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Comment by zippoxer | Hacker News Reader

I stopped reading where it said "never".

I'm usually against absolutes, but I'm really in agreement here. It's almost always better to use a relational database, and in the rare cases where you wouldn't want a relational database, MongoDB is the worst of the major options. It literally memory leaks and drops data without warning or provocation. If you're not going to go with a relational database, RethinkDB, Cassandra, Redis, or BerkeleyDB would all be better choices (which one depends on situation).

Do you know which role each of those (RethinkDB, Cassandra, Redis, and BerkeleyDB) excel at?

RethinkDB is targeted at soft real time applications which have a need to update data across many clients. Additionally, although its a schemaless document DB, it offers functionality similar to joins/relational queries. This is a good presentation on RethinkDB: https://www.youtube.com/watch?v=Ee1v_SuECRk

I'd usually start from a problem and choose a store rather than looking at what stores excel at. So I won't speak to places where I haven't seen something solved, but here are some examples:

1. Redis is great for caching in front of a relational database, and also for running task queues. I've used it personally in both cases. Unlike MongoDB it drops data based on cache invalidation criteria, rather than dropping it randomly. Caching is one of the cases other people are proposing MongoDB for, but it's ridiculous to use MongoDB here when Redis exists.

2. Cassandra is used by Reddit and Twitter--search around and you can find lots of good writing about how they use it. Personally I've only used it indirectly via Stream.io.

So you never read articles if they say never?

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