High Level IDP API will be deployed to a web farm so does it require a DB support for IIDCache?
That’s correct. If you don’t use a central repository such as a database then each server would maintain its own ID cache and there’s the risk of an assertion replay attack.
We do supply a DatabaseIDCache to support ID caches maintained in a database. This is described in our Developer Guide. Alternatively you can write your own custom IIDCache implementation.