PostgreSQL主备切换

备库如何激活

在PostgreSQL(HOT-Standby)如主库出现异常。备库如何激活；来替换主库工作。有下列2种方式

1. 备库在recovery.conf文件中有个配置项trigger_file。它是激活standby的触发文件。当它存在；就会激活standby。
2. 使用pg_ctl promote来激活。

演示场景

模拟在主库异常挂掉。备库来替换主库工作一段时间。并原主库切换成新备库并恢复操作。
环境
主机名    IP地址    角色    数据目录
postgres202    192.168.1.202    primary    /home/postgres/data
postgres203    192.168.1.203    standby    /home/postgres/data

postgres=# select * from pg_stat_replication ;
-[ RECORD 1 ]----+------------------------------
pid              | 7687
usesysid         | 24576
usename          | replica
application_name | walreceiver
client_addr      | 192.168.1.203
client_hostname  | 
client_port      | 60234
backend_start    | 2017-09-04 22:08:26.249452+08
backend_xmin     | 1680
state            | streaming
sent_location    | 0/B0000D0
write_location   | 0/B0000D0
flush_location   | 0/B0000D0
replay_location  | 0/B0000D0
sync_priority    | 0
sync_state       | async

2.1 模拟主库关机
[postgres@postgres202 ~]$ pg_stop
waiting for server to shut down........ done
server stopped

2.2 激活备库；
模拟新主库工作一段时间。操作删除T2表和新建T3表
[postgres@postgres203 data]$ pg_ctl promote
server promoting

[postgres@postgres203 ~]$ psql lottu lottu
psql (9.6.0)
Type "help" for help.

lottu=> \dt
       List of relations
 Schema | Name | Type  | Owner 
--------+------+-------+-------
 lottu  | t    | table | lottu
 lottu  | t2   | table | lottu
(2 rows)

lottu=> drop table t2;
DROP TABLE
                                        
lottu=> create table t3 as select * from t;
SELECT 12

#切换日志；进行归档。
[postgres@postgres203 ~]$ psql
psql (9.6.0)
Type "help" for help.

postgres=# select pg_switch_xlog();
 pg_switch_xlog 
----------------
 0/C018FC0
(1 row)

postgres=# select pg_switch_xlog();
 pg_switch_xlog 
----------------
 0/D000078
(1 row)

postgres=# select pg_switch_xlog();
 pg_switch_xlog 
----------------
 0/E000000
(1 row)

2.3 恢复主库
1. 用pg_rewind命令同步新备库。
[postgres@postgres203 ~]$ pg_rewind -?
pg_rewind resynchronizes a PostgreSQL cluster with another copy of the cluster.

Usage:
  pg_rewind [OPTION]...

Options:
  -D, --target-pgdata=DIRECTORY  existing data directory to modify
      --source-pgdata=DIRECTORY  source data directory to synchronize with
      --source-server=CONNSTR    source server to synchronize with
  -n, --dry-run                  stop before modifying anything
  -P, --progress                 write progress messages
      --debug                    write a lot of debug messages
  -V, --version                  output version information, then exit
  -?, --help                     show this help, then exit

Report bugs to @postgresql.org>.

[postgres@postgres202 ~]$ pg_rewind --target-pgdata $PGDATA --source-server='host=192.168.1.203 port=5432 user=postgres dbname=postgres' -P
connected to server
servers diverged at WAL position 0/C000098 on timeline 1
rewinding from last common checkpoint at 0/C000028 on timeline 1
reading source file list
reading target file list
reading WAL in target
need to copy 203 MB (total source directory size is 221 MB)
208264/208264 kB (100%) copied
creating backup label and updating control file
syncing target data directory
Done!

[postgres@postgres202 data]$ mv recovery.done recovery.conf
[postgres@postgres202 data]$ vi recovery.conf
standby_mode = on  # 这个说明这台机器为从库
primary_conninfo = 'host=192.168.1.203 port=5432 user=replica password=replica'  # 这个说明这台机器对应主库的信息
recovery_target_timeline = 'latest' # 这个说明这个流复制同步到最新的数据

由于postgresql.conf是同步过来的；所有不做修改。也有几个地方要进行修改
max_connections = 200              # 一般查多于写的应用从库的最大连接数要比较大
hot_standby = on                   # 说明这台机器不仅仅是用于数据归档，也用于数据查询
max_standby_streaming_delay = 30s  # 数据流备份的最大延迟时间
wal_receiver_status_interval = 10s # 多久向主报告一次从的状态，当然从每次数据复制都会向主报告状态，这里只是设置最长的间隔时间
hot_standby_feedback = on          # 如果有错误的数据复制，是否向主进行反馈

【验证】
[postgres@postgres202 data]$ pg_start
server starting
[postgres@postgres202 data]$ psql lottu lottu
psql (9.6.0)
Type "help" for help.

lottu=> \dt
       List of relations
 Schema | Name | Type  | Owner 
--------+------+-------+-------
 lottu  | t    | table | lottu
 lottu  | t3   | table | lottu
(2 rows)

[postgres@postgres203 data]$ psql
psql (9.6.0)
Type "help" for help.

postgres=# \x
Expanded display is on.
postgres=# select * from pg_stat_replication ;
-[ RECORD 1 ]----+------------------------------
pid              | 4193
usesysid         | 24576
usename          | replica
application_name | walreceiver
client_addr      | 192.168.1.202
client_hostname  | 
client_port      | 60078
backend_start    | 2017-09-02 07:56:08.390662+08
backend_xmin     | 1688
state            | streaming
sent_location    | 0/F01EEA8
write_location   | 0/F01EEA8
flush_location   | 0/F01EEA8
replay_location  | 0/F01EEA8
sync_priority    | 0
sync_state       | async

参考文献：http://www.cnblogs.com/songyuejie/p/4976171.html