It is said in Cache apidoc, that several methods like purge() or flush() operates dependent on persistence storage configured.
Unfortunately, I can't find, how to configure one?
Is it really possible?
Older versions of cache2k had persistence support baked in. It was working, but, however it never made it to a level that I would fully trust for production.
The actual issue was the clear() operation, which had a quite complex implementation. The clear should be fast, regardless of the storage implementation needing some time to remove the data. So, my idea was to switch to a write back scheme, where operations get queued and executed when the storage is available again. Implementing a partial write back scheme just for the clear, is quite some over engineering...
For the moment I dropped persistence from the feature set, since I don't want a 1.0 version which has a stabilized API and provides already a lot of useful features.
As you can see from the roadmap on the cache2k homepage the current plan is first to add bulk and async features and then get back to storage. Probably the storage interface needs to look totally different after the async capabilities are done.
Inside the current cache2k implementation there are still the interfaces where the storage will be hooked in, so that I do not completely abandon what already is achieved. flush() and purge() are still some remnants of this. So I better consequently will remove those two methods for the 1.0 version, to avoid confusion.
BTW: Since I saw your question on Guava, cache2k has support for a CacheWriter which is the counterpart for CacheLoader. With the cache loader and writer you can read and write to a storage by yourself, but it is not identical to storage support inside the cache itself. For example cache.contains(...) would check the storage, but it does not check the cache loader at least according to JSR107 and in every cache implementation I know of.