DAO using a ConcurrentHashMap

Question

Is it ok to have a REST Webservice (Spring and Jersey) that uses a DAO with a ConcurrentHashMap to store the data, or should I avoid it and use some kind of in-memory DB?

It's an sample application, so I don't mind losing the data every time the application stops.

Answer 1

You can use ConcurrentHashMap, but you will have some difficulties when:

• trying to do 2 and more actions in same "transaction", you should synchronize such actions with other threads, as ConcurrentHashMap is successfully works only with one operation;
• trying to search not by Map key but some other field of the Map.Entry.value Object.

ConcurrentHashMap is for other purposes. So, I'll advise to use any in-memory DB.

Answer 2

ConcurrentHashMap is fine if you pretty much just need to create, read, update and delete entities. I'm actually using ConcurrentHashMap in an application that runs in Jetty and emulates some system that our application integrates.

But, as Soitorios Delimanolis and omickron mentioned, things would get hairy if you need to to rely on the atomicity of transactions involving multiple database operations.

To safeguard myself from that situation, I defined interfaces for my DAOs and wrote ConcurrentHashMap-backed implementation. If time comes when that would not be sufficient, I'd be able to swap out that implementation with an implementation based on HSQLDB or SQLite.