[Zen2] Implement basic cluster formation (#33668)

This PR integrates the following pieces of machinery in the Coordinator:

- discovery
- pre-voting
- randomised election scheduling
- joining (of a new master)
- publication of cluster state updates

Together, these things are everything needed to form a cluster. We therefore
also add the start of a test suite that allows us to assert higher-level
properties of the interactions between all these pieces of machinery, with as
little fake behaviour as possible. We assert one such property: "a cluster
successfully forms".

server/src/main/java/org/elasticsearch/cluster/coordination/ApplyCommitRequest.java

@@ -49,6 +49,7 @@ public class ApplyCommitRequest extends TermVersionRequest {
         return "ApplyCommitRequest{" +
             "term=" + term +
             ", version=" + version +
+            ", sourceNode=" + sourceNode +
             '}';
     }
 }

server/src/main/java/org/elasticsearch/cluster/coordination/Coordinator.java

@@ -19,18 +19,35 @@
 package org.elasticsearch.cluster.coordination;
 
 import org.apache.lucene.util.SetOnce;
+import org.elasticsearch.cluster.ClusterState;
 import org.elasticsearch.cluster.node.DiscoveryNode;
 import org.elasticsearch.cluster.routing.allocation.AllocationService;
 import org.elasticsearch.cluster.service.MasterService;
+import org.elasticsearch.common.Nullable;
+import org.elasticsearch.common.Randomness;
 import org.elasticsearch.common.component.AbstractLifecycleComponent;
+import org.elasticsearch.common.lease.Releasable;
 import org.elasticsearch.common.settings.Settings;
+import org.elasticsearch.common.unit.TimeValue;
+import org.elasticsearch.discovery.HandshakingTransportAddressConnector;
+import org.elasticsearch.discovery.PeerFinder;
+import org.elasticsearch.discovery.UnicastConfiguredHostsResolver;
+import org.elasticsearch.discovery.zen.UnicastHostsProvider;
+import org.elasticsearch.threadpool.ThreadPool.Names;
+import org.elasticsearch.transport.TransportResponse.Empty;
 import org.elasticsearch.transport.TransportService;
 
+import java.util.ArrayList;
+import java.util.List;
 import java.util.Optional;
+import java.util.concurrent.atomic.AtomicLong;
 import java.util.function.Supplier;
 
 public class Coordinator extends AbstractLifecycleComponent {
 
+    public static final String PUBLISH_STATE_ACTION_NAME = "internal:cluster/coordination/publish_state";
+    public static final String COMMIT_STATE_ACTION_NAME = "internal:cluster/coordination/commit_state";
+
     private final TransportService transportService;
     private final JoinHelper joinHelper;
     private final Supplier<CoordinationState.PersistedState> persistedStateSupplier;
@@ -38,6 +55,17 @@ public class Coordinator extends AbstractLifecycleComponent {
     // These tests can be rewritten to use public methods once Coordinator is more feature-complete
     final Object mutex = new Object();
     final SetOnce<CoordinationState> coordinationState = new SetOnce<>(); // initialized on start-up (see doStart)
+    private volatile Optional<ClusterState> lastCommittedState = Optional.empty();
+
+    private final PeerFinder peerFinder;
+    private final PreVoteCollector preVoteCollector;
+    private final ElectionSchedulerFactory electionSchedulerFactory;
+    private final UnicastConfiguredHostsResolver configuredHostsResolver;
+    @Nullable
+    private Releasable electionScheduler;
+    @Nullable
+    private Releasable prevotingRound;
+    private AtomicLong maxTermSeen = new AtomicLong();
 
     private Mode mode;
     private Optional<DiscoveryNode> lastKnownLeader;
@@ -45,15 +73,99 @@ public class Coordinator extends AbstractLifecycleComponent {
     private JoinHelper.JoinAccumulator joinAccumulator;
 
     public Coordinator(Settings settings, TransportService transportService, AllocationService allocationService,
-                       MasterService masterService, Supplier<CoordinationState.PersistedState> persistedStateSupplier) {
+                       MasterService masterService, Supplier<CoordinationState.PersistedState> persistedStateSupplier,
+                       UnicastHostsProvider unicastHostsProvider) {
         super(settings);
         this.transportService = transportService;
         this.joinHelper = new JoinHelper(settings, allocationService, masterService, transportService,
-            this::getCurrentTerm, this::handleJoinRequest);
+            this::getCurrentTerm, this::handleJoinRequest, this::joinLeaderInTerm);
         this.persistedStateSupplier = persistedStateSupplier;
         this.lastKnownLeader = Optional.empty();
         this.lastJoin = Optional.empty();
         this.joinAccumulator = joinHelper.new CandidateJoinAccumulator();
+
+        this.electionSchedulerFactory = new ElectionSchedulerFactory(settings, Randomness.get(), transportService.getThreadPool());
+        this.preVoteCollector = new PreVoteCollector(settings, transportService, this::startElection, this::updateMaxTermSeen);
+        configuredHostsResolver = new UnicastConfiguredHostsResolver(settings, transportService, unicastHostsProvider);
+        this.peerFinder = new CoordinatorPeerFinder(settings, transportService,
+            new HandshakingTransportAddressConnector(settings, transportService), configuredHostsResolver);
+
+        transportService.registerRequestHandler(PUBLISH_STATE_ACTION_NAME, Names.GENERIC, false, false,
+            in -> new PublishRequest(in, transportService.getLocalNode()),
+            (request, channel, task) -> channel.sendResponse(handlePublishRequest(request)));
+
+        transportService.registerRequestHandler(COMMIT_STATE_ACTION_NAME, Names.GENERIC, false, false,
+            ApplyCommitRequest::new,
+            (request, channel, task) -> {
+                handleApplyCommit(request);
+                channel.sendResponse(Empty.INSTANCE);
+            });
+    }
+
+    private void handleApplyCommit(ApplyCommitRequest applyCommitRequest) {
+        synchronized (mutex) {
+            logger.trace("handleApplyCommit: applying commit {}", applyCommitRequest);
+
+            coordinationState.get().handleCommit(applyCommitRequest);
+            lastCommittedState = Optional.of(coordinationState.get().getLastAcceptedState());
+        }
+    }
+
+    private PublishWithJoinResponse handlePublishRequest(PublishRequest publishRequest) {
+        synchronized (mutex) {
+            final DiscoveryNode sourceNode = publishRequest.getAcceptedState().nodes().getMasterNode();
+            logger.trace("handlePublishRequest: handling [{}] from [{}]", publishRequest, sourceNode);
+            ensureTermAtLeast(sourceNode, publishRequest.getAcceptedState().term());
+            final PublishResponse publishResponse = coordinationState.get().handlePublishRequest(publishRequest);
+
+            if (sourceNode.equals(getLocalNode()) == false) {
+                becomeFollower("handlePublishRequest", sourceNode);
+            }
+
+            return new PublishWithJoinResponse(publishResponse,
+                joinWithDestination(lastJoin, sourceNode, publishRequest.getAcceptedState().term()));
+        }
+    }
+
+    private static Optional<Join> joinWithDestination(Optional<Join> lastJoin, DiscoveryNode leader, long term) {
+        if (lastJoin.isPresent()
+            && lastJoin.get().getTargetNode().getId().equals(leader.getId())
+            && lastJoin.get().getTerm() == term) {
+            return lastJoin;
+        }
+
+        return Optional.empty();
+    }
+
+    private void closePrevotingAndElectionScheduler() {
+        if (prevotingRound != null) {
+            prevotingRound.close();
+            prevotingRound = null;
+        }
+
+        if (electionScheduler != null) {
+            electionScheduler.close();
+            electionScheduler = null;
+        }
+    }
+
+    private void updateMaxTermSeen(final long term) {
+        maxTermSeen.updateAndGet(oldMaxTerm -> Math.max(oldMaxTerm, term));
+        // TODO if we are leader here, and there is no publication in flight, then we should bump our term
+        // (if we are leader and there _is_ a publication in flight then doing so would cancel the publication, so don't do that, but
+        // do check for this after the publication completes)
+    }
+
+    private void startElection() {
+        synchronized (mutex) {
+            // The preVoteCollector is only active while we are candidate, but it does not call this method with synchronisation, so we have
+            // to check our mode again here.
+            if (mode == Mode.CANDIDATE) {
+                final StartJoinRequest startJoinRequest
+                    = new StartJoinRequest(getLocalNode(), Math.max(getCurrentTerm(), maxTermSeen.get()) + 1);
+                getDiscoveredNodes().forEach(node -> joinHelper.sendStartJoinRequest(startJoinRequest, node));
+            }
+        }
     }
 
