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Re: Re: [COMMITTERS] pgsql: Efficient transaction-controlled synchronous replication.

From: Andrew Dunstan <andrew(at)dunslane(dot)net>
To: Aidan Van Dyk <aidan(at)highrise(dot)ca>
Cc: Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>, Simon Riggs <simon(at)2ndquadrant(dot)com>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: Re: [COMMITTERS] pgsql: Efficient transaction-controlled synchronous replication.
Date: 2011-03-07 15:03:10
Message-ID: 4D74F3AE.9000802@dunslane.net (view raw or flat)
Thread:
Lists: pgsql-committerspgsql-hackers

On 03/07/2011 09:29 AM, Aidan Van Dyk wrote:
> On Mon, Mar 7, 2011 at 2:21 PM, Andrew Dunstan<andrew(at)dunslane(dot)net>  wrote:
>
>>> For me, that's enough to call it "synchronous replication". It provides a
>>> useful guarantee to the client. But you could argue for an even stricter
>>> definition, requiring atomicity so that if a transaction is not successfully
>>> replicated for any reason, including crash, it is rolled back in the master
>>> too. That would require 2PC.
>>>
>> My worry is that the stricter definition is what many people will expect,
>> without reading the fine print.
> They they are either already hosed or already using 2PC.
>
>


This is about expectations. The thing that worries me is that the use of 
this term might cause some people NOT to use 2PC because they think they 
are getting an equivalent guarantee, when in fact they are not. And 
that's hardly unreasonable. Here for example is what wikipedia says 
<http://en.wikipedia.org/wiki/Replication_%28computer_science%29>:

    Synchronous replication - guarantees "zero data loss" by the means
    of atomic write operation, i.e. write either completes on both sides
    or not at all. Write is not considered complete until
    acknowledgement by both local and remote storage.


cheers

andrew

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