How to fix FATAL: database system identifier differs between primary and standby in PostgreSQL

sudo systemctl stop postgresql
# or for a specific version
sudo systemctl stop postgresql@15-main

sudo systemctl status postgresql

# Backup the existing standby data
sudo mv /var/lib/postgresql/15/main /var/lib/postgresql/15/main.bak

# Or if using a different version/path:
sudo mv /path/to/postgres/data /path/to/postgres/data.bak

sudo mkdir -p /var/lib/postgresql/15/main
sudo chown postgres:postgres /var/lib/postgresql/15/main
sudo chmod 700 /var/lib/postgresql/15/main

sudo -u postgres pg_basebackup \
  -h <primary_ip_or_hostname> \
  -p 5432 \
  -U replication \
  -D /var/lib/postgresql/15/main \
  -Fp \
  -Xs \
  -R

sudo -u postgres pg_basebackup \
  -h 192.168.1.10 \
  -p 5432 \
  -U replication \
  -D /var/lib/postgresql/15/main \
  -Fp \
  -Xs \
  -R

# Check standby.signal exists (indicates standby mode)
sudo ls -la /var/lib/postgresql/15/main/standby.signal

# Check postgresql.auto.conf was created
sudo cat /var/lib/postgresql/15/main/postgresql.auto.conf

primary_conninfo = 'host=192.168.1.10 port=5432 user=replication password=...'

sudo systemctl start postgresql
# or for specific version
sudo systemctl start postgresql@15-main

sudo systemctl status postgresql

sudo tail -f /var/log/postgresql/postgresql-15-main.log

LOG: database system was interrupted while in recovery at log time 2024-01-15 10:30:45 UTC
LOG: entering standby mode
LOG: started streaming WAL from primary at 0/3000000 on timeline 1

# On primary
psql -U postgres

SELECT slot_name, slot_type, active FROM pg_replication_slots;
SELECT pid, usename, application_name, client_addr, state FROM pg_stat_replication;

pid   | usename |  application_name  | client_addr  | state
---------+---------+--------------------+--------------+----------
  12345  | replication | walreceiver | 192.168.1.20 | streaming