Comments on: Document replication: CouchDB vs. DVCS

By: Tony Garnock-Jones

Tony Garnock-Jones — Fri, 06 Mar 2009 09:45:51 +0000

Having some kind of programmable merge capability would be a great addition to CouchDB. The current system, if I understand it correctly, is a kind of two-way merge in which one branch always wins. In order to support a more sophisticated approach, one would need to be able to identify the historical revision that the two branches have in common in order to do a three-way merge, and then to be able to supply a javascript function to actually do the merge. I've been experimenting with Javascript DVCS, including adding history-and-merging into an experimental TiddlyWiki; it'd be wonderful to have hooks within CouchDB for programmable 3-way conflict resolution.

By: lennon

lennon — Fri, 31 Oct 2008 18:24:25 +0000

I think that eschewing “magic” is commendable, and it may well be that automatic merging doesn’t belong in the CouchDB core. When I say that you need a good “story,” though, I mean precisely that: some compelling explanation for the “best practices” for multi-way merging that doesn’t simply leave it as an exercise for the reader.

Perhaps I’ll get a chance to play with some simple implementations of automatic merging once I start digging into CouchDB a little more deeply. If so, you can bet I’ll be coming to you with questions, Chris.

By: Chris Anderson

Chris Anderson — Fri, 31 Oct 2008 18:22:11 +0000

Oh and thanks for the insightful commentary. I do think that many merging use cases can be captured by just a few lines of code. Hopefully we’ll come up with a good way to reuse that code. What will be the jQuery of CouchDB?

BTW you’re site stores a cookie but gives me this error when I try to post the comment without refilling my name etc. “Error: please fill the required fields (name, email).”

By: Chris Anderson

Chris Anderson — Fri, 31 Oct 2008 18:20:13 +0000

Part of the CouchDB philosophy is to avoid RPC style “server magic” whenever possible. If you think of documents as just resources, of course they should be manipulated by the client. Some apps may have fields with complex interdependencies (like an invoice’s line items and it’s total). Pushing merging back to the app is the only solution for the breadth of uses out there.