Below are setup details and the same will be used in this demonstration.

Sr No.	Hostname	IP	Role
1	postgres@rac09-p	192.168.1.43	Master / Primary Server
2	postgres@rac10-p	192.168.1.44	Standby / Secondary Server

Basic Crosscheck :

postgres=# show listen_addresses;
listen_addresses
------------------
*
(1 row)

postgres=# show wal_level;
wal_level
-----------
replica
(1 row)

postgres=# show max_wal_senders;
max_wal_senders
-----------------
10
(1 row)

postgres=# show max_replication_slots;
max_replication_slots
-----------------------
10
(1 row)

postgres=# show max_wal_size;
max_wal_size
--------------
1GB
(1 row)

postgres=# show wal_log_hints;
wal_log_hints
---------------
off
(1 row)
postgres=#
postgres=# show archive_mode;
archive_mode
--------------
on
(1 row)

postgres=# show archive_command;
archive_command
-------------------------------------------------------------------------------
test ! -f /backup/PostgreSQL13_arch/%f && cp %p //backup/PostgreSQL13_arch/%f
(1 row)

1. Check whether replication process and the sync status:

-- Run on master server(192.168.1.43 )

postgres=# \x

Expanded display is on.

postgres=#

postgres=# select * from pg_stat_replication;

-[ RECORD 1 ]----+---------------------------------

pid | 16331

usesysid | 16385

usename | repuser

application_name | walreceiver

client_addr | 192.168.1.44

client_hostname |

client_port | 23210

backend_start | 2024-08-11 13:06:18.175803+05:30

backend_xmin |

state | streaming

sent_lsn | 0/4019BC0

write_lsn | 0/4019BC0

flush_lsn | 0/4019BC0

replay_lsn | 0/4019BC0

write_lag |

flush_lag |

replay_lag |

sync_priority | 0

sync_state | async

reply_time | 2024-08-11 18:50:57.943598+05:30

--Check the lag difference

postgres=# SELECT pg_wal_lsn_diff(sent_lsn, replay_lsn) from pg_stat_replication;

-[ RECORD 1 ]---+--

pg_wal_lsn_diff | 0

2. Check Replication Status on the secondary server:-

Now the replication is up and running. You can get the status of your replication from the pg_stat_wal_receiver table on the secondary server.

postgres=# SELECT "status", "last_msg_send_time", "slot_name", "sender_host" FROM pg_stat_wal_receiver;

-[ RECORD 1 ]------+---------------------------------
status | streaming
last_msg_send_time | 2024-08-11 18:39:26.562249+05:30
slot_name |
sender_host | 192.168.1.43

postgres=# select pg_last_wal_receive_lsn(), pg_last_wal_replay_lsn(), pg_last_xact_replay_timestamp();

-[ RECORD 1 ]-----------------+---------------------------------

pg_last_wal_receive_lsn | 0/4019BC0

pg_last_wal_replay_lsn | 0/4019BC0

pg_last_xact_replay_timestamp | 2024-08-11 18:14:17.652363+05:30

3. Check the status of Recovery process on Slave database.

postgres=# select pg_is_in_recovery();

pg_is_in_recovery

-------------------

(1 row)

postgres=#

postgres=# select pg_current_wal_lsn();

ERROR: recovery is in progress

HINT: WAL control functions cannot be executed during recovery.

postgres=#

As our database is running in recovery process hence it is not allowing us to see the current WAL lsn number.

Now we will proceed with switchover

2. Shutdown the MASTER: [ ON MASTER SERVER 192.168.1.43 ]

[postgres@rac09-p ~]$ /usr/pgsql-13/bin/pg_ctl stop -D /var/lib/pgsql/13/data
waiting for server to shut down.... done
server stopped

2024-08-11 19:04:13.852 IST [24646] FATAL: could not connect to the primary server: could not connect to server: Connection refused

Is the server running on host "192.168.1.43" and accepting