Question

I need to replicate a PostgreSQL database server as follows:

1. Two servers are adjacent to each-other – one is the master and the other standby. If the master fails, the standby takes over. Replication from master to slave needs to be failsafe, hence, synchronous. The standby will not be used for any querying unless it has become a master. So, no high-availability/load-balancing is required.

2. There is another backup server at a remote location. Data from the master server mentioned above will be replicated to this remote server asynchronously and in batches. Time is not a factor at all in this replication – a couple of hours is just fine. This server would be used just for backup.

I’ve studied the currently available replication solutions from the PostgreSQL docs as well as from Google, but can’t decide which combination of synchronous-asynchronous solutions would I need.

The closest I came up with is using pgpool-II for scenario 1 and Mammoth for scenario 2. However, as pgpool is statement-based, what would happen to queries containing rand() and now()?

Please note that I’d rather use free and open-source replication tools.

Also, just a side question – according to scenario 1 above, when the master fails, the standby will take over. Would the master-slave role be reversed after that, or would after the recovery of the master server the slave would go back to its standby state?

Any suggestion would be highly appreciated. Thanks.

Answer 1

You didn’t mention if the server in question was on a specific version or if this was a new project with the freedom to choose the version. The answers vary based on that information.

If you are starting with a clean slate, I would recommend designing based on the PostgreSQL 9.1 beta. The final version will be released long before you would be ready to go into a production environment and it has binary synchronous replication built-in.

I’ve been using the built-in asynchronous replication in PostgreSQL for years in almost the exact same scenario you describe and it has always been rock-solid for me. It’s become even better with 9.0 with Hot standby and it’s become much easier to configure and maintain. 9.1 provides the only missing piece you require.

However, if you are trying to replicate an existing server, built-in asynchronous replication with aggressive settings for “checkpoint_timeout” a very frequent backup of unarchived WAL files could be sufficient until you can upgrade to 9.1.

The bottom line here is that you can get exactly what you want is with stock PostgreSQL 9.1–no third-party products required.

As for failover, it is not an automatic process, you’ll need to handle that yourself. I would recommend that after a failover, switching the roles of the two machines until either the next failover event or until a controlled manual failover during a scheduled outage during a slow period of use. Again, this is not automatic and much be managed by the administrator (via shell scripts, presumably).