Re: The way to know whether the standby has caught up with the master

On 25.05.2011 07:42, Fujii Masao wrote:
> For reliable high-availability, when the master crashes, the clusterware must
> know whether it can promote the standby safely without any data loss,
> before actually promoting it. IOW, it must know whether the standby has
> already caught up with the primary. Otherwise, failover might cause data loss.
> We can know that from pg_stat_replication on the master. But the problem
> is that pg_stat_replication is not available since the master is not running at
> that moment. So that info should be available also on the standby.
>
> To achieve that, I'm thinking to change walsender so that, when the standby
> has caught up with the master, it sends back the message indicating that to
> the standby. And I'm thinking to add new function (or view like
> pg_stat_replication)
> available on the standby, which shows that info.

By the time the standby has received that message, it might not be
caught-up anymore because new WAL might've been generated in the master
already.

--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com