When to use NoSQL/key-value store?

I’ve been meaning to look into this for some time to get my head around these fancy new databases and find out when I should consider using them. I’ve come to the conclusion that you generally wouldn’t use an object/key-value database on it’s own.  You have your main relational database for important data that is definitely required, and then you can have multiple key-value databases to do a lot of the firehose-data grunt work across multiple servers.

A setup might look like:

Relational Database:

You need your relational database for things that a user absolutely has to have every request (e.g. you always need to access the users login details).

NoSQL/Key-Value Database:

However for things that don’t really matter (like posts, tweets, comments) then you can use a key-value store because who cares if all the comments/tweets etc aren’t listed every request as long as you show something and you know it’s going to be correct at some point.

I believe this is how reddit works it has postgres server(s) and then multiple cassandra servers for submissions.  I assume it’s similar for facebook except, I think, they use mysql & cassandra.


For your next web app I’d start with your usual relational database of choice.  Once you get to the stage when you have the performance/scalability issues to deal with then you might take a bigger look into the NoSQL/key-value stores.

Some interesting links:

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