Why would someone need an in-memory database?

Question

I read that a few databases can be used in-memory but can't think of reason why someone would want to use this feature. I always use a database to persist data and memory caches for fast access.

Answer 1

Cache is also a kind of database, like a file system is. 'Memory cache' is just a specific application of an in-memory database and some in-memory databases are specialized as memory caches.

Other uses of in-memory databases have already been included in other answers, but let me enumerate the uses too:

1. Memory cache. Usually a database system specialized for that use (and probably known as 'a memory cache' rather than 'a database') will be used.
2. Testing database-related code. In this case often an 'in-memory' mode of some generic database system will be used, but also a dedicated 'in-memory' database may be used to replace other 'on-disk' database for faster testing.
3. Sophisticated data manipulation. In-memory SQL databases are often used this way. SQL is a great tool for data manipulation and sometimes there is no need to write the data on disk while computing the final result.
4. Storing of transient runtime state. There are application that need to store their state in some kind of database but do not need to persist that over application restart. Think of some kind of process manager – it needs to keep track of sub-processes running, but that data is only valid as long as the application and the sub-processes run.