNAL_FILE);fast_promote = false;    //  先执行checkpoint再promote}ereport(LOG, (errmsg("received promote request")));ResetPromoteTriggered();triggered = true;return true;}if (TriggerFile == NULL)   //  检查recovery.conf是否配置了trigger_filereturn false;if (stat(TriggerFile, &stat_buf) == 0){ereport(LOG,(errmsg("trigger file found: %s", TriggerFile)));unlink(TriggerFile);triggered = true;fast_promote = true;    //   快速promotereturn true;}else if (errno != ENOENT)ereport(ERROR,(errcode_for_file_access(),errmsg("could not stat trigger file \"%s\": %m",TriggerFile)));return false;
}src/backend/postmaster/startup.cpqsignal(SIGUSR2, StartupProcTriggerHandler);  // 注册SIGUSR2信号处理函数/* SIGUSR2: set flag to finish recovery */
static void
StartupProcTriggerHandler(SIGNAL_ARGS)
{int                     save_errno = errno;promote_triggered = true;WakeupRecovery();errno = save_errno;
}bool
IsPromoteTriggered(void)
{return promote_triggered;
}

postmaster收到SIGUSER1信号后，检查是否收到promote信号，判断当前的状态是否处于恢复中的任意状态，然后向startup进程发一个SIGUSR2的信号，触发promote。

src/backend/postmaster/postmaster.cpqsignal(SIGUSR1, sigusr1_handler); /* message from child process */  // 注册SIGUSR1信号处理函数/** sigusr1_handler - handle signal conditions from child processes*/
static void
sigusr1_handler(SIGNAL_ARGS)
{
......if (CheckPromoteSignal() && StartupPID != 0 &&(pmState == PM_STARTUP || pmState == PM_RECOVERY ||pmState == PM_HOT_STANDBY || pmState == PM_WAIT_READONLY)){/* Tell startup process to finish recovery */signal_child(StartupPID, SIGUSR2);  //  向startup进程发SIGUSR2信号，通知它处理promote}
......src/backend/access/transam/xlog.c
/** Check to see if a promote request has arrived. Should be* called by postmaster after receiving SIGUSR1.*/
bool
CheckPromoteSignal(void)
{struct stat stat_buf;if (stat(PROMOTE_SIGNAL_FILE, &stat_buf) == 0 ||stat(FALLBACK_PROMOTE_SIGNAL_FILE, &stat_buf) == 0)return true;return false;
}

最后提一点， 9.3以前，曾经出现过pg_ctl promote -m 来指定是否需要fast promote或者fallback promote。