Merge pull request hashicorp#326 from james-lawrence/observations/pee…

…r-changes

adds peer changes to observations.

observer.go

@@ -9,7 +9,10 @@ type Observation struct {
 	// Raft holds the Raft instance generating the observation.
 	Raft *Raft
 	// Data holds observation-specific data. Possible types are
-	// *RequestVoteRequest and RaftState.
+	// *RequestVoteRequest
+	// RaftState
+	// PeerObservation
+	// LeaderObservation
 	Data interface{}
 }
 
@@ -18,6 +21,12 @@ type LeaderObservation struct {
 	leader ServerAddress
 }
 
+// PeerObservation is sent to observers when peers change.
+type PeerObservation struct {
+	Removed bool
+	Peer    Server
+}
+
 // nextObserverId is used to provide a unique ID for each observer to aid in
 // deregistration.
 var nextObserverID uint64

raft.go

@@ -450,6 +450,7 @@ func (r *Raft) startStopReplication() {
 			r.leaderState.replState[server.ID] = s
 			r.goFunc(func() { r.replicate(s) })
 			asyncNotifyCh(s.triggerCh)
+			r.observe(PeerObservation{Peer: server, Removed: false})
 		}
 	}
 
@@ -463,6 +464,7 @@ func (r *Raft) startStopReplication() {
 		repl.stopCh <- lastIdx
 		close(repl.stopCh)
 		delete(r.leaderState.replState, serverID)
+		r.observe(PeerObservation{Peer: repl.peer, Removed: true})
 	}
 }