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Re: Sync Rep Design

From: Robert Haas <robertmhaas(at)gmail(dot)com>
To: Simon Riggs <simon(at)2ndquadrant(dot)com>
Cc: Stefan Kaltenbrunner <stefan(at)kaltenbrunner(dot)cc>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: Sync Rep Design
Date: 2010-12-30 21:27:03
Message-ID: AANLkTi=SvN2KQ1Tzh-tRKcXD0i2FyWFBbbR=kXXdepqN@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-hackers
On Thu, Dec 30, 2010 at 2:04 PM, Simon Riggs <simon(at)2ndquadrant(dot)com> wrote:
> We use a single parameter to enable synchronous replication, set in
> postgresql.conf on both primary and standby servers:
>
> synchronous_replication = off (default) | on
>
> On the primary, synchronous_replication can be set for particular users
> or databases, or dynamically by applications programs.
>
> If more than one standby server specifies synchronous_replication, then
> whichever standby replies first will release waiting commits.
>
> Turning this setting off for a standby allows the administrator to
> exclude certain standby servers from releasing waiting transactions.
> This is useful if not all standby servers are designated as potential
> future primary servers. On the standby, this parameter only takes effect
> at server start.

I think it's a bad idea to use the same parameter to mean different
things on the master and standby.  You proposed this kind of double
meaning for the hot_standby parameter (possibly back when it was
called standby_connections, or something like that) and we (rightly, I
think) did not adopt that, instead ending up with wal_level to control
the master's behavior and hot_standby to control the slave's behavior.

> synchronous_replication (boolean)
>        Specifies whether transaction commit will wait for WAL records
>        to be replicated before the command returns a "success"
>        indication to the client.

The word "replicated" here could be taken to mean different things,
most obviously:

- slave has received the WAL
- slave has fsync'd the WAL
- slave has applied the WAL

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

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