     private Optional<Join> ensureTermAtLeast(DiscoveryNode sourceNode, long targetTerm) {
@@ -65,19 +177,20 @@ public class Coordinator extends AbstractLifecycleComponent {
     }
 
     private Join joinLeaderInTerm(StartJoinRequest startJoinRequest) {
-        assert Thread.holdsLock(mutex) : "Coordinator mutex not held";
+        synchronized (mutex) {
-        logger.debug("joinLeaderInTerm: from [{}] with term {}", startJoinRequest.getSourceNode(), startJoinRequest.getTerm());
+            logger.debug("joinLeaderInTerm: for [{}] with term {}", startJoinRequest.getSourceNode(), startJoinRequest.getTerm());
-        Join join = coordinationState.get().handleStartJoin(startJoinRequest);
+            final Join join = coordinationState.get().handleStartJoin(startJoinRequest);
-        lastJoin = Optional.of(join);
+            lastJoin = Optional.of(join);
-        if (mode != Mode.CANDIDATE) {
+            if (mode != Mode.CANDIDATE) {
-            becomeCandidate("joinLeaderInTerm");
+                becomeCandidate("joinLeaderInTerm");
+            }
+            return join;
         }
-        return join;
     }
 
     private void handleJoinRequest(JoinRequest joinRequest, JoinHelper.JoinCallback joinCallback) {
         assert Thread.holdsLock(mutex) == false;
-        logger.trace("handleJoin: as {}, handling {}", mode, joinRequest);
+        logger.trace("handleJoinRequest: as {}, handling {}", mode, joinRequest);
         transportService.connectToNode(joinRequest.getSourceNode());
 
         final Optional<Join> optionalJoin = joinRequest.getOptionalJoin();
@@ -85,29 +198,11 @@ public class Coordinator extends AbstractLifecycleComponent {
             final CoordinationState coordState = coordinationState.get();
             final boolean prevElectionWon = coordState.electionWon();
 
-            if (optionalJoin.isPresent()) {
+            optionalJoin.ifPresent(this::handleJoin);
-                Join join = optionalJoin.get();
-                // if someone thinks we should be master, let's add our vote and try to become one
-                // note that the following line should never throw an exception
-                ensureTermAtLeast(getLocalNode(), join.getTerm()).ifPresent(coordState::handleJoin);
-
-                if (coordState.electionWon()) {
-                    // if we have already won the election then the actual join does not matter for election purposes,
-                    // so swallow any exception
-                    try {
-                        coordState.handleJoin(join);
-                    } catch (CoordinationStateRejectedException e) {
-                        logger.trace("failed to add join, ignoring", e);
-                    }
-                } else {
-                    coordState.handleJoin(join); // this might fail and bubble up the exception
-                }
-            }
-
             joinAccumulator.handleJoinRequest(joinRequest.getSourceNode(), joinCallback);
 
             if (prevElectionWon == false && coordState.electionWon()) {
-                becomeLeader("handleJoin");
+                becomeLeader("handleJoinRequest");
             }
         }
     }
@@ -120,7 +215,11 @@ public class Coordinator extends AbstractLifecycleComponent {
             mode = Mode.CANDIDATE;
             joinAccumulator.close(mode);
             joinAccumulator = joinHelper.new CandidateJoinAccumulator();
+
+            peerFinder.activate(coordinationState.get().getLastAcceptedState().nodes());
         }
+
+        preVoteCollector.update(getPreVoteResponse(), null);
     }
 
     void becomeLeader(String method) {
@@ -129,9 +228,13 @@ public class Coordinator extends AbstractLifecycleComponent {
         logger.debug("{}: becoming LEADER (was {}, lastKnownLeader was [{}])", method, mode, lastKnownLeader);
 
         mode = Mode.LEADER;
-        lastKnownLeader = Optional.of(getLocalNode());
         joinAccumulator.close(mode);
         joinAccumulator = joinHelper.new LeaderJoinAccumulator();
+
+        lastKnownLeader = Optional.of(getLocalNode());
+        peerFinder.deactivate(getLocalNode());
+        closePrevotingAndElectionScheduler();
+        preVoteCollector.update(getPreVoteResponse(), getLocalNode());
     }
 
     void becomeFollower(String method, DiscoveryNode leaderNode) {
@@ -145,6 +248,14 @@ public class Coordinator extends AbstractLifecycleComponent {
         }
 
         lastKnownLeader = Optional.of(leaderNode);
+        peerFinder.deactivate(leaderNode);
+        closePrevotingAndElectionScheduler();
+        preVoteCollector.update(getPreVoteResponse(), leaderNode);
+    }
+
+    private PreVoteResponse getPreVoteResponse() {
+        return new PreVoteResponse(getCurrentTerm(), coordinationState.get().getLastAcceptedTerm(),
+            coordinationState.get().getLastAcceptedVersion());
     }
 
     // package-visible for testing
@@ -170,6 +281,7 @@ public class Coordinator extends AbstractLifecycleComponent {
     protected void doStart() {
         CoordinationState.PersistedState persistedState = persistedStateSupplier.get();
         coordinationState.set(new CoordinationState(settings, getLocalNode(), persistedState));
+        configuredHostsResolver.start();
     }
 
     public void startInitialJoin() {
@@ -180,7 +292,7 @@ public class Coordinator extends AbstractLifecycleComponent {
 
     @Override
     protected void doStop() {
-
+        configuredHostsResolver.stop();
     }
 
     @Override
@@ -190,22 +302,115 @@ public class Coordinator extends AbstractLifecycleComponent {
 
     public void invariant() {
         synchronized (mutex) {
+            final Optional<DiscoveryNode> peerFinderLeader = peerFinder.getLeader();
             if (mode == Mode.LEADER) {
                 assert coordinationState.get().electionWon();
                 assert lastKnownLeader.isPresent() && lastKnownLeader.get().equals(getLocalNode());
                 assert joinAccumulator instanceof JoinHelper.LeaderJoinAccumulator;
+                assert peerFinderLeader.equals(lastKnownLeader) : peerFinderLeader;
+                assert electionScheduler == null : electionScheduler;
+                assert prevotingRound == null : prevotingRound;
             } else if (mode == Mode.FOLLOWER) {
                 assert coordinationState.get().electionWon() == false : getLocalNode() + " is FOLLOWER so electionWon() should be false";
                 assert lastKnownLeader.isPresent() && (lastKnownLeader.get().equals(getLocalNode()) == false);
                 assert joinAccumulator instanceof JoinHelper.FollowerJoinAccumulator;
+                assert peerFinderLeader.equals(lastKnownLeader) : peerFinderLeader;
+                assert electionScheduler == null : electionScheduler;
+                assert prevotingRound == null : prevotingRound;
             } else {
                 assert mode == Mode.CANDIDATE;
                 assert joinAccumulator instanceof JoinHelper.CandidateJoinAccumulator;
+                assert peerFinderLeader.isPresent() == false : peerFinderLeader;
+                assert prevotingRound == null || electionScheduler != null;
             }
         }
     }
 
