From Streaming Replication in the PostgreSQL documentation:
If you use streaming replication without file-based continuous
archiving, the server might recycle old WAL segments before the
standby has received them. If this occurs, the standby will need to be
reinitialized from a new base backup. You can avoid this by setting
wal_keep_segments to a value large enough to ensure that WAL segments
are not recycled too early, or by configuring a replication slot for
the standby. If you set up a WAL archive that's accessible from the
standby, these solutions are not required, since the standby can
always use the archive to catch up provided it retains enough
segments.
To fix the issue, you have to reinitialize the data from primary server. Remove data directory on slave:
root@replica:~# su postgres
postgres@replica:~# mv /var/lib/postgresql/12/main /var/lib/postgresql/12/main_old
Copy all data from the primary server:
sudo -u postgres pg_basebackup -h [PRIMARY_IP] -D /var/lib/postgresql/12/main -U replication -P -v
if version is 12, Create the standby.signal file, otherwise configure replica.conf:
touch /var/lib/postgresql/12/main/standby.signal
Slave configuration:
listen_addresses = 'localhost,[IP_ADDRESS_OF_REPLIACA_ON_LAN]' # what IP address(es) to listen on;
max_connections = 100 # Ensure that this value is the same as the primary's
wal_level = 'replica'
archive_mode = on
archive_command = 'cd .'
primary_conninfo = 'host=[PRIMARY_IP] port=5432 user=replication password=[REPLICATION PASSWORD]'
hot_standby = on
max_wal_senders = 48
How long is the pg_basebackup taking? Remember that segments are generated about every 5 minutes, so if the backup takes an hour, you need at least 12 segments stored. At 2 hours, you need 24 etc., I'd set the value to about 12.2 segments/hour of backup.
https://www.gab.lc/articles/postgresql-12-replication/
