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.
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()); }
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 clusterid
- 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 machinegossip_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 nodesThen 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();
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!