+    // for tests
+    boolean hasJoinVoteFrom(DiscoveryNode localNode) {
+        return coordinationState.get().containsJoinVoteFor(localNode);
+    }
+
+    void handleJoin(Join join) {
+        synchronized (mutex) {
+            ensureTermAtLeast(getLocalNode(), join.getTerm()).ifPresent(this::handleJoin);
+
+            if (coordinationState.get().electionWon()) {
+                // if we have already won the election then the actual join does not matter for election purposes,
+                // so swallow any exception
+                try {
+                    coordinationState.get().handleJoin(join);
+                } catch (CoordinationStateRejectedException e) {
+                    logger.debug("failed to add join, ignoring", e);
+                }
+            } else {
+                coordinationState.get().handleJoin(join); // this might fail and bubble up the exception
+            }
+        }
+    }
+
+    public ClusterState getLastAcceptedState() {
+        synchronized (mutex) {
+            return coordinationState.get().getLastAcceptedState();
+        }
+    }
+
+    public Optional<ClusterState> getLastCommittedState() {
+        return lastCommittedState;
+    }
+
+    private List<DiscoveryNode> getDiscoveredNodes() {
+        final List<DiscoveryNode> nodes = new ArrayList<>();
+        nodes.add(getLocalNode());
+        peerFinder.getFoundPeers().forEach(nodes::add);
+        return nodes;
+    }
+
     public enum Mode {
         CANDIDATE, LEADER, FOLLOWER
     }
+
+    private class CoordinatorPeerFinder extends PeerFinder {
+
+        CoordinatorPeerFinder(Settings settings, TransportService transportService, TransportAddressConnector transportAddressConnector,
+                              ConfiguredHostsResolver configuredHostsResolver) {
+            super(settings, transportService, transportAddressConnector, configuredHostsResolver);
+        }
+
+        @Override
+        protected void onActiveMasterFound(DiscoveryNode masterNode, long term) {
+            // TODO
+        }
+
+        @Override
+        protected void onFoundPeersUpdated() {
+            synchronized (mutex) {
+                if (mode == Mode.CANDIDATE) {
+                    final CoordinationState.VoteCollection expectedVotes = new CoordinationState.VoteCollection();
+                    getFoundPeers().forEach(expectedVotes::addVote);
+                    expectedVotes.addVote(Coordinator.this.getLocalNode());
+                    final ClusterState lastAcceptedState = coordinationState.get().getLastAcceptedState();
+                    final boolean foundQuorum = CoordinationState.isElectionQuorum(expectedVotes, lastAcceptedState);
+
+                    if (foundQuorum) {
+                        if (electionScheduler == null) {
+                            final TimeValue gracePeriod = TimeValue.ZERO; // TODO variable grace period
+                            electionScheduler = electionSchedulerFactory.startElectionScheduler(gracePeriod, () -> {
+                                synchronized (mutex) {
+                                    if (mode == Mode.CANDIDATE) {
+                                        if (prevotingRound != null) {
+                                            prevotingRound.close();
+                                        }
+                                        prevotingRound = preVoteCollector.start(lastAcceptedState, getDiscoveredNodes());
+                                    }
+                                }
+                            });
+                        }
+                    } else {
+                        closePrevotingAndElectionScheduler();
+                    }
+                }
+            }
+        }
+    }
 }

server/src/main/java/org/elasticsearch/cluster/coordination/JoinHelper.java

@@ -18,6 +18,7 @@
  */
 package org.elasticsearch.cluster.coordination;
 
+import org.apache.logging.log4j.message.ParameterizedMessage;
 import org.elasticsearch.cluster.ClusterState;
 import org.elasticsearch.cluster.ClusterStateTaskConfig;
 import org.elasticsearch.cluster.ClusterStateTaskListener;
@@ -27,21 +28,29 @@ import org.elasticsearch.cluster.routing.allocation.AllocationService;
 import org.elasticsearch.cluster.service.MasterService;
 import org.elasticsearch.common.Priority;
 import org.elasticsearch.common.component.AbstractComponent;
+import org.elasticsearch.common.io.stream.StreamInput;
 import org.elasticsearch.common.settings.Settings;
 import org.elasticsearch.threadpool.ThreadPool;
+import org.elasticsearch.threadpool.ThreadPool.Names;
+import org.elasticsearch.transport.TransportException;
 import org.elasticsearch.transport.TransportResponse;
+import org.elasticsearch.transport.TransportResponse.Empty;
+import org.elasticsearch.transport.TransportResponseHandler;
 import org.elasticsearch.transport.TransportService;
 
 import java.io.IOException;
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
+import java.util.Optional;
 import java.util.function.BiConsumer;
+import java.util.function.Function;
 import java.util.function.LongSupplier;
 
 public class JoinHelper extends AbstractComponent {
 
     public static final String JOIN_ACTION_NAME = "internal:cluster/coordination/join";
+    public static final String START_JOIN_ACTION_NAME = "internal:cluster/coordination/start_join";
 
     private final MasterService masterService;
     private final TransportService transportService;
@@ -49,7 +58,7 @@ public class JoinHelper extends AbstractComponent {
 
     public JoinHelper(Settings settings, AllocationService allocationService, MasterService masterService,
                       TransportService transportService, LongSupplier currentTermSupplier,
-                      BiConsumer<JoinRequest, JoinCallback> joinHandler) {
+                      BiConsumer<JoinRequest, JoinCallback> joinHandler, Function<StartJoinRequest, Join> joinLeaderInTerm) {
         super(settings);
         this.masterService = masterService;
         this.transportService = transportService;
@@ -100,6 +109,62 @@ public class JoinHelper extends AbstractComponent {
                     return "JoinCallback{request=" + request + "}";
                 }
             }));
+
+        transportService.registerRequestHandler(START_JOIN_ACTION_NAME, Names.GENERIC, false, false,
+            StartJoinRequest::new,
+            (request, channel, task) -> {
+                final DiscoveryNode destination = request.getSourceNode();
+                final JoinRequest joinRequest
+                    = new JoinRequest(transportService.getLocalNode(), Optional.of(joinLeaderInTerm.apply(request)));
+                logger.debug("attempting to join {} with {}", destination, joinRequest);
+                this.transportService.sendRequest(destination, JOIN_ACTION_NAME, joinRequest, new TransportResponseHandler<Empty>() {
+                    @Override
+                    public Empty read(StreamInput in) {
+                        return Empty.INSTANCE;
+                    }
+
+                    @Override
+                    public void handleResponse(Empty response) {
+                        logger.debug("successfully joined {} with {}", destination, joinRequest);
+                    }
+
+                    @Override
+                    public void handleException(TransportException exp) {
+                        logger.debug(() -> new ParameterizedMessage("failed to join {} with {}", destination, joinRequest), exp);
+                    }
+
+                    @Override
+                    public String executor() {
+                        return Names.SAME;
+                    }
+                });
+                channel.sendResponse(Empty.INSTANCE);
+            });
+    }
+
+    public void sendStartJoinRequest(final StartJoinRequest startJoinRequest, final DiscoveryNode destination) {
+        transportService.sendRequest(destination, START_JOIN_ACTION_NAME,
+            startJoinRequest, new TransportResponseHandler<Empty>() {
+                @Override
+                public Empty read(StreamInput in) {
+                    return Empty.INSTANCE;
+                }
+
+                @Override
+                public void handleResponse(Empty response) {
+                    logger.debug("successful response to {} from {}", startJoinRequest, destination);
+                }
+
+                @Override
+                public void handleException(TransportException exp) {
+                    logger.debug(new ParameterizedMessage("failure in response to {} from {}", startJoinRequest, destination), exp);
+                }
+
+                @Override
+                public String executor() {
+                    return ThreadPool.Names.SAME;
+                }
+            });
     }
 
     public interface JoinCallback {
@@ -211,7 +276,8 @@ public class JoinHelper extends AbstractComponent {
 
         @Override
         public String toString() {
-            return "CandidateJoinAccumulator{" + joinRequestAccumulator.keySet() + '}';
+            return "CandidateJoinAccumulator{" + joinRequestAccumulator.keySet() +
+                ", closed=" + closed + '}';
         }
     }
 }

server/src/main/java/org/elasticsearch/cluster/coordination/PreVoteCollector.java

@@ -23,6 +23,7 @@ import org.apache.logging.log4j.message.ParameterizedMessage;
 import org.elasticsearch.cluster.ClusterState;
 import org.elasticsearch.cluster.coordination.CoordinationState.VoteCollection;
 import org.elasticsearch.cluster.node.DiscoveryNode;
+import org.elasticsearch.common.Nullable;
 import org.elasticsearch.common.collect.Tuple;
 import org.elasticsearch.common.component.AbstractComponent;
 import org.elasticsearch.common.lease.Releasable;
@@ -34,6 +35,7 @@ import org.elasticsearch.transport.TransportService;
 
 import java.util.Set;
 import java.util.concurrent.atomic.AtomicBoolean;
+import java.util.function.LongConsumer;
 
 import static org.elasticsearch.cluster.coordination.CoordinationState.isElectionQuorum;
 import static org.elasticsearch.common.util.concurrent.ConcurrentCollections.newConcurrentSet;
@@ -44,16 +46,17 @@ public class PreVoteCollector extends AbstractComponent {
 
     private final TransportService transportService;
     private final Runnable startElection;
+    private final LongConsumer updateMaxTermSeen;
 
-    // Tuple for simple atomic updates
+    // Tuple for simple atomic updates. null until the first call to `update()`.
-    private volatile Tuple<DiscoveryNode, PreVoteResponse> state; // DiscoveryNode component is null if there is currently no known leader
+    private volatile Tuple<DiscoveryNode, PreVoteResponse> state; // DiscoveryNode component is null if there is currently no known leader.
 
