PostgreSQL master/slave for dotCloud

Warning

This recipe is alpha quality.

Basic Setup

git clone git://github.com/jpetazzo/postgresql-on-dotcloud.git
dotcloud create myapp
dotcloud push

The superuser will be dotcloud (not root nor postgres). The password will be shown at the end of the build process. You can also retrieve it by running:

dotcloud run db cat password

You can then inject that password in your dotCloud environment, by running the following command:

dotcloud env set POSTGRESQL_PASSWORD=$(dotcloud run db.0 cat password)

Change PostgreSQL Version

The version of PostgreSQL is specified in the dotcloud.yml file. You can change it at any time, and repush. The requested version will be downloaded and compiled. Previous builds are kept, so rolling back to an old version (which was already installed before) won't require a full rebuild.

Master/Slave Setup

By default, this recipe deploys a single PostgreSQL master. To provision a slave, you need to do this:

dotcloud scale db=2
dotcloud run db.1 enslave 0

This last line will make db.1 a slave of db.0. The first argument to the enslave script is the instance number of the master. The script will retrieve the SSH and SQL connection parameters from the dotCloud environment, stop the slave postgres process, start a backup process on the master, transfer the PGDATA directory from the master to the slave, complete the backup process, create the replication parameters (the recovery.conf file), and restart the slave postgres process.

Failover

The failover is not automatic. You must do this:

dotcloud run db.1 rm /home/dotcloud/data/recovery.conf
dotcloud run db.1 supervisorctl restart postgres

And then update your app configuration to point to the promoted slave instead of the master. You can rerun the "enslave" procedure described above at anytime, but remember that the master data will overwrite the slave data. Remember to use the correct instance numbers!

Multi-Slave Setup

It should work with multiple slaves without a problem. Just provision each slave individually.

Note

Don't try to cascade slaves unless you know what you're doing. If you setup N slaves, if you promote 1 slave to be master, the other slaves should be reconfigured to talk to the new master. Yes, this could mean a lengthy reprovisioning process.