Is it mandatory parameter recover_target_timeline='latest' in switchover and switchback in PostgreSQL 9.4.1? - postgresql

Question

I have followed below steps for switchover and switchback.
Step 1:-
Disconnect application services from 10.x.x.10 and do the following
#Master(10.x.x.10)
pg_ctl -D /DATA_VEC/pgdata stop --mode=fast
#DR(20.x.x.20)
promote DR as read write mode
Step 2:- Start master as DR from new primary
#Master(10.x.x.10)
create 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.
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.x.x.10)
Pg_xlog content
-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:-
-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.historyarrived at standby still not starting streaming from new primary.
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?
2.If yes, is it for all PostgreSQL Version? like from 9.3 to 11.5

Answer 1

If you want switch-back functionality, you will have to set recovery_target_timeline='latest', as any promotion will increment the timeline. Using a fixed target timeline is usually reserved for very specific cases (i.e., you need to recover changes after a split-brain, diverged-timeline scenario).
To answer your specific questions:
Yes
Yes