Wednesday, October 23, 2013

PostgreSQL Streaming Replication Hot Standby

Streaming replication allows a standby server to stay up-to-date with primary. The standby connects to the primary, which streams WAL records to the standby as they're generated.

Hot standby is the term used to describe the ability to connect to the server and run read-only queries.

This method of replication is completely transparent to the client, it doesn't require any changes to database, allows query information from standby server and requires minimum administrative effort.

Prerequisites (consider take a look how to install postgresql here):
  • Master and slave must run the same version of PostgreSQL on the same platform
  • Standby is identical to primary and performs failover and readonly queries
  • Primary IP address:
  • Standby IP address:

Primary / Master

  1. Configure streaming replication (file /etc/postgresql/9.1/main/postgresql.conf):
    wal_level = hot_standby
    max_wal_senders = 1
    checkpoint_segments = 10
    wal_keep_segments = 10
  2. Create a user used for replication:
    su - postgres
  3. Allow standby server access the primary (file /etc/postgresql/9.1/main/pg_hba.conf):
  4. host  replication  repl  md5
  5. Restart postgresql
    /etc/init.d/postgresql restart

Standby / Slave

  1. Configure streaming replication (file /etc/postgresql/9.1/main/postgresql.conf):
    wal_level = hot_standby
    hot_standby = on
  2. Copy primary server base:
    /etc/init.d/postgresql stop
    su - postgres
    cd /var/lib/postgresql/9.1/
    mv main/ main-backup/
    pg_basebackup -h -D main -U repl
    cp -d main-backup/server.* main/
  3. Configure primary connection (file /var/lib/postgresql/9.1/main/recovery.conf):
    standby_mode = on
    primary_conninfo = 'host= user=repl password=CHANGE_ME'
    trigger_file = '/tmp/postgresql-repl.trigger'
  4. Restart postgresql
    /etc/init.d/postgresql restart
Now you should be able access master in read/write transactions, slave is read only.


You can monitor replication status on master:
su - postgres
psql -x -c "SELECT * FROM pg_stat_replication"
Here is a sample output:
-[ RECORD 1 ]----+------------------------------
procpid          | 4002
usesysid         | 16384
usename          | repl
application_name | walreceiver
client_addr      |
client_hostname  |
client_port      | 51967
backend_start    | 2013-10-23 16:03:25.340673+03
state            | streaming
sent_location    | 0/3001324
write_location   | 0/3001324
flush_location   | 0/3001324
replay_location  | 0/3001324
sync_priority    | 0
sync_state       | async
You can read more about streaming replication here.


Most likely after install you will get the following hint:
Server instrumentation not installed
This requires install contrib package:
apt-get install postgresql-contrib
and create adminpack extension, here is sql command:

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