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Re: memcached and PostgreSQL

From: Patrick B Kelly <pbk(at)patrickbkelly(dot)org>
To: Sean Chittenden <sean(at)chittenden(dot)org>
Cc: Michael Adler <adler(at)pobox(dot)com>,Darcy Buskermolen <darcy(at)wavefire(dot)com>, josh(at)agliodbs(dot)com,pgsql-performance(at)postgresql(dot)org,Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us>
Subject: Re: memcached and PostgreSQL
Date: 2004-11-22 07:59:12
Message-ID: 659B27BE-3C5C-11D9-8252-000A958A3956@patrickbkelly.org (view raw or flat)
Thread:
Lists: pgsql-performance
On Nov 21, 2004, at 11:55 PM, Sean Chittenden wrote:

>
>> This is similar to sending email in a trigger or on commit where you 
>> can't be certain you send email always
>> and only on a commit.
>
> While this is certainly a possibility, it's definitely closer to the 
> exception and not the normal instance.

While an exception, this is a very real possibility in day to day 
operations. The absence of any feedback or balancing mechanism between 
the database and cache makes it impossible to know that they are in 
sync and even a small error percentage multiplied over time will lead 
to an ever increasing likelihood of error.

More dangerous is that this discrepancy will NOT always be apparent 
because without active verification of the correctness of the cache, we 
will not know about any errors unless the error grows to an obvious 
point. The errors may cause material damage long before they become 
obvious. This is a common failure pattern with caches.



Patrick B. Kelly
------------------------------------------------------
                               http://patrickbkelly.org


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Subject: Re: memcached and PostgreSQL
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Subject: Big number of schemas (3500) into a single database

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