My requirement is to utilize 2 different database sources(persistent) and serve to frontend. To make api response faster can I utilize in-memory database like H2 or gemfire, to store data that is known to be frequently accessed (like a cron job at some time does that) and for other calls go to the databases. Here, the challenge for me is transferring the data from persistent to in-memory as Spring needs same 2 POJO's with different annotation(For e.g @Document for mongo, for h2,gemfire @Entity). as of now it does not make sense manually go through each record from an array received from mongo and save it in in-mem.
You can utilize the Spring Framework's Cache Abstraction and different forms of caching patterns when using Apache Geode (or alternatively, VMware Tanzu GemFire). See a more general description about caching starting here.
I also provide many Samples of the different caching patterns in action, Guide and Source included, which are reference in the relevant section in the reference documentation.
Finally, in the Spring Boot for Apache Geode project, I also have 2 test classes testing the Inline Caching Pattern, which you may refer to:
The primary, backend data store used in Inline Caching is made irrelevant since it uses the Spring Data Repository abstraction to interface with any data store (referred to as "modules" in the Spring Data portfolio; see here) supported by the SD Repository abstraction.
Caching is but 1 technique to keep relevant data in-memory in order to improve response times in a Spring (Boot) application using a backing data store, such as an RDBMS, as the primary System of Record (SOR). Of course, there are other approaches, too.
Apache Geode can even be configured for durability (persistence) and redundancy. In some cases, it might even completely replace your database for some function. So, you have a lot of options.
Hopefully this will get you started and give you more ideas.