RendezvousHash

An alternative to the ring based, consistent hashing. This is a fast thread safe implementation of Rendezvous (Highest Random Weight, HRW) hashing. An algorithm that allows clients to achieve distributed agreement on which node (or proxy) a given key is to be placed in. This implementation has the following properties.

• Non-blocking reads : Determining which node a key belongs to is always non-blocking. Adding and removing nodes however blocks each other.
• Low overhead: providing using a hash function of low overhead. Throughput can be computed as (hashes computable per sec)/node count
• Load balancing: Since the hash function is randomizing, each of the n nodes is equally likely to receive the key K. Loads are uniform across the sites.
• High hit rate: Since all clients agree on placing an key K into the same node N , each fetch or placement of K into N yields the maximum utility in terms of hit rate. The key K will always be found unless it is evicted by some replacement algorithm at N.
• Minimal disruption: When a node is removed, only the keys mapped to that node need to be remapped and they will be distributed evenly

Source: https://en.wikipedia.org/wiki/Rendezvous_hashing

In comparison (source code) of Consistent hashing and Rendezvous hashing, consider the following load distribution after removing a couple nodes in a 5 node ring:

Only node4 takes the load of the 2 that were removed. However using HRW the distribution remains even

This example uses a rather simple ring for consistent hash implementation however and this extreme unbalance can be mitigated by adding the nodes many times (ie ~200) throughout the ring. These virtual nodes (or vnodes) are used in databases like Riak and Cassandra. Many libraries however do not implement vnodes.

Example:

    private static final Funnel<CharSequence> strFunnel = Funnels.stringFunnel(Charset.defaultCharset());
    
    // prepare 5 initial nodes "node1", "node2" ... "node5"
    List<String> nodes = Lists.newArrayList();
    for(int i = 0 ; i < 5; i ++) {
        nodes.add("node"+i); 
    }
    
    // create HRW instance
    RendezvousHash<String, String> h = new RendezvousHash(Hashing.murmur3_128(), strFunnel, strFunnel, nodes);
    
    String node = h.get("key");  // returns "node1"
    // remove "node1" from pool
    h.remove(node);
    h.get("key"); // returns "node2"
    
    // add "node1" back into pool
    h.add(node);  
    h.get("key"); // returns "node1"