First of all: what is an adapter?

Adapter is a bridge between cache instance and storage it (adapter) uses.

stash-it doesn't come with any adapter built in, you need to provide one.

For example: stash-it-adapter-memory provides access to memory, in which data is stored; stash-it-adapter-mongo provides access to mongo DB, etc.

Adapter provides API for IO operations on given storage. APIs of all adapters are, by design, the same. Therefore you can switch your adapter, and the API will stay consistent.