將stanby host(slave)提升為master

若master因意外導致無法正常服務，為能夠正常繼續提供服務，此時就需要將slave提升為master，確保DB能夠正常寫入資料。本章節將模擬master無法提供服務之情況，並透過repmgr standby promote功能，將slave提升為master角色，流程大致如下列：

1. 確認repmgr是否運作正常
2. 關閉mater postgreql服務
3. 於slave手動執行repmgr standby promote，提升為mater角色
4. 確認DB可正常被寫入

1.確認repmgr是否運作正常

[On master & slave]

先確認repmgr運作是否正常運作

repmgr=# SELECT * FROM repmgr_dbc.repl_nodes ORDER BY id;
 id |  type   | upstream_node_id | cluster | name  |                   conninfo                    |   slot_name   | priority | active 
----+---------+------------------+---------+-------+-----------------------------------------------+---------------+----------+--------
  1 | master  |                  | dbc     | node1 | host=192.168.252.50 user=repmgr dbname=repmgr | repmgr_slot_1 |      100 | t
  2 | standby |                1 | dbc     | node2 | host=192.168.252.51 user=repmgr dbname=repmgr | repmgr_slot_2 |      100 | t
(2 rows)

2.關閉mater postgreql服務

[on master]

~$ sudo service postgresql stop
or
~$ sudo pg_ctlcluster 10 main stop --force (On Debian/Ubuntu)

[on slave]

確認節點狀態

repmgr=# SELECT * FROM repmgr_dbc.repl_nodes ORDER BY id;
 id |  type   | upstream_node_id | cluster | name  |                   conninfo                    |   slot_name   | priority | active 
----+---------+------------------+---------+-------+-----------------------------------------------+---------------+----------+--------
  1 | master  |                  | dbc     | node1 | host=192.168.252.50 user=repmgr dbname=repmgr | repmgr_slot_1 |      100 | t
  2 | standby |                1 | dbc     | node2 | host=192.168.252.51 user=repmgr dbname=repmgr | repmgr_slot_2 |      100 | t
(2 rows)

此時在slave上會發現mater狀態仍是active，這個狀態我也疑惑了一陣子，後來在repmgr的文件中得到了幹話般解釋：

At this point the replication cluster will be in a partially disabled state with both standbys accepting read-only connections while attempting to connect to the stopped master. Note that the repl_nodes table will not yet have been updated and will still show the master as active.

3. 於slave手動執行repmgr standby promote，提升為mater角色

[on slave]

postgres@DBC-slave:/home/tw_it$ repmgr standby promote
ERROR: connection to database failed: could not connect to server: Connection refused
    Is the server running on host "192.168.252.50" and accepting
    TCP/IP connections on port 5432?

NOTICE: promoting standby
NOTICE: promoting server using 'pg_ctl -D /var/lib/postgresql/10/main promote'
waiting for server to promote.... done
server promoted
NOTICE: STANDBY PROMOTE successful

再次確認節點狀態，會發現原本master狀態已變為"f", slave的type也會轉變為"master"。

repmgr=# SELECT * FROM repmgr_dbc.repl_nodes ORDER BY id;
 id |  type  | upstream_node_id | cluster | name  |                   conninfo                    |   slot_name   | priority | active 
----+--------+------------------+---------+-------+-----------------------------------------------+---------------+----------+--------
  1 | master |                  | dbc     | node1 | host=192.168.252.50 user=repmgr dbname=repmgr | repmgr_slot_1 |      100 | f
  2 | master |                  | dbc     | node2 | host=192.168.252.51 user=repmgr dbname=repmgr | repmgr_slot_2 |      100 | t

Note1: 若有出現"sh: 1: pg_ctl: not found"訊息，請參閱上一章節"加入pg_ctl搜尋路徑" Note2: repmgr會先確認master是否可以正常連線，若在master安好的狀態下，repmgr會拒絕將slave promote為master。所以第一筆輸出連線失敗的ERROR訊息是正常的。