Simple CouchDB multi-master clustering via Nginx | Hacker News Reader

Simple CouchDB multi-master clustering via Nginx

ephemera.karmi.cz

39 pointsva_coder8 comments

I totally agree with the sentiment of the post - using CouchDB has simply been unbelievable amounts of fun (and certain types of applications would be painful if not impossible to design without it).

The existence of projects like couchdb-python and couchdb-lucene are just icing on a very tasty cake.

seconded. it's interesting how it forces you to build for scale right away.

I'm not too familiar with multi master replication. What happens when a request handled by server A is followed by a subsequent request that is routed to server B before server A has replicated the changes from the first request? The client would expect a certain state on the "server" given the first request but it certainly seems possible to invalidate that.

If you attempt to update a record, you send a uuid to server B representing the document you're attempting to update; if sever B doesn't have that document yet (either because it hasn't been created on server B or because it is not yet updated to the latest changes on server B), that change will be rejected.

If you are just expecting the document to be a member of a list (say, you put an order on server A, then look for all orders belonging to a customer on server B), you're not guaranteed to see it. This is 'eventual consistency' -- in practice, you want to be sure that you can live with this as a tradeoff.

Thank you.

The google tech talk on CouchDB talks about this problem :

http://www.youtube.com/watch?v=ESDBM9-U804

54:14 in, someone asks a similar question

Great writeup. I use CouchDB behind nginx anyway for simple authentication so I'll have to try this. The only problem is "continuous replication currently does not survive server restart" - that is not so good.

OK, apparently this is not such a problem; from the book: "At the time of writing, CouchDB doesn’t remember continuous replications over a server restart. For the time being, you are required to trigger them again, when you restart CouchDB."

So, put something in /etc/rc.local for each server to start the replication again.

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