I saw in the tests spring cloud dataflow used to store the SpringDefinition - HashMap, is it possible to override the configuration of DateFlowServerConfiguration for storing streams and Tasks in an InMemory, for example in the same HashMap, if so, how?
I don't think it would be a trivial change. The server needs a backend to store it's metadata. By default it actually uses H2 in memory, and it relies on Spring Data JPA abstraction to give users the chance to select their RDBMS.
Storing on a different storage engine, would require not only replacing all the *Repository definitions on several configuration modules, but we do as well some pre population of data. It would become a bit hard to maintain this over time.
Is there a reason why a traditional RDBMS is not suitable here? or if you want in-memory just go with the ephemeral approach of H2?