I've installed postgres master and slave nodes by this manual:
At the master (10.0.0.1):
sudo -u postgres initdb -D /data/postgres/main
Change postgresql.conf:
listen_addresses = '*'
wal_level = hot_standby
max_wal_senders = 2
logging_collector = on
max_wal_senders = 5
Add record for replication to pg_hba.conf:
host replication replica 10.0.0.2/32 md5
Then start postgres
service postgresql start
Create replication user:
CREATE USER replica WITH REPLICATION ENCRYPTED PASSWORD '123456';
And add replication slot:
SELECT pg_create_physical_replication_slot('standby_slot');
At the slave (10.0.0.2):
pg_basebackup -h 10.0.0.1 -U replica -D /db/postgres/main -X s -P
Change state to standby in postgresql.conf:
hot_standby = on
Add recovery.conf:
standby_mode = 'on'
primary_conninfo = 'host=10.0.0.1 port=5432 user=replica password=123456'
primary_slot_name = 'standby_slot'
trigger_file = 'trigger'
Start standby:
service postgresql start
So, replication is working fine. If I create database, it will appear both at the master and the slave.
If slave is going down and then starting, all changes come there. It all fine too.
But I have problem if master is down. When it stops, I create trigger file at the slave. Then I create another database at the slave, which now is primary. After that I start master, trigger file remove automatically, recovery.conf rename to recovery.done automatically too. Ok. But how to synchronize changes from slave (new primary) to master (old primary)?
I found that I've got to do these four steps:
At the master:
Stop postgres:
service postgresql stop
At the slave:
Switch backup mode on:
SELECT pg_backup_start('backup', true);
Then rsync new data from slave to master.
And then switch backup mode off:
SELECT pg_backup_stop();
After last step I get message
NOTICE: WAL archiving is not enabled; you must ensure that all required WAL segments are copied through other means to complete the backup
And then if I start master it doesn't start with log message:
FATAL: could not locate required checkpoint record
I don't understand, how to get new data at the master and make it primary again.
So at the future I plan to install pgpool2 for balancing and failover, but test installing did not work because I don't know, how this mechanism should work.
