A slave can be a wonderful thing if you want to scale up reads or if you want to have a backup of your data. But, a slave might not always have to stay a slave. At some point, you might need to turn a slave into a master.

PostgreSQL offers some simple ways to do that. The first and most likely the most convenient way to turn a slave into a master is by using pg_ctl:

iMac:slavehs$ pg_ctl -D . promote
server promoting
iMac:slavehs$ psql test
psql (9.2.4)
Type "help" for help.
test=# CREATE TABLE sample (id int4);
CREATE TABLE

The promote command will signal the postmaster and turn your slave into a master. Once this is complete, you can connect and create objects.

In addition to the promote command, there is a second option to turn a slave into a master. Especially when you are trying to integrate PostgreSQL with a high-availability software of your choice, it can be easier to create a simple file than to call an init script.

To use the file based method, you can add the trigger_file command to your recovery.conf file:

trigger_file = '/tmp/start_me_up.txt'

In our case, PostgreSQL will wait for a file called /tmp/start_me_up.txt to come into existence. The content of this file is totally irrelevant; PostgreSQL simply checks if the file is present, and if it is, it will stop recovery and turn itself into a master.

Creating an empty file is a rather simple task:

iMac:slavehs$ touch /tmp/start_me_up.txt

The database system will react to the new file start_me_up.txt

FATAL:  terminating walreceiver proced fire up:
LOG:  trigger file found: /tmp/start_ss due to 
administrator command
LOG:  redo done at 0/50000E0
LOG:  selected new timeline ID: 2
LOG:  archive recovery complete
LOG:  database system is ready to accept connections
LOG:  autovacuum launcher started

PostgreSQL will check for the file you have defined in recovery.conf every five seconds. For most cases, this is perfectly fine, and by far, fast enough.