Java – Google Guava’s CacheLoader loadAll() vs reload() semantics


Two things I really like about Guava 11's CacheLoader (thanks, Google!) are loadAll(), which allows me to load multiple keys at once, and reload(), which allows me to reload a key asynchronously when it's "stale" but an old value exists. I'm curious as to how they play together, since reload() operates on but a single key.

Concretely, extending the example from CachesExplained:

LoadingCache<Key, Graph> graphs = CacheBuilder.newBuilder()
   .refreshAfterWrite(1, TimeUnit.MINUTES)
       new CacheLoader<Key, Graph>() {
         public Graph load(Key key) { // no checked exception
           return getGraphFromDatabase(key);

         public Map<Key, Graph> loadAll(Iterable<? extends K> keys) {
           return getAllGraphsFromDatabase(keys);

         public ListenableFuture<Graph> reload(final Key key, Graph prevGraph) {
           if (neverNeedsRefresh(key)) {
             return Futures.immediateFuture(prevGraph);
           } else {
             // asynchronous!
             return ListenableFutureTask.create(new Callable<Graph>() {
               public Graph call() {
                 return getGraphFromDatabase(key);

…where "getAllGraphsFromDatabase()" does an aggregate database query rather than length(keys) individual queries.

How do these two components of a LoadingCache play together? If some keys in my request to getAll() aren't present in the cache, they are loaded as a group with loadAll(), but if some need refreshing, do they get reloaded individually with load()? If so, are there plans to support a reloadAll()?

Best Solution

Here's how refreshing works.

Refreshing on a cache entry can be triggered in two ways:

  1. Explicitly, with cache.refresh(key).
  2. Implicitly, if the cache is configured with refreshAfterWrite and the entry is queried after the specified amount of time after it was written.

If an entry that is eligible for reload is queried, then the old value is returned, and a (possibly asynchronous) refresh is triggered. The cache will continue to return the old value for the key while the refresh is in progress. (So if some keys in a getAll request are eligible for refresh, their old values will be returned, but the values for those keys will be (possibly asynchronously) reloaded.)

The default implementation of CacheLoader.reload(key, oldValue) just returns Futures.immediateFuture(load(key)), which (synchronously) recomputes the value. More sophisticated, asynchronous implementations are recommended if you expect to be doing cache refreshes.

I don't think we're inclined to provide reloadAll at the moment. I suspect it's possible, but things are complicated enough as it is, and I think we're inclined to wait until we see specific demand for such a thing.