Gossip

Gossip protocol is a method for a group of nodes to discover and check the liveliness of a cluster. More information can be found at http://en.wikipedia.org/wiki/Gossip_protocol.

The original implementation was forked from https://code.google.com/p/java-gossip/. Several bug fixes and changes have already been added.

A set of easily-run examples, illustrating various features of Gossip, are available in the gossip-examples module. The README.md file, in that module described how to run those examples.

Below, a list of code snippits which show how to incorproate Apache Gossip into your project.

Usage

To gossip you need one or more seed nodes. Seed is just a list of places to initially connect to.

  GossipSettings settings = new GossipSettings();
  int seedNodes = 3;
  List<GossipMember> startupMembers = new ArrayList<>();
  for (int i = 1; i < seedNodes+1; ++i) {
    URI uri = new URI("udp://" + "127.0.0.1" + ":" + (50000 + i));
    startupMembers.add(new RemoteGossipMember(cluster, uri, i + ""));
  }

Here we start five gossip processes and check that they discover each other. (Normally these are on different hosts but here we give each process a distinct local ip.

  List<GossipService> clients = new ArrayList<>();
  int clusterMembers = 5;
  for (int i = 1; i < clusterMembers+1; ++i) {
    URI uri = new URI("udp://" + "127.0.0.1" + ":" + (50000 + i));
   GossipService gossipService = new GossipService(cluster, uri, i + "",
             startupMembers, settings, (a,b) -> {});
  }

Later we can check that the nodes discover each other

  Thread.sleep(10000);
  for (int i = 0; i < clusterMembers; ++i) {
    Assert.assertEquals(4, clients.get(i).getGossipManager().getLiveMembers().size());
  }

Usage with Settings File

For a very simple client setup with a settings file you first need a JSON file such as:

[{
  "cluster":"9f1e6ddf-8e1c-4026-8fc6-8585d0132f77",
  "id":"447c5bec-f112-492d-968b-f64c8e36dfd7",
  "uri":"udp://127.0.0.1:50001",
  "gossip_interval":1000,
  "cleanup_interval":10000,
  "members":[
    {"cluster": "9f1e6ddf-8e1c-4026-8fc6-8585d0132f77","uri":"udp://127.0.0.1:5000"}
  ]
}]

where:

• cluster - is the name of the cluster
• id - is a unique id for this node (you can use any string, but above we use a UUID)
• uri - is a URI object containing IP/hostname and port to use on the default adapter on the node’s machine
• gossip_interval - how often (in milliseconds) to gossip list of members to other node(s)
• cleanup_interval - when to remove ‘dead’ nodes (in milliseconds) (deprecated may be coming back)
• members - initial seed nodes

Then starting a local node is as simple as:

GossipService gossipService = new GossipService(
  StartupSettings.fromJSONFile( "node_settings.json" )
);
gossipService.start();

And then when all is done, shutdown with:

gossipService.shutdown();

Event Listener

The status can be polled using the getters that return immutable lists.

   public List<LocalGossipMember> getLiveMembers()
   public List<LocalGossipMember> getDeadMembers()

These can be accessed from the GossipManager on your GossipService, e.g: gossipService.getGossipManager().getLiveMembers();

Users can also attach an event listener:

    GossipService gossipService = new GossipService(cluster, uri, i + "", startupMembers,
    settings, new GossipListener() {
      @Override
      public void gossipEvent(GossipMember member, GossipState state) {
        System.out.println(System.currentTimeMillis() + " Member " + j + " reports "
                + member + " " + state);
      }
  });
  //The lambda syntax is (a,b) -> { }  //NICE!