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PostgreSQL Automatic Fail-Over - User Documentation

Automatic Failover

repmgr allows setups for automatic failover when it detects the failure of the master node. Following is a quick setup for this.

Installation

For convenience, we define:

  • node1 is the hostname fully qualified of the Master server, IP 192.168.1.10
  • node2 is the hostname fully qualified of the Standby server, IP 192.168.1.11
  • witness is the hostname fully qualified of the server used for witness, IP 192.168.1.12
Note

It is not recommanded to use name defining status of a server like «masterserver», this is a name leading to confusion once a failover take place and the Master is now on the «standbyserver».

Summary

2 PostgreSQL servers are involved in the replication. Automatic fail-over need to vote to decide what server it should promote, thus an odd number is required and a witness-repmgrd is installed in a third server where it uses a PostgreSQL cluster to communicate with other repmgrd daemons.

1. Install PostgreSQL in all the servers involved (including the server used for witness) 2. Install repmgr in all the servers involved (including the server used for witness) 3. Configure the Master PostreSQL 4. Clone the Master to the Standby using "repmgr standby clone" command 5. Configure repmgr in all the servers involved (including the server used for witness) 6. Register Master and Standby nodes 7. Initiate witness server 8. Start the repmgrd daemons in all nodes

Note

A complete Hight-Availability design need at least 3 servers to still have a backup node after a first failure.

Install PostgreSQL

You can install PostgreSQL using any of the recommended methods. You should ensure it's 9.0 or superior.

Install repmgr

Install repmgr following the steps in the README.

Configure PostreSQL

Log in node1.

Edit the file postgresql.conf and modify the parameters:

listen_addresses='*'
wal_level = 'hot_standby'
archive_mode = on
archive_command = 'cd .'   # we can also use exit 0, anything that 
                           # just does nothing
max_wal_senders = 10
wal_keep_segments = 5000   # 80 GB required on pg_xlog
hot_standby = on
shared_preload_libraries = 'repmgr_funcs'

Edit the file pg_hba.conf and add lines for the replication:

host     repmgr           repmgr      127.0.0.1/32            trust
host     repmgr           repmgr      192.168.1.10/30         trust
host     replication      all         192.168.1.10/30         trust
Note

It is also possible to use a password authentication (md5), .pgpass file should be edited to allow connection between each node.

Create the user and database to manage replication:

su - postgres
createuser -s repmgr
createdb -O repmgr repmgr
psql -f /usr/share/postgresql/9.0/contrib/repmgr_funcs.sql repmgr

Restart the PostgreSQL server:

pg_ctl -D $PGDATA restart

And check everything is fine in the server log.

Create the ssh-key for the postgres user and copy it to other servers:

su - postgres
ssh-keygen             # /!\ do not use a passphrase /!\
cat ~/.ssh/id_rsa.pub > ~/.ssh/authorized_keys
chmod 600 ~/.ssh/authorized_keys
exit
rsync -avz ~postgres/.ssh/authorized_keys node2:~postgres/.ssh/
rsync -avz ~postgres/.ssh/authorized_keys witness:~postgres/.ssh/
rsync -avz ~postgres/.ssh/id_rsa* node2:~postgres/.ssh/
rsync -avz ~postgres/.ssh/id_rsa* witness:~postgres/.ssh/

Clone Master

Log in node2.

Clone the node1 (the current Master):

su - postgres
repmgr -d repmgr -U repmgr -h node1 standby clone

Start the PostgreSQL server:

pg_ctl -D $PGDATA start

And check everything is fine in the server log.

Configure repmgr

Log in each server and configure repmgr by editing the file /etc/repmgr/repmgr.conf:

cluster=my_cluster
node=1
node_name=earth
conninfo='host=192.168.1.10 dbname=repmgr user=repmgr'
master_response_timeout=60
reconnect_attempts=6
reconnect_interval=10
failover=automatic
promote_command='promote_command.sh'
follow_command='repmgr standby follow -f /etc/repmgr/repmgr.conf'
  • cluster is the name of the current replication.
  • node is the number of the current node (1, 2 or 3 in the current example).
  • node_name is an identifier for every node.
  • conninfo is used to connect to the local PostgreSQL server (where the configuration file is) from any node. In the witness server configuration it is needed to add a 'port=5499' to the conninfo.
  • master_response_timeout is the maximum amount of time we are going to wait before deciding the master has died and start failover procedure.
  • reconnect_attempts is the number of times we will try to reconnect to master after a failure has been detected and before start failover procedure.
  • reconnect_interval is the amount of time between retries to reconnect to master after a failure has been detected and before start failover procedure.
  • failover configure behavior : manual or automatic.
  • promote_command the command executed to do the failover (including the PostgreSQL failover itself). The command must return 0 on success.
  • follow_command the command executed to address the current standby to another Master. The command must return 0 on success.

Register Master and Standby

Log in node1.

Register the node as Master:

su - postgres
repmgr -f /etc/repmgr/repmgr.conf master register

Log in node2.

Register the node as Standby:

su - postgres
repmgr -f /etc/repmgr/repmgr.conf standby register

Initialize witness server

Log in witness.

Initialize the witness server:

su - postgres
repmgr -d repmgr -U repmgr -h 192.168.1.10 -D $WITNESS_PGDATA -f /etc/repmgr/repmgr.conf witness create

It needs information to connect to the master to copy the configuration of the cluster, also it needs to know where it should initialize it's own $PGDATA. As part of the procees it also ask for the superuser password so it can connect when needed.

Start the repmgrd daemons

Log in node2 and witness.

su - postgres repmgrd -f /etc/repmgr/repmgr.conf > /var/log/postgresql/repmgr.log 2>&1

Note

The Master does not need a repmgrd daemon.

Suspend Automatic behavior

Edit the repmgr.conf of the node to remove from automatic processing and change:

failover=manual

Then, signal repmgrd daemon:

su - postgres
kill -HUP `pidoff repmgrd`
TODO : -HUP configuration update is not implemented and it should check its

configuration file against its configuration in DB, updating accordingly the SQL conf (especialy the failover manual or auto) this allow witness-standby and standby-not-promotable features and simpler usage of the tool ;)

Usage

The repmgr documentation is in the README file (how to build, options, etc.)