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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