I have a question regarding some NoSQL databases. In Ehcache we have for example the JCache API, in MapDB the Map Interface and in Riak KV we have a own process with clusters. How do I exactly find out which database fits to which implementation type? For example for RocksDB (I assume that it is a process) and same for LevelDB.
For reference, RocksDB and LevelDB perform very similar functions and can be interchangeable in some situations.
Given your question of Is RocksDB and LevelDB just like Riak?, I can say that they are not the same as Riak provides a scalable distributed platform to run on that can connect to one or more backend databases simultaneoulsy (currently supported backends are Bitcask, LevelDB, Leveled and memory). RocksDB and LevelDB are essentially stand alone database platforms that can be used as such or can utilised by other software such as Riak as a backend. While you could technically implement RocksDB as a backend for Riak KV without needing a mountain of custom code, you probably wouldn't want to as RocksDB does not scale well.
How do I exactly find out which database fits to which implementation type? is rather a broad question. I think you might want to rephrase it as Which databases offer me {my list of desired implementations/functions}? to make it easier for community members to answer. Please note that some NoSQL databases have multiple uses available e.g. as you mentioned Riak KV, we have Maps, Sets, GSets, Flags, Registers, Solr Search, 2i and the standard CRDT options as well but some of those may be tied to other requirements e.g. 2i only works with a LevelDB/Leveled backend, Solr Search requires the Yokozuna package version of Riak KV 3.0.0 and above but is built in for all Riak 2.x.x versions etc.
What you may also want to try to do is download a few different options to a VM or bare metal rig, have a play and see how it works out. There are often cases where two competing products do something very similar on paper but in your specific use case, one outperforms the other significantly.
To get you started, here are links to Riak 2.9.8 (the latest release of the 2.x.x series) and to the Riak 2.2.6 docs (the 2.9.x docs should be out later this month).
I'm not sure if this has directly answered your question but, hopefully, it will give you some pointers as to where to go next.