数据库ACID中的持久化如何实现
数据库ACID里面的D，持久化。 指的是对于用户来说提交的事务，数据是可靠的，即使数据库crash了，在硬件完好的情况下，也能恢复回来。
PostgreSQL是怎么做到的呢，看一幅图，画得比较丑，凑合看吧。
假设一个事务，对数据库做了一些操作，并且产生了一些脏数据，首先这些脏数据会在数据库的shared buffer中。
同时，产生这些脏数据的同时也会产生对应的redo信息，产生的REDO会有对应的LSN号（你可以理解为REDO 的虚拟地址空间的一个唯一的OFFSET，每一笔REDO都有），这个LSN号也会记录到shared buffer中对应的脏页中。
walwriter是负责将wal buffer flush到持久化设备的进程，同时它会更新一个全局变量，记录已经flush的最大的LSN号。
bgwriter是负责将shared buffer的脏页持久化到持久化设备的进程，它在flush时，除了要遵循LRU算法之外，还要通过LSN全局变量的比对，来保证脏页对应的REDO记录已经flush到持久化设备了，如果发现还对应的REDO没有持久化，会触发WAL writer去flush wal buffer。 (即确保日志比脏数据先落盘)
当用户提交事务时，也会产生一笔提交事务的REDO，这笔REDO也携带了LSN号。backend process 同样需要等待对应LSN flush到磁盘后才会返回给用户提交成功的信号。(保证日志先落盘，然后返回给用户) 
数据库同步复制原理浅析
同步流复制，即保证standby节点和本地节点的日志双双落盘。 
PostgreSQL使用另一组全局变量，记录同步流复制节点已经接收到的XLOG LSN，以及已经持久化的XLOG LSN。
用户在发起提交请求后，backend process除了要判断本地wal有没有持久化，同时还需要判断同步流复制节点的XLOG有没有接收到或持久化（通过synchronous_commit参数控制）。
如果同步流复制节点的XLOG还没有接收或持久化，backend process会进入等待状态。
数据库同步复制代码浅析
对应的代码和解释如下：
CommitTransaction @ src/backend/access/transam/xact.c
RecordTransactionCommit @ src/backend/access/transam/xact.c
/*
* If we didn't create XLOG entries, we're done here; otherwise we
* should trigger flushing those entries the same as a commit record
* would. This will primarily happen for HOT pruning and the like; we
* want these to be flushed to disk in due time.
*/
if (!wrote_xlog) // 没有产生redo的事务，直接返回
goto cleanup;
if (wrote_xlog && markXidCommitted) // 如果产生了redo, 等待同步流复制
SyncRepWaitForLSN(XactLastRecEnd);SyncRepWaitForLSN @ src/backend/replication/syncrep.c
/*
* Wait for synchronous replication, if requested by user.
*
* Initially backends start in state SYNC_REP_NOT_WAITING and then
* change that state to SYNC_REP_WAITING before adding ourselves
* to the wait queue. During SyncRepWakeQueue() a WALSender changes
* the state to SYNC_REP_WAIT_COMPLETE once replication is confirmed.
* This backend then resets its state to SYNC_REP_NOT_WAITING.
*/
void
SyncRepWaitForLSN(XLogRecPtr XactCommitLSN)
{
...
/*
* Fast exit if user has not requested sync replication, or there are no
* sync replication standby names defined. Note that those standbys don't
* need to be connected.
*/
if (!SyncRepRequested() || !SyncStandbysDefined()) // 如果不是同步事务或者没有定义同步流复制节点，直接返回
return;
...
/*
* We don't wait for sync rep if WalSndCtl->sync_standbys_defined is not
* set. See SyncRepUpdateSyncStandbysDefined.
*
* Also check that the standby hasn't already replied. Unlikely race
* condition but we'll be fetching that cache line anyway so it's likely
* to be a low cost check.
