Example application illustrating how to communicate with Mikromarc 3 using the OData-based MMWebApi.
OData is an attempt add "best practices" for REST-like web-services. It mainly standardizes resource access and manipulation, filtering and search. Adding to that it introduces a convention for RPC-like calls (called "actions").
The official basic tutorial is a good introduction to most core concepts.
Apache Olingo is the Apache Foundations implementation of the OData standard. It seems stable and has all the functionality, it's also actively maintained (unlike odata4j).
The official documentation can be a bit hard to digest. Fortunately enough, these two blog posts exist and are a big help for those just getting started.