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From Damien Katz <dam...@apache.org>
Subject Re: handling simultaneous identical replications
Date Thu, 05 Mar 2009 01:47:06 GMT

On Mar 4, 2009, at 8:34 PM, Adam Kocoloski 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
>

Option 3 is the best I think.

-Damien

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