-    PreVoteCollector(final Settings settings, final PreVoteResponse preVoteResponse,
+    PreVoteCollector(final Settings settings, final TransportService transportService, final Runnable startElection,
-                     final TransportService transportService, final Runnable startElection) {
+                     final LongConsumer updateMaxTermSeen) {
         super(settings);
-        state = new Tuple<>(null, preVoteResponse);
         this.transportService = transportService;
         this.startElection = startElection;
+        this.updateMaxTermSeen = updateMaxTermSeen;
 
         // TODO does this need to be on the generic threadpool or can it use SAME?
         transportService.registerRequestHandler(REQUEST_PRE_VOTE_ACTION_NAME, Names.GENERIC, false, false,
@@ -74,7 +77,7 @@ public class PreVoteCollector extends AbstractComponent {
         return preVotingRound;
     }
 
-    public void update(final PreVoteResponse preVoteResponse, final DiscoveryNode leader) {
+    public void update(final PreVoteResponse preVoteResponse, @Nullable final DiscoveryNode leader) {
         logger.trace("updating with preVoteResponse={}, leader={}", preVoteResponse, leader);
         state = new Tuple<>(leader, preVoteResponse);
     }
@@ -156,7 +159,7 @@ public class PreVoteCollector extends AbstractComponent {
                 return;
             }
 
-            // TODO the response carries the sender's current term. If an election starts then it should be in a higher term.
+            updateMaxTermSeen.accept(response.getCurrentTerm());
 
             if (response.getLastAcceptedTerm() > clusterState.term()
                 || (response.getLastAcceptedTerm() == clusterState.term()

server/src/main/java/org/elasticsearch/cluster/coordination/Publication.java

@@ -156,7 +156,7 @@ public abstract class Publication extends AbstractComponent {
 
     protected abstract Optional<ApplyCommitRequest> handlePublishResponse(DiscoveryNode sourceNode, PublishResponse publishResponse);
 
-    protected abstract void onPossibleJoin(DiscoveryNode sourceNode, PublishWithJoinResponse response);
+    protected abstract void onJoin(Join join);
 
     protected abstract void sendPublishRequest(DiscoveryNode destination, PublishRequest publishRequest,
                                                ActionListener<PublishWithJoinResponse> responseActionListener);
@@ -287,8 +287,10 @@ public abstract class Publication extends AbstractComponent {
                     return;
                 }
 
-                // TODO: check if we need to pass the full response here or if it's sufficient to just pass the optional join.
+                response.getJoin().ifPresent(join -> {
-                onPossibleJoin(discoveryNode, response);
+                    assert discoveryNode.equals(join.getSourceNode());
+                    onJoin(join);
+                });
 
                 assert state == PublicationTargetState.SENT_PUBLISH_REQUEST : state + " -> " + PublicationTargetState.WAITING_FOR_QUORUM;
                 state = PublicationTargetState.WAITING_FOR_QUORUM;

server/src/main/java/org/elasticsearch/discovery/PeerFinder.java

@@ -72,8 +72,8 @@ public abstract class PeerFinder extends AbstractComponent {
     private final Map<TransportAddress, Peer> peersByAddress = newConcurrentMap();
     private Optional<DiscoveryNode> leader = Optional.empty();
 
-    PeerFinder(Settings settings, TransportService transportService,
+    public PeerFinder(Settings settings, TransportService transportService, TransportAddressConnector transportAddressConnector,
-               TransportAddressConnector transportAddressConnector, ConfiguredHostsResolver configuredHostsResolver) {
+                      ConfiguredHostsResolver configuredHostsResolver) {
         super(settings);
         findPeersDelay = DISCOVERY_FIND_PEERS_INTERVAL_SETTING.get(settings);
         this.transportService = transportService;
@@ -95,6 +95,8 @@ public abstract class PeerFinder extends AbstractComponent {
             leader = Optional.empty();
             handleWakeUp(); // return value discarded: there are no known peers, so none can be disconnected
         }
+
+        onFoundPeersUpdated(); // trigger a check for a quorum already
     }
 
     public void deactivate(DiscoveryNode leader) {
@@ -116,7 +118,7 @@ public abstract class PeerFinder extends AbstractComponent {
         return Thread.holdsLock(mutex);
     }
 
-    boolean assertInactiveWithNoKnownPeers() {
+    private boolean assertInactiveWithNoKnownPeers() {
         assert active == false;
         assert peersByAddress.isEmpty() : peersByAddress.keySet();
         return true;
@@ -125,13 +127,24 @@ public abstract class PeerFinder extends AbstractComponent {
     PeersResponse handlePeersRequest(PeersRequest peersRequest) {
         synchronized (mutex) {
             assert peersRequest.getSourceNode().equals(getLocalNode()) == false;
+            final List<DiscoveryNode> knownPeers;
             if (active) {
+                assert leader.isPresent() == false : leader;
                 startProbe(peersRequest.getSourceNode().getAddress());
                 peersRequest.getKnownPeers().stream().map(DiscoveryNode::getAddress).forEach(this::startProbe);
-                return new PeersResponse(Optional.empty(), getFoundPeersUnderLock(), currentTerm);
+                knownPeers = getFoundPeersUnderLock();
             } else {
-                return new PeersResponse(leader, Collections.emptyList(), currentTerm);
+                assert leader.isPresent();
+                knownPeers = Collections.emptyList();
             }
+            return new PeersResponse(leader, knownPeers, currentTerm);
+        }
+    }
+
+    // exposed for checking invariant in o.e.c.c.Coordinator (public since this is a different package)
+    public Optional<DiscoveryNode> getLeader() {
+        synchronized (mutex) {
+            return leader;
         }
     }
 
@@ -247,7 +260,7 @@ public abstract class PeerFinder extends AbstractComponent {
 
             @Override
             public String toString() {
-                return "PeerFinder::handleWakeUp";
+                return "PeerFinder handling wakeup";
             }
         });
 

server/src/main/java/org/elasticsearch/transport/TransportService.java

@@ -733,6 +733,11 @@ public class TransportService extends AbstractLifecycleComponent implements Tran
                                     "failed to notify channel of error message for action [{}]", action), inner);
                         }
                     }
+
+                    @Override
+                    public String toString() {
+                        return "processing of [" + action + "][" + requestId + "]: " + request;
+                    }
                 });
             }
 
@@ -1049,6 +1054,11 @@ public class TransportService extends AbstractLifecycleComponent implements Tran
                 "cancel must be called after the requestId [" + requestId + "] has been removed from clientHandlers";
             FutureUtils.cancel(future);
         }
+
+        @Override
+        public String toString() {
+            return "TimeoutHandler for [" + action + "][" + requestId + "]";
+        }
     }
 
     static class TimeoutInfoHolder {
@@ -1176,7 +1186,17 @@ public class TransportService extends AbstractLifecycleComponent implements Tran
                 if (ThreadPool.Names.SAME.equals(executor)) {
                     processResponse(handler, response);
                 } else {
-                    threadPool.executor(executor).execute(() -> processResponse(handler, response));
+                    threadPool.executor(executor).execute(new Runnable() {
+                        @Override
+                        public String toString() {
+                            return "delivery of response to [" + action + "][" + requestId + "]: " + response;
+                        }
+
+                        @Override
+                        public void run() {
+                            DirectResponseChannel.this.processResponse(handler, response);
+                        }
+                    });
                 }
             }
         }

server/src/test/java/org/elasticsearch/action/admin/indices/create/ShrinkIndexIT.java

@@ -162,7 +162,8 @@ public class ShrinkIndexIT extends ESIntegTestCase {
         final List<Integer> factors = Arrays.asList(2, 3, 5, 7);
         final List<Integer> numberOfShardsFactors = randomSubsetOf(scaledRandomIntBetween(1, factors.size() - 1), factors);
         final int numberOfShards = numberOfShardsFactors.stream().reduce(1, (x, y) -> x * y);
-        final int numberOfTargetShards = randomSubsetOf(numberOfShardsFactors).stream().reduce(1, (x, y) -> x * y);
+        final int numberOfTargetShards = randomSubsetOf(randomInt(numberOfShardsFactors.size() - 1), numberOfShardsFactors)
+            .stream().reduce(1, (x, y) -> x * y);
         internalCluster().ensureAtLeastNumDataNodes(2);
         prepareCreate("source").setSettings(Settings.builder().put(indexSettings()).put("number_of_shards", numberOfShards)).get();
 

server/src/test/java/org/elasticsearch/cluster/coordination/CoordinatorTests.java

