I have a legacy app that uses CouchDB under the hood. It's not actually appropriate for how it's being used, but I can see how it could be used in an appropriate way. Over time I got to respect and even like CouchDB (just not the way we're using it ;-) ). However before CouchDB apparently we started with MongoDB and then migrated to Couch after problems with data integrity in Mongo (this was a long time ago and I've heard that problems that we had have been fixed).
But I've always been curious: as a person who like MongoDB, do you have an opinion about Mongo vs. Couch? Keep in mind that you don't have do convince me that there are niches where that style of DB is appropriate ;-) I'm mostly just interested in the comparison since I've never spent any time looking at MongoDB.
IMO, MongoDB is really good if all your indexes and your data (after sharding) can fit in memory, and you need queries that are a simple match query against a small number of keys or a range query against a date or float type. And also your data doesn't fit into a row/column structure very easily (otherwise go SQL). The aggregation framework, joins, other features are all nice to have for occasional use, but you can't be doing those 10k times a second.
It's great for things like a realtime layer of some app that merges data with a slower and more historical layer of data running on a SQL engine or something safer. Or for services that provide realtime or recent-time analytics, while storing your historical data somewhere else (see any patterns here so far? :P ). In my case the main usage is for an advertising bid and content serving engine, which was pretty much the ideal example use-case for MongoDB mentioned in books I read years ago when first learning it.
Just to note, yes the data integrity problems are "fixed", but only if you configure your instances properly and your read and write statements properly. It's not terribly hard to do, but I don't know if I would really go recommending MongoDB for newbies. If you know how to configure it properly for your data-safety needs, and would benefit from being able to have a flexible schema in early development.... I'd still maybe suggest looking at other document DBs unless you need the read/write speed Mongo can give on simple queries (and fresh projects probably do not need that)
But I've always been curious: as a person who like MongoDB, do you have an opinion about Mongo vs. Couch? Keep in mind that you don't have do convince me that there are niches where that style of DB is appropriate ;-) I'm mostly just interested in the comparison since I've never spent any time looking at MongoDB.