#FifoOffHeapLongCache

###A lightweight concurrent off-heap cache with FIFO eviction semantics written in Java.

As its name suggests, FifoOffHeapLongCache maps byte array keys to long values and utilizes a FIFO eviction policy. It is not a general purpose Object cache. FifoOffHeapLongCache uses Hopscotch hashing (a form of linear probing) for collision resolution and uses the 128 bit version of MurmurHash3 as its hashing function (note however that we only use the lower 64 bits of the generated hash). This class is thread-safe.

The bookkeeping strategy for managing linked list pointers and eviction is modeled on the ConcurrentLinkedHashMap design which amortizes bookkeeping and eviction across threads. The default target load factor is 0.8, but can be configured.

Puts into FifoOffHeapLongCache are only written if the key does not currently exist in the cache. Note that FifoOffHeapLongCache explicitly does NOT offer delete/remove key functionality.

FifoOffHeapLongCache was built for large (>10GB) caches and stores all key, value, and pointer data off-heap in pre-allocated memory in order to minimize GC overhead. There are also APIs provided for saving the cache to and restoring the cache from disk if persistence is required. NOTE: you must call destroy on the cache or use it in a try-with-resources block (in which case destroy will be called for you) or any allocated memory will not be released.

####Usage

// add an Eviction listener to be called when entries are evicted
FifoOffHeapLongCache.EvictionListener listener = new FifoOffHeapLongCache.EvictionListener() {
  @Override
  public void onEvict(long key, long value) {
    System.out.println(String.format("Evicted entry %d => %d", key, value));
  }
};

// build the cache
try (FifoOffHeapLongCache cache = new FifoOffHeapLongCache.Builder()
  .setSize(100000000L)
  .setEvictionThreshold(0.80)
  .setEvictionListener(listener)
  .build()
) {
  String key = "key";
  long value = 1L;
  cache.put(key.getBytes(), value);
  assert 1 == cache.get(key.getBytes());
}