@@ -0,0 +1,466 @@
+/*
+ * Licensed to Elasticsearch under one or more contributor
+ * license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright
+ * ownership. Elasticsearch licenses this file to you under
+ * the Apache License, Version 2.0 (the "License"); you may
+ * not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.elasticsearch.cluster.coordination;
+
+import org.apache.logging.log4j.message.ParameterizedMessage;
+import org.elasticsearch.Version;
+import org.elasticsearch.action.ActionListener;
+import org.elasticsearch.cluster.ClusterState;
+import org.elasticsearch.cluster.ClusterState.VotingConfiguration;
+import org.elasticsearch.cluster.ClusterStateUpdateTask;
+import org.elasticsearch.cluster.ESAllocationTestCase;
+import org.elasticsearch.cluster.coordination.CoordinationState.PersistedState;
+import org.elasticsearch.cluster.coordination.CoordinationState.VoteCollection;
+import org.elasticsearch.cluster.coordination.CoordinationStateTests.InMemoryPersistedState;
+import org.elasticsearch.cluster.coordination.CoordinatorTests.Cluster.ClusterNode;
+import org.elasticsearch.cluster.node.DiscoveryNode;
+import org.elasticsearch.cluster.node.DiscoveryNode.Role;
+import org.elasticsearch.cluster.service.MasterService;
+import org.elasticsearch.common.UUIDs;
+import org.elasticsearch.common.component.AbstractComponent;
+import org.elasticsearch.common.settings.Settings;
+import org.elasticsearch.common.transport.TransportAddress;
+import org.elasticsearch.discovery.zen.UnicastHostsProvider.HostsResolver;
+import org.elasticsearch.indices.cluster.FakeThreadPoolMasterService;
+import org.elasticsearch.test.ESTestCase;
+import org.elasticsearch.test.junit.annotations.TestLogging;
+import org.elasticsearch.test.transport.CapturingTransport;
+import org.elasticsearch.threadpool.ThreadPool.Names;
+import org.elasticsearch.transport.RequestHandlerRegistry;
+import org.elasticsearch.transport.TransportChannel;
+import org.elasticsearch.transport.TransportException;
+import org.elasticsearch.transport.TransportRequest;
+import org.elasticsearch.transport.TransportResponse;
+import org.elasticsearch.transport.TransportResponse.Empty;
+import org.elasticsearch.transport.TransportResponseHandler;
+import org.elasticsearch.transport.TransportResponseOptions;
+import org.elasticsearch.transport.TransportService;
+import org.hamcrest.Matcher;
+
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.EnumSet;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Optional;
+import java.util.Set;
+import java.util.concurrent.atomic.AtomicReference;
+import java.util.function.Consumer;
+import java.util.function.Predicate;
+import java.util.stream.Collectors;
+
+import static java.util.Collections.emptySet;
+import static org.elasticsearch.cluster.coordination.CoordinationStateTests.clusterState;
+import static org.elasticsearch.cluster.coordination.CoordinationStateTests.setValue;
+import static org.elasticsearch.cluster.coordination.CoordinationStateTests.value;
+import static org.elasticsearch.cluster.coordination.Coordinator.COMMIT_STATE_ACTION_NAME;
+import static org.elasticsearch.cluster.coordination.Coordinator.Mode.FOLLOWER;
+import static org.elasticsearch.cluster.coordination.Coordinator.PUBLISH_STATE_ACTION_NAME;
+import static org.elasticsearch.node.Node.NODE_NAME_SETTING;
+import static org.elasticsearch.transport.TransportService.HANDSHAKE_ACTION_NAME;
+import static org.elasticsearch.transport.TransportService.NOOP_TRANSPORT_INTERCEPTOR;
+import static org.hamcrest.Matchers.empty;
+import static org.hamcrest.Matchers.equalTo;
+import static org.hamcrest.Matchers.is;
+import static org.hamcrest.Matchers.not;
+
+@TestLogging("org.elasticsearch.cluster.coordination:TRACE,org.elasticsearch.cluster.discovery:TRACE")
+public class CoordinatorTests extends ESTestCase {
+
+    public void testCanUpdateClusterStateAfterStabilisation() {
+        final Cluster cluster = new Cluster(randomIntBetween(1, 5));
+        cluster.stabilise();
+
+        final ClusterNode leader = cluster.getAnyLeader();
+        long finalValue = randomLong();
+        leader.submitValue(finalValue);
+        cluster.stabilise(); // TODO this should only need a short stabilisation
+
+        for (final ClusterNode clusterNode : cluster.clusterNodes) {
+            final String nodeId = clusterNode.getId();
+            final ClusterState committedState = clusterNode.coordinator.getLastCommittedState().get();
+            assertThat(nodeId + " has the committed value", value(committedState), is(finalValue));
+        }
+    }
+
+    private static String nodeIdFromIndex(int nodeIndex) {
+        return "node" + nodeIndex;
+    }
+
+    class Cluster {
+
+        static final long DEFAULT_STABILISATION_TIME = 3000L; // TODO use a real stabilisation time - needs fault detection and disruption
+
+        final List<ClusterNode> clusterNodes;
+        final DeterministicTaskQueue deterministicTaskQueue = new DeterministicTaskQueue(
+            Settings.builder().put(NODE_NAME_SETTING.getKey(), "deterministic-task-queue").build());
+        private final VotingConfiguration initialConfiguration;
+
+        Cluster(int initialNodeCount) {
+            logger.info("--> creating cluster of {} nodes", initialNodeCount);
+
+            Set<String> initialNodeIds = new HashSet<>(initialNodeCount);
+            for (int i = 0; i < initialNodeCount; i++) {
+                initialNodeIds.add(nodeIdFromIndex(i));
+            }
+            initialConfiguration = new VotingConfiguration(initialNodeIds);
+
+            clusterNodes = new ArrayList<>(initialNodeCount);
+            for (int i = 0; i < initialNodeCount; i++) {
+                final ClusterNode clusterNode = new ClusterNode(i);
+                clusterNodes.add(clusterNode);
+            }
+        }
+
+        void stabilise() {
+            final long stabilisationStartTime = deterministicTaskQueue.getCurrentTimeMillis();
+            while (deterministicTaskQueue.getCurrentTimeMillis() < stabilisationStartTime + DEFAULT_STABILISATION_TIME) {
+
+                while (deterministicTaskQueue.hasRunnableTasks()) {
+                    try {
+                        deterministicTaskQueue.runRandomTask(random());
+                    } catch (CoordinationStateRejectedException e) {
+                        logger.debug("ignoring benign exception thrown when stabilising", e);
+                    }
+                    for (final ClusterNode clusterNode : clusterNodes) {
+                        clusterNode.coordinator.invariant();
+                    }
+                }
+
+                if (deterministicTaskQueue.hasDeferredTasks() == false) {
+                    break; // TODO when fault detection is enabled this should be removed, as there should _always_ be deferred tasks
+                }
+
+                deterministicTaskQueue.advanceTime();
+            }
+
+            assertUniqueLeaderAndExpectedModes();
+        }
+
+        private void assertUniqueLeaderAndExpectedModes() {
+            final ClusterNode leader = getAnyLeader();
+            final long leaderTerm = leader.coordinator.getCurrentTerm();
+            Matcher<Optional<Long>> isPresentAndEqualToLeaderVersion
+                = equalTo(Optional.of(leader.coordinator.getLastAcceptedState().getVersion()));
+
+            assertThat(leader.coordinator.getLastCommittedState().map(ClusterState::getVersion), isPresentAndEqualToLeaderVersion);
+
+            for (final ClusterNode clusterNode : clusterNodes) {
+                if (clusterNode == leader) {
+                    continue;
+                }
+
+                final String nodeId = clusterNode.getId();
+                assertThat(nodeId + " has the same term as the leader", clusterNode.coordinator.getCurrentTerm(), is(leaderTerm));
+                assertTrue("leader should have received a vote from " + nodeId,
+                    leader.coordinator.hasJoinVoteFrom(clusterNode.getLocalNode()));
+
+                assertThat(nodeId + " is a follower", clusterNode.coordinator.getMode(), is(FOLLOWER));
+                assertThat(nodeId + " is at the same accepted version as the leader",
+                    Optional.of(clusterNode.coordinator.getLastAcceptedState().getVersion()), isPresentAndEqualToLeaderVersion);
+                assertThat(nodeId + " is at the same committed version as the leader",
+                    clusterNode.coordinator.getLastCommittedState().map(ClusterState::getVersion), isPresentAndEqualToLeaderVersion);
+            }
+        }
+
+        ClusterNode getAnyLeader() {
+            List<ClusterNode> allLeaders = clusterNodes.stream().filter(ClusterNode::isLeader).collect(Collectors.toList());
+            assertThat(allLeaders, not(empty()));
+            return randomFrom(allLeaders);
+        }
+
+        class ClusterNode extends AbstractComponent {
+            private final int nodeIndex;
+            private Coordinator coordinator;
+            private DiscoveryNode localNode;
+            private final PersistedState persistedState;
+            private MasterService masterService;
+            private TransportService transportService;
+            private CapturingTransport capturingTransport;
+
+            ClusterNode(int nodeIndex) {
+                super(Settings.builder().put(NODE_NAME_SETTING.getKey(), nodeIdFromIndex(nodeIndex)).build());
+                this.nodeIndex = nodeIndex;
+                localNode = createDiscoveryNode();
+                persistedState = new InMemoryPersistedState(1L,
+                    clusterState(1L, 1L, localNode, initialConfiguration, initialConfiguration, 0L));
+                setUp();
+            }
+
+            private DiscoveryNode createDiscoveryNode() {
+                final TransportAddress transportAddress = buildNewFakeTransportAddress();
+                // Generate the ephemeral ID deterministically, for repeatable tests. This means we have to pass everything else into the
+                // constructor explicitly too.
+                return new DiscoveryNode("", nodeIdFromIndex(nodeIndex), UUIDs.randomBase64UUID(random()),
+                    transportAddress.address().getHostString(),
+                    transportAddress.getAddress(), transportAddress, Collections.emptyMap(),
+                    EnumSet.allOf(Role.class), Version.CURRENT);
+            }
+
+            private void setUp() {
+                capturingTransport = new CapturingTransport() {
+                    @Override
+                    protected void onSendRequest(long requestId, String action, TransportRequest request, DiscoveryNode destination) {
+                        assert destination.equals(localNode) == false : "non-local message from " + localNode + " to itself";
+                        super.onSendRequest(requestId, action, request, destination);
+
+                        // connecting and handshaking with a new node happens synchronously, so we cannot enqueue these tasks for later
+                        final Consumer<Runnable> scheduler;
+                        final Predicate<ClusterNode> matchesDestination;
+                        if (action.equals(HANDSHAKE_ACTION_NAME)) {
+                            scheduler = Runnable::run;
+                            matchesDestination = n -> n.getLocalNode().getAddress().equals(destination.getAddress());
+                        } else {
+                            scheduler = deterministicTaskQueue::scheduleNow;
+                            matchesDestination = n -> n.getLocalNode().equals(destination);
+                        }
+
+                        scheduler.accept(new Runnable() {
+                            @Override
+                            public String toString() {
+                                return "delivery of [" + action + "][" + requestId + "]: " + request;
+                            }
+
+                            @Override
+                            public void run() {
