PostgreSQL的同步级别与MySQL的半同步after_sync比较

MySQL的半同步中通过binlog进行流复制，同步级别和PostgreSQL对比可以发现：

PostgreSQL                MySQL

off

local                         after_commit         

remote_write            after_sync

remote_apply

on

所以，整体来说MySQL的日志同步上还是没有PostgreSQL做的严谨，在金融系统中，PostgreSQL的日志同步级别都是设置为on，即日志接收到，apply，然后等待数据刷盘才返回commit的ack。

Specifies whether transaction commit will wait for WAL records to be written to disk before the command returns a “success” indication to the client. Valid values are on, remote_apply, remote_write, local, and off. The default, and safe, setting is on. When off, there can be a delay between when success is reported to the client and when the transaction is really guaranteed to be safe against a server crash. (The maximum delay is three times wal_writer_delay.) Unlike fsync, setting this parameter to off does not create any risk of database inconsistency: an operating system or database crash might result in some recent allegedly-committed transactions being lost, but the database state will be just the same as if those transactions had been aborted cleanly. So, turning synchronous_commit off can be a useful alternative when performance is more important than exact certainty about the durability of a transaction. For more discussion see Section 30.3.

If synchronous_standby_names is non-empty, this parameter also controls whether or not transaction commits will wait for their WAL records to be replicated to the standby server(s). When set to on, commits will wait until replies from the current synchronous standby(s) indicate they have received the commit record of the transaction and flushed it to disk. This ensures the transaction will not be lost unless both the primary and all synchronous standbys suffer corruption of their database storage. When set to remote_apply, commits will wait until replies from the current synchronous standby(s) indicate they have received the commit record of the transaction and applied it, so that it has become visible to queries on the standby(s). When set to remote_write, commits will wait until replies from the current synchronous standby(s) indicate they have received the commit record of the transaction and written it out to their operating system. This setting is sufficient to ensure data preservation even if a standby instance of PostgreSQL were to crash, but not if the standby suffers an operating-system-level crash, since the data has not necessarily reached stable storage on the standby. Finally, the setting local causes commits to wait for local flush to disk, but not for replication. This is not usually desirable when synchronous replication is in use, but is provided for completeness.

If synchronous_standby_names is empty, the settings on, remote_apply, remote_write and local all provide the same synchronization level: transaction commits only wait for local flush to disk.

This parameter can be changed at any time; the behavior for any one transaction is determined by the setting in effect when it commits. It is therefore possible, and useful, to have some transactions commit synchronously and others asynchronously. For example, to make a single multistatement transaction commit asynchronously when the default is the opposite, issue SET LOCAL synchronous_commit TO OFF within the transaction.

下面转自：https://www.jianshu.com/p/3bfb0bfb8b34

今天主要剖析一下MySQL 5.7增强半同步的AFTER SYNC和AFTER COMMIT的区别。

如果我们生产库对数据的一致性要求比较高，那么我们一般会开启了半同步复制，但在MySQL5.5/5.6里，会存在数据不一致的风险。比如有如下场景，客户端提交了一个事务，master把binlog发送给slave，在发送的期间，网络出现波动，此时Binlog Dump线程发送就会卡住，要等待slave把binlog写到本地的relay-log里，然后给master一个反馈，等待的时间以rpl_semi_sync_master_timeout参数为准，默认为10秒。在这等待的10秒钟里，在其他会话里，查看刚才的事务是可以看见的，此时一旦master发生宕机，由于binlog没有发送给slave，前端app切到slave查看，就会发现刚才已提交的事务不见了。盗用两张图讲解一下两者的区别。

• AFTER SYNC
  after sync是MySQL5.7官方新加的用以解决MySQL5.6半同步缺陷的选项，也是官方推荐的方式。下面我结合图来说明一下AFTER SYNC是怎么回事。
   

  

  image

  实际上，客户端发出commit请求后，在主库上写入binlog并推送给slave，slave接收到binlog并写入relaylog，发送ACK确认已经接收binlog后，master在引擎层commit，客户端接收commit完成，此时其他会话才可以看见已提交的数据。
      故障分析：假设master在接收ACK确认时宕机，因为在引擎层并没有提交，HA切换到从库，因为binlog已经写入从库的relaylog，因此不会造成数据丢失，个人认为是目前比较完美的解决方式。

• AFTER COMMIT
   

  

  image

  after commit是MySQL5.6半同步参数，区别于after sync，after sync是在接收ack确认以后主库在引擎层做提交，而after commit是先在引擎层做提交后等待ACK确认。因此，在写入数据后并且在从库确认之前，其他的客户端可以看到在这一事务。
      故障分析：
  1.binlog 未发送到从库:
      事务B获取到事务A提交的内容， 此时宕机故障切换到slave，事务B获取到的内容却丢失了。事务A commit没有收到反馈信息(则需要业务判断了)。
  2.binlog 已经发送给从库 :
      事务B获取到事务A提交的内容，故障切换到salve ，B仍然获取到A提交的内容，没毛病。事务A commit没有收到反馈信息，若重新执行该事务，则相当于执行两次A事务(则需要业务判断了)。

dump thread过程分析:

• mysql5.6版本之前：
  1. master dump thread 发送binlog events 给 slave 的IO thread，等待 slave 的ack反馈
  2. slave 接受binlog events 写入relay log ，返回 ack 消息给master dump thread
  3. master dump thread 收到ack消息，给session返回commit ok，然后继续发送写一个事务的binlog。

• mysql5.7之后新增ack线程：
  1. master dump thread 发送binlog events 给 slave 的IO thread，开启ack线程等待 slave 的ack反馈，dump 线程继续向slaveIO thread发送下一个事务的binlog。
  2. slave 接受binlog events 写入relay log ，返回 ack 消息给master ack线程，然后给session返回commit ok。

过程总结:

1.Master在收到slave的应答后才Commit事务--after_sync(5.6上Master在commit后，才等待Slave的应答--after commit).
因此在确认事务复制到Slave上之前，并发的事务看不到当前事务的数据.当Master出现故障时,所有已经提交的事务都复制到了Slave上.
2.缺省采用无数据丢失的应答等待机制after_sync。用户也可以选择使用5.6的应答等待机制after_commit

设置方法:

mysql> SET rpl_semi_sync_master_wait_point= AFTER_SYNC;

Master接收到N个slave的应答后，才commit 事务.
用户可以设置应答Slave的数量:

mysql> SET GLOBAL rpl_semi_sync_master_wait_for_slave_count= N；