Guava LoadingCache - how to handle keys which don't exist in backing store

Question

I am using CacheBuilder and LoadingCache to implement an in-memory cache of database data.

Suppose a client queries the cache for an item that does not exist in the backing store. I want the client to know that no data was found for the specified key. What is the best approach for handling this?

• Store special value in cache which signifies "no data".
• Store nothing in cache and raise exception.
• Other ideas?

Answer 1

I've always solved this in the following way.

interface KeyValueService<K,V> {
    V get(K key);
}

class CachingKeyValueService<K,V> {
    Cache<K,Optional<V>> cache;
    V get(K key) {
        return cache.get(key).orNull();
    }

}

Ideally you would change the interface for KeyValueService to always return Optional, but sometimes thats not possible.

You can use weighting to cause all Optional.ABSENT references to be evicted quickly.