+                                clusterNodes.stream().filter(matchesDestination).findAny().ifPresent(
+                                    destinationNode -> {
+
+                                        final RequestHandlerRegistry requestHandler
+                                            = destinationNode.capturingTransport.getRequestHandler(action);
+
+                                        final TransportChannel transportChannel = new TransportChannel() {
+                                            @Override
+                                            public String getProfileName() {
+                                                return "default";
+                                            }
+
+                                            @Override
+                                            public String getChannelType() {
+                                                return "coordinator-test-channel";
+                                            }
+
+                                            @Override
+                                            public void sendResponse(final TransportResponse response) {
+                                                scheduler.accept(new Runnable() {
+                                                    @Override
+                                                    public String toString() {
+                                                        return "delivery of response " + response
+                                                            + " to [" + action + "][" + requestId + "]: " + request;
+                                                    }
+
+                                                    @Override
+                                                    public void run() {
+                                                        handleResponse(requestId, response);
+                                                    }
+                                                });
+                                            }
+
+                                            @Override
+                                            public void sendResponse(TransportResponse response, TransportResponseOptions options) {
+                                                sendResponse(response);
+                                            }
+
+                                            @Override
+                                            public void sendResponse(Exception exception) {
+                                                scheduler.accept(new Runnable() {
+                                                    @Override
+                                                    public String toString() {
+                                                        return "delivery of error response " + exception.getMessage()
+                                                            + " to [" + action + "][" + requestId + "]: " + request;
+                                                    }
+
+                                                    @Override
+                                                    public void run() {
+                                                        handleRemoteError(requestId, exception);
+                                                    }
+                                                });
+                                            }
+                                        };
+
+                                        try {
+                                            processMessageReceived(request, requestHandler, transportChannel);
+                                        } catch (Exception e) {
+                                            scheduler.accept(new Runnable() {
+                                                @Override
+                                                public String toString() {
+                                                    return "delivery of processing error response " + e.getMessage()
+                                                        + " to [" + action + "][" + requestId + "]: " + request;
+                                                }
+
+                                                @Override
+                                                public void run() {
+                                                    handleRemoteError(requestId, e);
+                                                }
+                                            });
+                                        }
+                                    }
+                                );
+                            }
+                        });
+                    }
+                };
+
+                masterService = new FakeThreadPoolMasterService("test", deterministicTaskQueue::scheduleNow);
+                AtomicReference<ClusterState> currentState = new AtomicReference<>(getPersistedState().getLastAcceptedState());
+                masterService.setClusterStateSupplier(currentState::get);
+                masterService.setClusterStatePublisher((event, publishListener, ackListener) -> {
+                    final PublishRequest publishRequest;
+                    try {
+                        publishRequest = coordinator.coordinationState.get().handleClientValue(event.state());
+                    } catch (CoordinationStateRejectedException e) {
+                        publishListener.onFailure(new FailedToCommitClusterStateException("rejected client value", e));
+                        return;
+                    }
+
+                    final Publication publication = new Publication(settings, publishRequest, ackListener,
+                        deterministicTaskQueue::getCurrentTimeMillis) {
+
+                        @Override
+                        protected void onCompletion(boolean committed) {
+                            if (committed) {
+                                currentState.set(event.state());
+                                publishListener.onResponse(null);
+                            } else {
+                                publishListener.onFailure(new FailedToCommitClusterStateException("not committed"));
+                            }
+                        }
+
+                        @Override
+                        protected boolean isPublishQuorum(VoteCollection votes) {
+                            return coordinator.coordinationState.get().isPublishQuorum(votes);
+                        }
+
+                        @Override
+                        protected Optional<ApplyCommitRequest> handlePublishResponse(DiscoveryNode sourceNode,
+                                                                                     PublishResponse publishResponse) {
+                            return coordinator.coordinationState.get().handlePublishResponse(sourceNode, publishResponse);
+                        }
+
+                        @Override
+                        protected void onJoin(Join join) {
+                            coordinator.handleJoin(join);
+                        }
+
+                        @Override
+                        protected void sendPublishRequest(DiscoveryNode destination, PublishRequest publishRequest,
+                                                          ActionListener<PublishWithJoinResponse> responseActionListener) {
+                            transportService.sendRequest(destination, PUBLISH_STATE_ACTION_NAME, publishRequest,
+
+                                new TransportResponseHandler<PublishWithJoinResponse>() {
+
+                                    @Override
+                                    public void handleResponse(PublishWithJoinResponse response) {
+                                        responseActionListener.onResponse(response);
+                                    }
+
+                                    @Override
+                                    public void handleException(TransportException exp) {
+                                        responseActionListener.onFailure(exp);
+                                    }
+
+                                    @Override
+                                    public String executor() {
+                                        return Names.GENERIC;
+                                    }
+                                });
+                        }
+
+                        @Override
+                        protected void sendApplyCommit(DiscoveryNode destination, ApplyCommitRequest applyCommit,
+                                                       ActionListener<Empty> responseActionListener) {
+                            transportService.sendRequest(destination, COMMIT_STATE_ACTION_NAME, applyCommit,
+
+                                new TransportResponseHandler<Empty>() {
+
+                                    @Override
+                                    public void handleResponse(Empty response) {
+                                        responseActionListener.onResponse(response);
+                                    }
+
+                                    @Override
+                                    public void handleException(TransportException exp) {
+                                        responseActionListener.onFailure(exp);
+                                    }
+
+                                    @Override
+                                    public String executor() {
+                                        return Names.GENERIC;
+                                    }
+                                });
+                        }
+                    };
+                    publication.start(emptySet());
+                });
+                masterService.start();
+
+                transportService = capturingTransport.createCapturingTransportService(
+                    settings, deterministicTaskQueue.getThreadPool(), NOOP_TRANSPORT_INTERCEPTOR, a -> localNode, null, emptySet());
+                transportService.start();
+                transportService.acceptIncomingRequests();
+
+                coordinator = new Coordinator(settings, transportService, ESAllocationTestCase.createAllocationService(Settings.EMPTY),
+                    masterService, this::getPersistedState, Cluster.this::provideUnicastHosts);
+
+                coordinator.start();
+                coordinator.startInitialJoin();
+            }
+
+            private PersistedState getPersistedState() {
+                return persistedState;
+            }
+
+            String getId() {
+                return localNode.getId();
+            }
+
+            public DiscoveryNode getLocalNode() {
+                return localNode;
+            }
+
+            boolean isLeader() {
+                return coordinator.getMode() == Coordinator.Mode.LEADER;
+            }
+
+            void submitValue(final long value) {
+                masterService.submitStateUpdateTask("new value [" + value + "]", new ClusterStateUpdateTask() {
+                    @Override
+                    public ClusterState execute(ClusterState currentState) {
+                        return setValue(currentState, value);
+                    }
+
+                    @Override
+                    public void onFailure(String source, Exception e) {
+                        logger.debug(() -> new ParameterizedMessage("failed to publish: [{}]", source), e);
+                    }
+                });
+            }
+        }
+
+        private List<TransportAddress> provideUnicastHosts(HostsResolver ignored) {
+            return clusterNodes.stream().map(ClusterNode::getLocalNode).map(DiscoveryNode::getAddress).collect(Collectors.toList());
+        }
+    }
+
+    @SuppressWarnings("unchecked")
+    private static void processMessageReceived(TransportRequest request, RequestHandlerRegistry requestHandler,
+                                               TransportChannel transportChannel) throws Exception {
+        requestHandler.processMessageReceived(request, transportChannel);
+    }
+}