*/
if (!WalSndCtl->sync_standbys_defined ||
XactCommitLSN <= WalSndCtl->lsn[mode]) // 如果没有定义同步流复制节点，或者判断到commit lsn小于已同步的LSN，说明XLOG已经flush了，直接返回。
{
LWLockRelease(SyncRepLock);
return;
}
...
// 进入循环等待状态，说明本地的xlog已经flush了，只是等待同步流复制节点的REDO同步状态。
/*
* Wait for specified LSN to be confirmed.
*
* Each proc has its own wait latch, so we perform a normal latch
* check/wait loop here.
*/
for (;;) // 进入等待状态，检查latch是否满足释放等待的条件（wal sender会根据REDO的同步情况，实时更新对应的latch）
{
int syncRepState;
/* Must reset the latch before testing state. */
ResetLatch(&MyProc->procLatch);
syncRepState = MyProc->syncRepState;
if (syncRepState == SYNC_REP_WAITING)
{
LWLockAcquire(SyncRepLock, LW_SHARED);
syncRepState = MyProc->syncRepState;
LWLockRelease(SyncRepLock);
}
if (syncRepState == SYNC_REP_WAIT_COMPLETE) // 说明XLOG同步完成，退出等待
break;
// 如果本地进程挂了，输出的消息内容是，本地事务信息已持久化，但是远程也许还没有持久化
if (ProcDiePending)
{
ereport(WARNING,
(errcode(ERRCODE_ADMIN_SHUTDOWN),
errmsg("canceling the wait for synchronous replication and terminating connection due to administrator command"),
errdetail("The transaction has already committed locally, but might not have been replicated to the standby.")));
whereToSendOutput = DestNone;
SyncRepCancelWait();
break;
}
// 如果用户主动cancel query，输出的消息内容是，本地事务信息已持久化，但是远程也许还没有持久化
if (QueryCancelPending)
{
QueryCancelPending = false;
ereport(WARNING,
(errmsg("canceling wait for synchronous replication due to user request"),
errdetail("The transaction has already committed locally, but might not have been replicated to the standby.")));
SyncRepCancelWait();
break;
}
// 如果postgres主进程挂了，进入退出流程。
if (!PostmasterIsAlive())
{
ProcDiePending = true;
whereToSendOutput = DestNone;
SyncRepCancelWait();
break;
}
// 等待wal sender来修改对应的latch
/*
* Wait on latch. Any condition that should wake us up will set the
* latch, so no need for timeout.
*/
WaitLatch(&MyProc->procLatch, WL_LATCH_SET | WL_POSTMASTER_DEATH, -1);注意用户进入等待状态后，只有主动cancel , 或者kill(terminate) , 或者主进程die才能退出无限的等待状态。后面会讲到如何将同步级别降级为异步。
前面提到了，用户端需要等待LATCH的释放信号。
那么谁来给它这个信号了，是wal sender进程，源码和解释如下 :
src/backend/replication/walsender.c
StartReplication
WalSndLoop
ProcessRepliesIfAny
ProcessStandbyMessage
ProcessStandbyReplyMessage
if (!am_cascading_walsender) // 非级联流复制节点，那么它将调用SyncRepReleaseWaiters修改backend process等待队列中它们对应的 latch。
SyncRepReleaseWaiters();SyncRepReleaseWaiters @ src/backend/replication/syncrep.c
/*
* Update the LSNs on each queue based upon our latest state. This
* implements a simple policy of first-valid-standby-releases-waiter.
*
* Other policies are possible, which would change what we do here and what
* perhaps also which information we store as well.
*/
void
SyncRepReleaseWaiters(void)
{
...
// 释放满足条件的等待队列
/*
* Set the lsn first so that when we wake backends they will release up to
* this location.
