From: | Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com> |
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To: | Fujii Masao <masao(dot)fujii(at)gmail(dot)com> |
Cc: | PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: The way to know whether the standby has caught up with the master |
Date: | 2011-05-25 05:16:48 |
Message-ID: | 4DDC90C0.60800@enterprisedb.com |
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Lists: | pgsql-hackers |
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
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