caching/write_through_cache library

Write-through and write-back caches over async loaders — roadmap #508.

Both wrap an async backing store (a remote API, a database, a file) behind an in-memory map so repeated reads of the same key are served locally, and both coordinate writes to that store — the capability the read-through WriteThroughCache (roadmap #523) does not provide. They differ only in when writes reach the backing store:

• WriteThroughStore writes synchronously to the store on every put, so the store is always current and a crash loses nothing — at the cost of one round-trip per write.
• WriteBackStore buffers writes in memory and only persists them on WriteBackStore.flush, coalescing repeated writes to the same key into one store call — faster under write-heavy load, but unflushed entries are lost on a crash.

Neither evicts on its own; pair with a size-bounded map (see lru_cache) if the key space is unbounded.

Classes

WriteBackStore<K, V>

A cache that buffers writes in memory until flush persists them.

WriteThroughStore<K, V>

A cache that persists every write to the backing store immediately.

Typedefs

CacheLoader<K, V> = Future<V?> Function(K key)

Loads the value for a key from the backing store (null = absent).

CacheStorer<K, V> = Future<void> Function(K key, V value)

Persists a key/value pair to the backing store.