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From Paul Davis <paul.joseph.da...@gmail.com>
Subject Re: handling simultaneous identical replications
Date Thu, 05 Mar 2009 06:31:42 GMT
On Wed, Mar 4, 2009 at 8:34 PM, Adam Kocoloski <adam.kocoloski@gmail.com> wrote:
> Hi folks, we've been running into a problem where multiple replications with
> the same source and target are running simultaneously.  This introduces
> quite a lot of unnecessary network traffic and causes real problems with
> update collisions on the local replication history documents.  If replicator
> A updates the source doc and replicator B updates the target doc, subsequent
> replications will decide that a full replication is necessary.
>
> I have some ideas about how to ensure only one is running at a time (more on
> that in a separate mail), but I'd like some feedback on how to handle the
> second..Nth request.  Let's call the initial POST to _replicate "A" and the
> second POST "B":
>
> Option 1 -- Respond to B with the results from A
> This option works fine if the source is remote.  However, if the source is
> local, the replication started by A will be missing updates to the source DB
> that occurred between A and B.  B may be surprised by that result.
>
> Option 2 -- Grab an updated DB and continue the replication
> This option will include updates to the source that occurred between A and B
> in the response to both requests.
>
> Option 3 -- Respond to A, then trigger another replication for B
> In this case we wait till the replication started by A has completed, then
> do an incremental one and respond to B with the results of that incremental.
>
> I think I'd vote for 3.  Cheers, Adam
>
>

If I follow this correctly, the issue is, "POST to _replicate, a
second POST to _replicate occurs before the first request finishes"
(with the same source/target info).

My knowledge of replication is only cursory, but I could also see:

Option 4:

Same as views, we wait for replication to finish and return the same
result to all clients that made a request.

Option 5:

Return an error on B that says, "Yeah, yeah. Already on it."

But I could be missing something.

HTH,
Paul Davis

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