PostgreSQL启动以及关闭或运行过程中的状态包括以上七种。在pg_controldata获取的内容Database cluster state一栏显示的是DB的状态。其中：
DB_STARTUP：表示数据库正在启动状态，实际上没有使用该状态。
DB_SHUTDOWNED：数据库实例正常关闭（非standby）控制文件写入的状态就是这个状态
DB_SHUTDOWNED_IN_RECOVERY：standby实例正常关闭，控制文件写入的状态是这个状态。是由CreateRestartPoint修改该状态。
DB_SHUTDOWNING：非standby实例在关闭时，做checkpoint：CreateCheckPoint，开始做时修改为该状态，做完后修改为DB_SHUTDOWNED状态。
DB_IN_CRASH_RECOVERY：实例异常关闭，重启后，恢复时需要将实例先置为该状态
DB_IN_ARCHIVE_RECOVERY：standby实例重启后置为该状态。
DB_IN_PRODUCTION：非standby实例正常重启后就是这个状态，standby是DB_IN_ARCHIVE_RECOVERY
分析
1、DB_STARTUP
initdb->BootStrapXLOG: memset(ControlFile, 0, sizeof(ControlFileData)); ... ControlFile->state = DB_SHUTDOWNED; ... WriteControlFile();初始化时，首先将其状态初始化为DB_STARTUP，然后立即置成DB_SHUTDOWNED并将其刷写到磁盘。
2、StartupXLOG
StartupXLOG-> ReadControlFile(); ... readRecoveryCommandFile();-> |--... | for (item = head; item; item = item->next){ | if (strcmp(item->name, "restore_command") == 0){ | ... | }... | else if (strcmp(item->name, "standby_mode") == 0){ | if (!parse_bool(item->value, &StandbyModeRequested)) | }... | } | ... |-- ArchiveRecoveryRequested = true; ... if (ArchiveRecoveryRequested && (ControlFile->minRecoveryPoint != InvalidXLogRecPtr || ControlFile->backupEndRequired || ControlFile->backupEndPoint != InvalidXLogRecPtr || ControlFile->state == DB_SHUTDOWNED)){ InArchiveRecovery = true; if (StandbyModeRequested) StandbyMode = true; } ... record = ReadCheckpointRecord(xlogreader, checkPointLoc, 1, true); ... if (InRecovery){ if (InArchiveRecovery)//何时？ ControlFile->state = DB_IN_ARCHIVE_RECOVERY; else ControlFile->state = DB_IN_CRASH_RECOVERY; ... UpdateControlFile(); replay... } ... LWLockAcquire(ControlFileLock, LW_EXCLUSIVE); ControlFile->state = DB_IN_PRODUCTION; UpdateControlFile(); LWLockRelease(ControlFileLock); ...只要有recovery.conf文件，ArchiveRecoveryRequested即为TRUE->InArchiveRecovery = true，配置了standby_mode=on,那么StandbyMode=TRUE。这样standby启动后，ControlFile->state为DB_IN_ARCHIVE_RECOVERY状态。
3、checkpoint
CheckpointerMain-> for (;;){ ... if (shutdown_requested){ ShutdownXLOG(0, 0);-> |--if (RecoveryInProgress()){ | CreateRestartPoint(CHECKPOINT_IS_SHUTDOWN | CHECKPOINT_IMMEDIATE); | }else{ | CreateCheckPoint(CHECKPOINT_IS_SHUTDOWN | CHECKPOINT_IMMEDIATE); | } |--... proc_exit(0); } ... if (do_checkpoint){ do_restartpoint = RecoveryInProgress(); ... if (flags & CHECKPOINT_END_OF_RECOVERY)//flags从哪来？ do_restartpoint = false; ... if (!do_restartpoint){ CreateCheckPoint(flags); ckpt_performed = true; } else ckpt_performed = CreateRestartPoint(flags); } }备机上做checkpoint调用CreateRestartPoint，主机做checkpoint调用CreateCheckPoint
CreateCheckPoint(int flags)-> if (flags & (CHECKPOINT_IS_SHUTDOWN | CHECKPOINT_END_OF_RECOVERY)) shutdown = true; else shutdown = false; ... if (shutdown){ LWLockAcquire(ControlFileLock, LW_EXCLUSIVE); ControlFile->state = DB_SHUTDOWNING; ControlFile->time = (pg_time_t) time(NULL); UpdateControlFile(); LWLockRelease(ControlFileLock); } ... LWLockAcquire(ControlFileLock, LW_EXCLUSIVE); if (shutdown) ControlFile->state = DB_SHUTDOWNED; ... UpdateControlFile(); LWLockRelease(ControlFileLock);shutdown时，先将状态置为DB_SHUTDOWNING，最后将状态置为DB_SHUTDOWNED
CreateRestartPoint(int flags)-> LWLockAcquire(CheckpointLock, LW_EXCLUSIVE); SpinLockAcquire(&XLogCtl->info_lck); lastCheckPointRecPtr = XLogCtl->lastCheckPointRecPtr; lastCheckPointEndPtr = XLogCtl->lastCheckPointEndPtr; lastCheckPoint = XLogCtl->lastCheckPoint; SpinLockRelease(&XLogCtl->info_lck); if (!RecoveryInProgress()){ LWLockRelease(CheckpointLock); return false; } ... if (XLogRecPtrIsInvalid(lastCheckPointRecPtr) ||lastCheckPoint.redo <= ControlFile->checkPointCopy.redo){ UpdateMinRecoveryPoint(InvalidXLogRecPtr, true); if (flags & CHECKPOINT_IS_SHUTDOWN){ LWLockAcquire(ControlFileLock, LW_EXCLUSIVE); ControlFile->state = DB_SHUTDOWNED_IN_RECOVERY; ControlFile->time = (pg_time_t) time(NULL); UpdateControlFile(); LWLockRelease(ControlFileLock); } LWLockRelease(CheckpointLock); return false; } ... LWLockAcquire(ControlFileLock, LW_EXCLUSIVE); if (ControlFile->state == DB_IN_ARCHIVE_RECOVERY && ControlFile->checkPointCopy.redo < lastCheckPoint.redo){ ... if (flags & CHECKPOINT_IS_SHUTDOWN) ControlFile->state = DB_SHUTDOWNED_IN_RECOVERY; UpdateControlFile(); } LWLockRelease(ControlFileLock); ...备机shutdown，将状态置为DB_SHUTDOWNED_IN_RECOVERY
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