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From Sho Fukamachi <sho.fukama...@gmail.com>
Subject Re: Replication Options for Offline Web App
Date Tue, 19 Aug 2008 05:15:31 GMT

On 19/08/2008, at 1:31 PM, Chris Anderson wrote:

> From what I understand about the replication process, the client won't
> have any trouble receiving a subset of the full replication.

Perhaps it would be a good idea for Someone Who Knows to write out a  
brief summary of exactly what goes on with document lifecycle and  
replication. I also have an (incomplete, assumed, probably wrong)  
understanding of how I *think* it works - I've read the technical  
overview but there's still a lot of black box stuff going on.

For example, where is CouchDB storing the list of "documents updated  
since last replication"? Is that list generated by a push replication  
as well? What if you push-replicate to more than one remote server, is  
another list created for that server? How are servers identified - do  
they have IDs? etc etc .. Seems a few people would like to know more  
about the internals of replication.

http://wiki.apache.org/couchdb/ConfiguringDistributedSystems whet my  
appetite but it stops just as it's getting good!


> However,
> it's worth looking out for what happens if you try to replicate again
> later (perhaps from a different view of the same database). Could
> _revs get out of sync?

I am also curious about that - does replication grab only the latest  
_rev, or all of them (if extant)? If the status is deleted, does it  
bother replicating it? What happens if the local DB is compacted and  
deleted docs expunged before replication - are they deleted/retained  
on the remote?

If someone knows of an existing document with the answers to these  
questions, please direct me to it, and forgive my delinquency.


> This feature has been on the roadmap for a while, so maybe Damien has
> some ideas for how it should be designed.

Well, in the Technical Overview at http://incubator.apache.org/couchdb/docs/overview.html

  it says:

"Partial replicas can be created and maintained. Replication can be  
filtered by a javascript function, so that only particular documents  
or those meeting specific criteria are replicated. This can allow  
users to take subsets of a large shared database application offline  
for their own use, while maintaining normal interaction with the  
application and that subset of data."

I think that's why I thought it actually DID work, thought I later  
realised that document is more of a "features CouchDB will have in the  
future" rather than a description of current functionality. Regardless  
it's obviously it's been intended to work like that for some time so I  
imagine a lot of that design work has already been done and it's just  
a matter of finding the time to finish it : )



> -- 
> Chris Anderson
> http://jchris.mfdz.com


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