server/src/test/java/org/elasticsearch/cluster/coordination/DeterministicTaskQueue.java

@@ -326,7 +326,7 @@ public class DeterministicTaskQueue extends AbstractComponent {
 
                     @Override
                     public boolean cancel(boolean mayInterruptIfRunning) {
-                        throw new UnsupportedOperationException();
+                        return false;
                     }
 
                     @Override

server/src/test/java/org/elasticsearch/cluster/coordination/ElectionSchedulerFactoryTests.java

@@ -144,37 +144,39 @@ public class ElectionSchedulerFactoryTests extends ESTestCase {
         {
             final Settings settings = Settings.builder().put(ELECTION_INITIAL_TIMEOUT_SETTING.getKey(), "0s").build();
             IllegalArgumentException e = expectThrows(IllegalArgumentException.class, () -> ELECTION_INITIAL_TIMEOUT_SETTING.get(settings));
-            assertThat(e.getMessage(), is("Failed to parse value [0s] for setting [cluster.election.initial_timeout] must be >= 1ms"));
+            assertThat(e.getMessage(), is("failed to parse value [0s] for setting [cluster.election.initial_timeout], must be >= [1ms]"));
         }
 
         {
             final Settings settings = Settings.builder().put(ELECTION_INITIAL_TIMEOUT_SETTING.getKey(), "10001ms").build();
             IllegalArgumentException e = expectThrows(IllegalArgumentException.class, () -> ELECTION_INITIAL_TIMEOUT_SETTING.get(settings));
-            assertThat(e.getMessage(), is("Failed to parse value [10001ms] for setting [cluster.election.initial_timeout] must be <= 10s"));
+            assertThat(e.getMessage(),
+                is("failed to parse value [10001ms] for setting [cluster.election.initial_timeout], must be <= [10s]"));
         }
 
         {
             final Settings settings = Settings.builder().put(ELECTION_BACK_OFF_TIME_SETTING.getKey(), "0s").build();
             IllegalArgumentException e = expectThrows(IllegalArgumentException.class, () -> ELECTION_BACK_OFF_TIME_SETTING.get(settings));
-            assertThat(e.getMessage(), is("Failed to parse value [0s] for setting [cluster.election.back_off_time] must be >= 1ms"));
+            assertThat(e.getMessage(), is("failed to parse value [0s] for setting [cluster.election.back_off_time], must be >= [1ms]"));
         }
 
         {
             final Settings settings = Settings.builder().put(ELECTION_BACK_OFF_TIME_SETTING.getKey(), "60001ms").build();
             IllegalArgumentException e = expectThrows(IllegalArgumentException.class, () -> ELECTION_BACK_OFF_TIME_SETTING.get(settings));
-            assertThat(e.getMessage(), is("Failed to parse value [60001ms] for setting [cluster.election.back_off_time] must be <= 1m"));
+            assertThat(e.getMessage(),
+                is("failed to parse value [60001ms] for setting [cluster.election.back_off_time], must be <= [60s]"));
         }
 
         {
             final Settings settings = Settings.builder().put(ELECTION_MAX_TIMEOUT_SETTING.getKey(), "199ms").build();
             IllegalArgumentException e = expectThrows(IllegalArgumentException.class, () -> ELECTION_MAX_TIMEOUT_SETTING.get(settings));
-            assertThat(e.getMessage(), is("Failed to parse value [199ms] for setting [cluster.election.max_timeout] must be >= 200ms"));
+            assertThat(e.getMessage(), is("failed to parse value [199ms] for setting [cluster.election.max_timeout], must be >= [200ms]"));
         }
 
         {
             final Settings settings = Settings.builder().put(ELECTION_MAX_TIMEOUT_SETTING.getKey(), "301s").build();
             IllegalArgumentException e = expectThrows(IllegalArgumentException.class, () -> ELECTION_MAX_TIMEOUT_SETTING.get(settings));
-            assertThat(e.getMessage(), is("Failed to parse value [301s] for setting [cluster.election.max_timeout] must be <= 5m"));
+            assertThat(e.getMessage(), is("failed to parse value [301s] for setting [cluster.election.max_timeout], must be <= [300s]"));
         }
 
         {

server/src/test/java/org/elasticsearch/cluster/coordination/NodeJoinTests.java

@@ -61,6 +61,7 @@ import java.util.concurrent.atomic.AtomicReference;
 import java.util.stream.Collectors;
 import java.util.stream.IntStream;
 
+import static java.util.Collections.emptyList;
 import static java.util.Collections.emptyMap;
 import static org.hamcrest.Matchers.containsString;
 import static org.mockito.Matchers.anyObject;
@@ -135,6 +136,7 @@ public class NodeJoinTests extends ESTestCase {
         this.masterService = masterService;
         TransportService transportService = mock(TransportService.class);
         when(transportService.getLocalNode()).thenReturn(initialState.nodes().getLocalNode());
+        when(transportService.getThreadPool()).thenReturn(threadPool);
         @SuppressWarnings("unchecked")
         ArgumentCaptor<TransportRequestHandler<JoinRequest>> joinRequestHandler = ArgumentCaptor.forClass(
             (Class) TransportRequestHandler.class);
@@ -142,7 +144,7 @@ public class NodeJoinTests extends ESTestCase {
             transportService,
             ESAllocationTestCase.createAllocationService(Settings.EMPTY),
             masterService,
-            () -> new CoordinationStateTests.InMemoryPersistedState(term, initialState));
+            () -> new CoordinationStateTests.InMemoryPersistedState(term, initialState), r -> emptyList());
         verify(transportService).registerRequestHandler(eq(JoinHelper.JOIN_ACTION_NAME), eq(ThreadPool.Names.GENERIC), eq(false), eq(false),
             anyObject(), joinRequestHandler.capture());
         transportRequestHandler = joinRequestHandler.getValue();

server/src/test/java/org/elasticsearch/cluster/coordination/PreVoteCollectorTests.java

