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From Adam Kocoloski <adam.kocolo...@gmail.com>
Subject handling simultaneous identical replications
Date Thu, 05 Mar 2009 01:34:16 GMT
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


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