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Re: Synchronous replication

Heikki Linnakangas <heikki.linnakangas@enterprisedb.com>

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From: Heikki Linnakangas <heikki.linnakangas@enterprisedb.com>
To: Joshua Tolley <eggyknap@gmail.com>
Cc: Fujii Masao <masao.fujii@gmail.com>, Yeb Havinga <yebhavinga@gmail.com>, Aidan Van Dyk <aidan@highrise.ca>, Heikki Linnakangas <heikki.linnakangas@enterprisedb.com>, PostgreSQL-development <pgsql-hackers@postgresql.org>
Date: 2010-08-01T06:11:25Z
Lists: pgsql-hackers 
On 27/07/10 16:12, Joshua Tolley wrote:
> My concern is that in a quorum system, if the quorum number is less than the
> total number of replicas, there's no way to know *which* replicas composed the
> quorum for any given transaction, so we can't know which servers to fail to if
> the master dies.

In fact, it's possible for one standby to sync up to X, then disconnect 
and reconnect, and have the master count it second time in the quorum. 
Especially if the master doesn't notice that the standby disconnected, 
e.g a network problem.

I don't think any of this quorum stuff makes much sense without 
explicitly registering standbys in the master.

That would also solve the fuzziness with wal_keep_segments - if the 
master knew what standbys exist, it could keep track of how far each 
standby has received WAL, and keep just enough WAL for each standby to 
catch up.

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