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Re: Snapshot synchronization, again...

From: Joachim Wieland <joe(at)mcknight(dot)de>
To: Florian Pflug <fgp(at)phlo(dot)org>
Cc: pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Snapshot synchronization, again...
Date: 2010-12-31 03:26:52
Message-ID: AANLkTimn+Coy4wUg73YPJqGxG-Sw6+HMqMSfLi6Tin0Q@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-hackers
On Thu, Dec 30, 2010 at 9:49 AM, Florian Pflug <fgp(at)phlo(dot)org> wrote:
> On Dec30, 2010, at 13:31 , Joachim Wieland wrote:
>> We return snapshot information as a chunk of data to the client. At
>> the same time however, we set a checksum in shared memory to protect
>> against modification of the snapshot. A publishing backend can revoke
>> its snapshot by deleting the checksum and a backend that is asked to
>> install a snapshot can verify that the snapshot is correct and current
>> by calculating the checksum and comparing it with the one in shared
>> memory.
>
> We'd still have to stream these checksums to the standbys though,
> or would they be exempt from the checksum checks?

I am not talking about having synchronized snapshots among standby
servers at all.

I am only proposing a client API that will work for this future idea as well.


> I still wonder whether these checks are worth the complexity. I
> believe we'd only allow snapshot modifications for read-only queries
> anyway, so what point is there in preventing clients from setting
> broken snapshots?

What's the use case for it? As soon as nobody comes up with a
reasonable use case for it, let's aim for the robust version.


Joachim

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