Transition Raft servers to candidate/leader immediately if single nod…

…e cluster (#1005)

* Transition Raft servers to candidate/leader immediately if single node cluster.

Co-authored-by: Johno Crawford <johno@hellface.com>

protocols/raft/src/main/java/io/atomix/protocols/raft/roles/CandidateRole.java

@@ -55,6 +55,13 @@ public RaftServer.Role role() {
 
   @Override
   public synchronized CompletableFuture<RaftRole> start() {
+    if (raft.getCluster().getActiveMemberStates().isEmpty()) {
+      log.debug("Single member cluster. Transitioning directly to leader.");
+      raft.setTerm(raft.getTerm() + 1);
+      raft.setLastVotedFor(raft.getCluster().getMember().memberId());
+      raft.transition(RaftServer.Role.LEADER);
+      return CompletableFuture.completedFuture(this);
+    }
     return super.start().thenRun(this::startElection).thenApply(v -> this);
   }
 
@@ -91,13 +98,6 @@ private void sendVoteRequests() {
     final AtomicBoolean complete = new AtomicBoolean();
     final Set<DefaultRaftMember> votingMembers = new HashSet<>(raft.getCluster().getActiveMemberStates().stream().map(RaftMemberContext::getMember).collect(Collectors.toList()));
 
-    // If there are no other members in the cluster, immediately transition to leader.
-    if (votingMembers.isEmpty()) {
-      log.debug("Single member cluster. Transitioning directly to leader.", raft.getCluster().getMember().memberId());
-      raft.transition(RaftServer.Role.LEADER);
-      return;
-    }
-
     // Send vote requests to all nodes. The vote request that is sent
     // to this node will be automatically successful.
     // First check if the quorum is null. If the quorum isn't null then that

protocols/raft/src/main/java/io/atomix/protocols/raft/roles/FollowerRole.java

@@ -62,6 +62,11 @@ public RaftServer.Role role() {
 
   @Override
   public synchronized CompletableFuture<RaftRole> start() {
+    if (raft.getCluster().getActiveMemberStates().isEmpty()) {
+      log.debug("Single member cluster. Transitioning directly to candidate.");
+      raft.transition(RaftServer.Role.CANDIDATE);
+      return CompletableFuture.completedFuture(this);
+    }
     raft.getMembershipService().addListener(clusterListener);
     return super.start().thenRun(this::resetHeartbeatTimeout).thenApply(v -> this);
   }