*/
if (walsndctl->lsn[SYNC_REP_WAIT_WRITE] < MyWalSnd->write)
{
walsndctl->lsn[SYNC_REP_WAIT_WRITE] = MyWalSnd->write;
numwrite = SyncRepWakeQueue(false, SYNC_REP_WAIT_WRITE);
}
if (walsndctl->lsn[SYNC_REP_WAIT_FLUSH] < MyWalSnd->flush)
{
walsndctl->lsn[SYNC_REP_WAIT_FLUSH] = MyWalSnd->flush;
numflush = SyncRepWakeQueue(false, SYNC_REP_WAIT_FLUSH);
}
...SyncRepWakeQueue @ src/backend/replication/syncrep.c
/*
* Walk the specified queue from head. Set the state of any backends that
* need to be woken, remove them from the queue, and then wake them.
* Pass all = true to wake whole queue; otherwise, just wake up to
* the walsender's LSN.
*
* Must hold SyncRepLock.
*/
static int
SyncRepWakeQueue(bool all, int mode)
{
...
while (proc) // 修改对应的backend process 的latch
{
/*
* Assume the queue is ordered by LSN
*/
if (!all && walsndctl->lsn[mode] < proc->waitLSN)
return numprocs;
/*
* Move to next proc, so we can delete thisproc from the queue.
* thisproc is valid, proc may be NULL after this.
*/
thisproc = proc;
proc = (PGPROC *) SHMQueueNext(&(WalSndCtl->SyncRepQueue[mode]),
&(proc->syncRepLinks),
offsetof(PGPROC, syncRepLinks));
/*
* Set state to complete; see SyncRepWaitForLSN() for discussion of
* the various states.
*/
thisproc->syncRepState = SYNC_REP_WAIT_COMPLETE; // 满足条件时，改成SYNC_REP_WAIT_COMPLETE
....如何设置事务可靠性级别
PostgreSQL 支持在会话中设置事务的可靠性级别。
off 表示commit 时不需要等待wal 持久化。
local 表示commit 是只需要等待本地数据库的wal 持久化。
remote_write 表示commit 需要等待本地数据库的wal 持久化，同时需要等待sync standby节点wal write buffer完成(不需要持久化)。
on 表示commit 需要等待本地数据库的wal 持久化，同时需要等待sync standby节点wal持久化。
提醒一点， synchronous_commit 的任何一种设置，都不影响wal日志持久化必须先于shared buffer脏数据持久化。 所以不管你怎么设置，都不好影响数据的一致性。
synchronous_commit = off # synchronization level;
# off, local, remote_write, or on如何实现同步复制降级
从前面的代码解析可以得知，如果 backend process 进入了等待循环，只接受几种信号降级。 并且降级后会告警，表示本地wal已持久化，但是sync standby节点不确定wal有没有持久化。
如果你只配置了1个standby，并且将它配置为同步流复制节点。一旦出现网络抖动，或者sync standby节点故障，将导致同步事务进入等待状态。
怎么降级呢？
方法1.
修改配置文件并重置
$ vi postgresql.conf
synchronous_commit = local
$ pg_ctl reload然后cancel 所有query .
postgres=# select pg_cancel_backend(pid) from pg_stat_activity where pid<>pg_backend_pid();收到这样的信号，表示事务成功提交，同时表示WAL不知道有没有同步到sync standby。
WARNING: canceling wait for synchronous replication due to user request
DETAIL: The transaction has already committed locally, but might not have been replicated to the standby.
COMMIT
postgres=# show synchronous_commit ;
synchronous_commit
--------------------
off
(1 row)同时它会读到全局变量synchronous_commit 已经是 local了。
这样就完成了降级的动作。
方法2.
方法1的降级需要对已有的正在等待wal sync的pid使用cancel进行处理，有点不人性化。
可以通过修改代码的方式，做到更人性化。
SyncRepWaitForLSN for循环中，加一个判断，如果发现全局变量sync commit变成local, off了，则告警并退出。这样就不需要人为的去cancel query了.
WARNING: canceling wait for synchronous replication due to user request
DETAIL: The transaction has already committed locally, but might not have been replicated to the standby.