Horton

Re: Configuring synchronous replication

Heikki Linnakangas <heikki.linnakangas@enterprisedb.com>

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From: Heikki Linnakangas <heikki.linnakangas@enterprisedb.com>
To: Simon Riggs <simon@2ndQuadrant.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>, Robert Haas <robertmhaas@gmail.com>, jd@commandprompt.com, Thom Brown <thom@linux.com>, Bruce Momjian <bruce@momjian.us>, Andrew Dunstan <andrew@dunslane.net>, Dave Page <dpage@pgadmin.org>, Peter Eisentraut <peter_e@gmx.net>, Fujii Masao <masao.fujii@gmail.com>, Josh Berkus <josh@agliodbs.com>, pgsql-hackers@postgresql.org
Date: 2010-09-24T11:12:29Z
Lists: pgsql-hackers 
On 24/09/10 13:57, Simon Riggs wrote:
> If you want high availability you need N+1 redundancy. If you want a
> standby server that is N=1. If you want a highly available standby
> configuration then N+1 = 2.

Yep. Synchronous replication with one standby gives you zero data loss. 
When you add a 2nd standby as you described, then you have a reasonable 
level of high availability as well, as you can continue processing 
transactions in the master even if one slave dies.

> Show me the textbook that describes what happens with 2 standbys. If one
> exists, I'm certain it would agree with my analysis.

I don't disagree with your analysis about multiple standbys and high 
availability. What I'm saying is that in a two standby situation, if 
you're willing to continue operation as usual in the master even if the 
standby is down, you're not doing synchronous replication. Extending 
that to a two standby situation, my claim is that if you're willing to 
continue operation as usual in the master when both standbys are down, 
you're not doing synchronous replication.

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