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From Michael Aufreiter ...@zive.at>
Subject Guys with deep knowledge about CouchDB Replication, I'd need your help :)
Date Thu, 28 Jul 2011 15:48:10 GMT
I'm currently working on a complete data-persistence solution for offline apps, involving CouchDB
and Data.js. I already introduced Data.js here at this mailing list the other day, but here's
a link again:

http://substance.io/#michael/data-js

I've setup a cleanroom example (tasks) that I want to test the new sync-functionality against.


http://tasks.substance.io (don't miss the sync button in the upper right corner)
https://github.com/michael/data/blob/7729d41677e48bd5132119997dc0cff53522bb55/examples/tasks/public/javascripts/views/app.js

It's currently just one way. It just writes changes to the server but does not pull in node-updates.
Now this should change.

The algorithm for a bi-directional sync I have in mind looks like so:

1. Pull: For all nodes I have in my local graph, check if there are updates (other users might
have updated them), and if yes, pull them in
 If conflicts occur the client/user decides how to resolve it (choose a revision or merge
it)

2. Push: Write all local dirty nodes to the server

If that succeeded, the sync is complete. Usually if there's not much time between the pull
and the push it's unlikely to run into conflicts when doing the push.

However I'm asking myself how CouchDB replication is implemented -- maybe I can re-use some
of the concepts. 

In order to perform the Pull, I thought about sending a list of ID's+revisions to the server.
The server (resp. Couch) should then check if there are updates for any of them. If yes, those
nodes should be fetched and delivered to the client. Given that number of ID/revision pairs,
what would be the best way to check for updates? Or do you have any other ideas on how to
do the pull?

An implication of this scenario is that application developers should do their best to keep
the local graph rather small (the bigger it gets the more overhead you have when doing the
push, also more memory is used). However this should suit a lot of scenarios (like in my case
making possible offline editing of Substance.io documents)

Would be great if some of you could help me out a bit here. I think such a framework (Data.js+Couch)
would be a great benefit for application developers who wan't to build offline apps. What
do you think?

Thanks!

-- Michael 

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