started on heartbeat nodes

src/tom/cocagne/paxos/HeartbeatCallback.java

@@ -0,0 +1,7 @@
+package tom.cocagne.paxos;
+
+public interface HeartbeatCallback {
+
+	public void execute();
+
+}

src/tom/cocagne/paxos/HeartbeatMessenger.java

@@ -0,0 +1,12 @@
+package tom.cocagne.paxos;
+
+public interface HeartbeatMessenger extends PracticalMessenger {
+
+	public void sendHeartbeat( ProposalID leaderProposalID);
+
+	public void schedule(int millisecondDelay, HeartbeatCallback callback);
+
+	public void onLeadershipLost();
+
+	public void onLeadershipChange(String previousLeaderUID, String newLeaderUID);
+}

src/tom/cocagne/paxos/HeartbeatNode.java

@@ -0,0 +1,65 @@
+package tom.cocagne.paxos;
+
+import java.util.HashSet;
+
+public class HeartbeatNode extends PracticalNode {
+
+	protected String          leaderUID;
+	protected ProposalID      leaderProposalID;
+	protected long            lastHeartbeatTimestamp;
+	protected long            lastPrepareTimestamp;
+	protected long            heartbeatPeriod         = 1000; // Milliseconds
+	protected long            livenessWindow          = 5000; // Milliseconds
+	protected boolean         acquiringLeadership     = false;
+	protected HashSet<String> acceptNACKs             = new HashSet<String>();
+
+
+	public HeartbeatNode(PracticalMessenger messenger, String proposerUID,
+			int quorumSize, String leaderUID, int heartbeatPeriod, int livenessWindow) {
+		super(messenger, proposerUID, quorumSize);
+
+		this.leaderUID       = leaderUID;
+		this.heartbeatPeriod = heartbeatPeriod;
+		this.livenessWindow  = livenessWindow;
+
+		leaderProposalID       = new ProposalID(1, leaderUID);
+		lastHeartbeatTimestamp = timestamp();
+		lastPrepareTimestamp   = timestamp();
+
+		if (leaderUID != null && proposerUID.equals(leaderUID))
+			setLeader(true);
+	}
+
+	public long timestamp() {
+		return System.currentTimeMillis();
+	}
+
+	@Override
+	public void prepare(boolean incrementProposalNumber) {
+		if (incrementProposalNumber)
+			acceptNACKs.clear();
+		super.prepare(incrementProposalNumber);
+	}
+
+	public boolean leaderIsAlive() {
+		return timestamp() - lastHeartbeatTimestamp <= livenessWindow;
+	}
+
+	public boolean observedRecentPrepare() {
+		return timestamp() - lastPrepareTimestamp <= livenessWindow * 1.5;
+	}
+
+	public void pollLiveness() {
+		if (!leaderIsAlive() && !observedRecentPrepare()) {
+			if (acquiringLeadership)
+				prepare();
+			else
+				acquireLeadership();
+		}
+	}
+
+	private void acquireLeadership() {
+
+
+	}
+}