# PostgreSQL 9.6 同步多副本 与 remote_apply事务同步级别 应用场景分析

## PostgreSQL 9.6 同步多副本 与 remote_apply事务同步级别

digoal

2016-10-06

### 标签

PostgreSQL , 9.6 , 同步多副本 , remote_apply , 全同步

## 背景

9.6以前的版本，同步复制是确保XLOG已经复制到备库，而不是已经在备库apply，虽然APPLY通常都很快，可能也在毫秒级别完成，但是以前没有apply级别的同步机制。

## 同步多副本配置方法

synchronous_standby_names (string)

num_sync ( standby_name [, ...] )

standby_name [, ...]


num_sync 数字，表示同步的副本数。

standby_name是standby配置在recovery.conf中的application_name，可以使用*代替所有standby。

2 (s1, s2, s3, s4)


Specifies a list of standby servers that can support synchronous replication, as described in Section 26.2.8.

There will be one or more active synchronous standbys;

transactions waiting for commit will be allowed to proceed after these standby servers confirm receipt of their data.

The synchronous standbys will be those whose names appear earlier in this list, and that are both currently connected and streaming data in real-time

(as shown by a state of streaming in the pg_stat_replication view).

Other standby servers appearing later in this list represent potential synchronous standbys.

If any of the current synchronous standbys disconnects for whatever reason, it will be replaced immediately with the next-highest-priority standby.

Specifying more than one standby name can allow very high availability.

This parameter specifies a list of standby servers using either of the following syntaxes:

num_sync ( standby_name [, ...] )

standby_name [, ...]

where num_sync is the number of synchronous standbys that transactions need to wait for replies from, and standby_name is the name of a standby server.

For example, a setting of 3 (s1, s2, s3, s4) makes transaction commits wait until their WAL records are received by three higher-priority standbys chosen from standby servers s1, s2, s3 and s4.

The second syntax was used before PostgreSQL version 9.6 and is still supported.

It's the same as the first syntax with num_sync equal to 1.

For example, 1 (s1, s2) and s1, s2 have the same meaning:

either s1 or s2 is chosen as a synchronous standby.

The name of a standby server for this purpose is the application_name setting of the standby, as set in the primary_conninfo of the standby's WAL receiver.

There is no mechanism to enforce uniqueness.

In case of duplicates one of the matching standbys will be considered as higher priority, though exactly which one is indeterminate.

The special entry * matches any application_name, including the default application name of walreceiver.

Note: Each standby_name should have the form of a valid SQL identifier, unless it is *.

You can use double-quoting if necessary.

But note that standby_names are compared to standby application names case-insensitively, whether double-quoted or not.

If no synchronous standby names are specified here, then synchronous replication is not enabled and transaction commits will not wait for replication.

This is the default configuration.

Even when synchronous replication is enabled, individual transactions can be configured not to wait for replication by setting the synchronous_commit parameter to local or off.


## 事务同步级别说明

SET [ SESSION | LOCAL ] configuration_parameter { TO | = } { value | 'value' | DEFAULT }


事务提交或回滚时，会产生一笔事务结束的commit/rollback redo record，在REDO的地址系统中，用LSN表示。

on,
事务commit或rollback时，等待其redo在primary、以及同步standby(s)已持久化（>=其lsn）。

remote_apply,
事务commit或rollback时，等待其redo在primary、以及同步standby(s)已持久化（>=其lsn）。 并且其redo在同步standby(s)已apply（>=其lsn）。

remote_write,
事务commit或rollback时，等待其redo在primary已持久化;  其redo在同步standby(s)已调用write接口(写到OS, 但是还没有调用持久化接口如fsync)（>=其lsn）。

local,
事务commit或rollback时，等待其redo在primary已持久化;

off
事务commit或rollback时，等待其redo在primary已写入wal buffer，不需要等待其持久化;


synchronous_commit (enum)

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.


## remote_apply事务同步级别配置方法

SET [ SESSION | LOCAL ] configuration_parameter { TO | = } { value | 'value' | DEFAULT }


1. 可靠性

2. 读写分离的读一致性保证

3. 切换时延

Count

+ 订阅