General discussion about an async caching API.

Current state and issues

At the moment the methods Cache.loadAll and Cache.reloadAll can be used in a read through (with cache loader) caching configuration. The interface is ugly and it does not return the loaded values.

In the presence of a CacheWriter the Cache.put operation will potentially have I/O latency as well. Which means Cache.put should have an async counterpart as well.

If cache2k will support a hierarchical caching, e.g. with persistence to a file, the cache operation Cache.containsKey would potentially have I/O latency as well.

There are potentially long running life cycle operations, which should run without blocking the caller, e.g.: Cache.clear, Cache.close or cache resize.

Requirements

I see two different requirements at the moment:

1. A better interface specifically for caches operated in read through, which is quite common

2. Be able to run every cache operation in async mode

Solutions / ideas

New async cache interface supporting every operation

Obvious approach. Should we use CompletableFuture everywhere or support different
async models?

Subset of cache methods returning CompletableFuture for loading/read through caches

The loader returns a CompletableFuture which is hold by the cache directly.

Async entry processor

Maybe a good first start. Add the following two methods to Cache:

<X> CompletableFuture<X> invokeAsync(K key, EntryProcessor<K, V, X> p)
<X> CompletableFuture<X> invokeAllAsync(Iterable<K> key, EntryProcessor<K, V, X> p)

This way every cache operation could be executed in an asynchronous processing model.