I have followed below steps for switchover and switchback.
(我已按照以下步骤进行切换和切回。)
Step 1:- Disconnect application services from 10.xx10 and do the following
(步骤1：-从10.xx10断开应用程序服务并执行以下操作)
@Master(10.xx10)
(@Master（10.xx10）)
pg_ctl -D /DATA_VEC/pgdata stop --mode=fast
@DR(20.xx20)
(@DR（20.xx20）)
promote DR as read write mode
(将DR提升为读写模式)
Step 2:- Start master as DR from new primary
(步骤2：-从新的主服务器启动主服务器作为DR)
@Master(10.xx10) create recovery.conf
(@Master（10.xx10）创建recovery.conf)
standby_mode = 'on'
primary_conninfo = 'user= password= host=20.x.x.20 port=9999
trigger_file = '/tmp/node1'
restore_command = 'cp /DATA_VEC/restore/%f "%p"'
pg_ctl -D /DATA_VEC/pgdata start
after promotion new standby, old primary not getting syncing with new primary server(old standby) Logs from DR server which was primary.
(升级新的备用数据库后，旧的主要数据库无法与新的主服务器（旧的备用数据库）同步。)
2019-12-01 18:46:56 IST LOG: database system was shut down in recovery at 2019-12-01 18:46:53 IST
2019-12-01 18:46:56 IST LOG: entering standby mode
cp: cannot stat `/DATA_VEC/restore/00000002.history': No such file or directory
2019-12-01 18:46:56 IST LOG:
2019-12-01 18:46:56 IST LOG: restored log file "00000002000000000000000C" from archive
2019-12-01 18:46:57 IST LOG: consistent recovery state reached at 0/C000090
2019-12-01 18:46:57 IST LOG: record with zero length at 0/C000090
2019-12-01 18:46:57 IST LOG: database system is ready to accept read only connections
2019-12-01 18:46:57 IST LOG: started streaming WAL from primary at 0/C000000 on timeline 2
2019-12-01 18:46:57 IST LOG: replication terminated by primary server
2019-12-01 18:46:57 IST DETAIL: End of WAL reached on timeline 2 at 0/C000090.
2019-12-01 18:46:57 IST LOG: restored log file "00000002000000000000000C" from archive
2019-12-01 18:46:57 IST LOG: record with zero length at 0/C000090
2019-12-01 18:46:57 IST LOG: restarted WAL streaming at 0/C000000 on timeline 2
2019-12-01 18:46:57 IST LOG: replication terminated by primary server
2019-12-01 18:46:57 IST DETAIL: End of WAL reached on timeline 2 at 0/C000090.
@Master(10.xx10)
(@Master（10.xx10）)
Pg_xlog content
(pg_xlog内容)
-bash-4.1$ cd pg_xlog
-bash-4.1$ ll
total 65552
-rw------- 1 postgres postgres 302 Dec 1 12:52 00000002000000000000000A.00000028.backup
-rw------- 1 postgres postgres 16777216 Dec 1 13:52 00000002000000000000000B
-rw------- 1 postgres postgres 16777216 Dec 1 14:28 00000002000000000000000C
-rw------- 1 postgres postgres 16777216 Dec 1 12:52 00000002000000000000000D
-rw------- 1 postgres postgres 16777216 Dec 1 12:52 00000002000000000000000E
-rw------- 1 postgres postgres 41 Dec 1 13:57 00000002.history
-rw------- 1 postgres postgres 83 Dec 1 13:57 00000003.history
drwx------ 2 postgres postgres 4096 Dec 1 13:57 archive_status
@in restore_command location content:-
(@在restore_command位置内容中：-)
-bash-4.1$ cd /DATA_VEC/restore/ -bash-4.1$ ll total 49156 -rw------- 1 postgres postgres 16777216 Dec 1 18:45 00000002000000000000000A -rw------- 1 postgres postgres 16777216 Nov 30 21:22 00000002000000000000000B -rw------- 1 postgres postgres 16777216 Dec 1 18:45 00000002000000000000000C -rw------- 1 postgres postgres 83 Dec 1 18:45 00000003.history -bash-4.1$
as per_pg_xlog timeline history file 00000003.history arrived at standby still not starting streaming from new primary.
(由于per_pg_xlog时间轴历史记录文件00000003.history到达备用数据库仍未从新的主数据库开始流式传输。)
Question:- 1. Is it mandatory parameter recover_target_timeline='latest' in recovery.conf file" to get latest timeline id from new primary through streaming to start streaming replication?
(问题： - 1.这是必须的参数recover_target_timeline='latest'中recovery.conf文件”，通过流开始流复制摆脱新主最新时间表ID？)
2.If yes, is it for all PostgreSQL Version?
(2.如果是，是否适用于所有PostgreSQL版本？)
like from 9.3 to 11.5 (从9.3到11.5)
ask by Adam Mulla translate from so 