@@ -109,10 +109,12 @@ public class PreVoteCollectorTests extends ESTestCase {
         transportService.start();
         transportService.acceptIncomingRequests();
 
-        preVoteCollector = new PreVoteCollector(settings, getLocalPreVoteResponse(), transportService, () -> {
+        preVoteCollector = new PreVoteCollector(settings, transportService, () -> {
             assert electionOccurred == false;
             electionOccurred = true;
-        });
+        }, l -> {
+        }); // TODO need tests that check that the max term seen is updated
+        preVoteCollector.update(getLocalPreVoteResponse(), null);
     }
 
     private PreVoteResponse getLocalPreVoteResponse() {

server/src/test/java/org/elasticsearch/cluster/coordination/PublicationTests.java

@@ -100,7 +100,7 @@ public class PublicationTests extends ESTestCase {
 
         Map<DiscoveryNode, ActionListener<PublishWithJoinResponse>> pendingPublications = new HashMap<>();
         Map<DiscoveryNode, ActionListener<TransportResponse.Empty>> pendingCommits = new HashMap<>();
-        Map<DiscoveryNode, PublishWithJoinResponse> possibleJoins = new HashMap<>();
+        Map<DiscoveryNode, Join> joins = new HashMap<>();
 
         MockPublication(Settings settings, PublishRequest publishRequest, Discovery.AckListener ackListener,
                                LongSupplier currentTimeSupplier) {
@@ -116,8 +116,8 @@ public class PublicationTests extends ESTestCase {
         }
 
         @Override
-        protected void onPossibleJoin(DiscoveryNode sourceNode, PublishWithJoinResponse response) {
+        protected void onJoin(Join join) {
-            assertNull(possibleJoins.put(sourceNode, response));
+            assertNull(joins.put(join.getSourceNode(), join));
         }
 
         @Override
@@ -180,13 +180,17 @@ public class PublicationTests extends ESTestCase {
             PublishResponse publishResponse = nodeResolver.apply(e.getKey()).coordinationState.handlePublishRequest(
                 publication.publishRequest);
             assertNotEquals(processedNode1PublishResponse.get(), publication.pendingCommits.isEmpty());
-            assertFalse(publication.possibleJoins.containsKey(e.getKey()));
+            assertFalse(publication.joins.containsKey(e.getKey()));
             PublishWithJoinResponse publishWithJoinResponse = new PublishWithJoinResponse(publishResponse,
                 randomBoolean() ? Optional.empty() : Optional.of(new Join(e.getKey(), randomFrom(n1, n2, n3), randomNonNegativeLong(),
                     randomNonNegativeLong(), randomNonNegativeLong())));
             e.getValue().onResponse(publishWithJoinResponse);
-            assertTrue(publication.possibleJoins.containsKey(e.getKey()));
+            if (publishWithJoinResponse.getJoin().isPresent()) {
-            assertEquals(publishWithJoinResponse, publication.possibleJoins.get(e.getKey()));
+                assertTrue(publication.joins.containsKey(e.getKey()));
+                assertEquals(publishWithJoinResponse.getJoin().get(), publication.joins.get(e.getKey()));
+            } else {
+                assertFalse(publication.joins.containsKey(e.getKey()));
+            }
             if (e.getKey().equals(n1)) {
                 processedNode1PublishResponse.set(true);
             }

server/src/test/java/org/elasticsearch/common/settings/SettingTests.java

@@ -799,7 +799,7 @@ public class SettingTests extends ESTestCase {
             = expectThrows(IllegalArgumentException.class,
             () -> settingWithLowerBound.get(Settings.builder().put("foo", "4999ms").build()));
 
-        assertThat(illegalArgumentException.getMessage(), equalTo("Failed to parse value [4999ms] for setting [foo] must be >= 5s"));
+        assertThat(illegalArgumentException.getMessage(), equalTo("failed to parse value [4999ms] for setting [foo], must be >= [5s]"));
 
         Setting<TimeValue> settingWithBothBounds = Setting.timeSetting("bar",
             TimeValue.timeValueSeconds(10), TimeValue.timeValueSeconds(5), TimeValue.timeValueSeconds(20));
@@ -810,12 +810,12 @@ public class SettingTests extends ESTestCase {
         illegalArgumentException
             = expectThrows(IllegalArgumentException.class,
             () -> settingWithBothBounds.get(Settings.builder().put("bar", "4999ms").build()));
-        assertThat(illegalArgumentException.getMessage(), equalTo("Failed to parse value [4999ms] for setting [bar] must be >= 5s"));
+        assertThat(illegalArgumentException.getMessage(), equalTo("failed to parse value [4999ms] for setting [bar], must be >= [5s]"));
 
         illegalArgumentException
             = expectThrows(IllegalArgumentException.class,
             () -> settingWithBothBounds.get(Settings.builder().put("bar", "20001ms").build()));
-        assertThat(illegalArgumentException.getMessage(), equalTo("Failed to parse value [20001ms] for setting [bar] must be <= 20s"));
+        assertThat(illegalArgumentException.getMessage(), equalTo("failed to parse value [20001ms] for setting [bar], must be <= [20s]"));
     }
 
     public void testSettingsGroupUpdater() {

server/src/test/java/org/elasticsearch/indices/cluster/FakeThreadPoolMasterService.java

@@ -84,20 +84,28 @@ public class FakeThreadPoolMasterService extends MasterService {
     private void scheduleNextTaskIfNecessary() {
         if (taskInProgress == false && pendingTasks.isEmpty() == false && scheduledNextTask == false) {
             scheduledNextTask = true;
-            onTaskAvailableToRun.accept(() -> {
+            onTaskAvailableToRun.accept(new Runnable() {
-                assert taskInProgress == false;
+                @Override
-                assert waitForPublish == false;
+                public String toString() {
-                assert scheduledNextTask;
+                    return "master service scheduling next task";
-                final int taskIndex = randomInt(pendingTasks.size() - 1);
+                }
-                logger.debug("next master service task: choosing task {} of {}", taskIndex, pendingTasks.size());
+
-                final Runnable task = pendingTasks.remove(taskIndex);
+                @Override
-                taskInProgress = true;
+                public void run() {
-                scheduledNextTask = false;
+                    assert taskInProgress == false;
-                task.run();
+                    assert waitForPublish == false;
-                if (waitForPublish == false) {
+                    assert scheduledNextTask;
-                    taskInProgress = false;
+                    final int taskIndex = randomInt(pendingTasks.size() - 1);
+                    logger.debug("next master service task: choosing task {} of {}", taskIndex, pendingTasks.size());
+                    final Runnable task = pendingTasks.remove(taskIndex);
+                    taskInProgress = true;
+                    scheduledNextTask = false;
+                    task.run();
+                    if (waitForPublish == false) {
+                        taskInProgress = false;
+                    }
+                    FakeThreadPoolMasterService.this.scheduleNextTaskIfNecessary();
                 }
-                scheduleNextTaskIfNecessary();
             });
         }
     }

x-pack/plugin/monitoring/src/test/java/org/elasticsearch/xpack/monitoring/integration/MonitoringIT.java

@@ -373,6 +373,9 @@ public class MonitoringIT extends ESSingleNodeTestCase {
         assertThat(clusterState.remove("cluster_uuid"), notNullValue());
         assertThat(clusterState.remove("master_node"), notNullValue());
         assertThat(clusterState.remove("nodes"), notNullValue());
+        assertThat(clusterState.remove("term"), notNullValue());
+        assertThat(clusterState.remove("last_committed_config"), notNullValue());
+        assertThat(clusterState.remove("last_accepted_config"), notNullValue());
         assertThat(clusterState.keySet(), empty());
     }
 
@@ -538,16 +541,16 @@ public class MonitoringIT extends ESSingleNodeTestCase {
         if (ti.getLockName() != null) {
           b.append(" on ").append(ti.getLockName());
         }
-        
+
         if (ti.getLockOwnerName() != null) {
           b.append(" owned by \"").append(ti.getLockOwnerName())
            .append("\" ID=").append(ti.getLockOwnerId());
         }
-        
+
         b.append(ti.isSuspended() ? " (suspended)" : "");
         b.append(ti.isInNative() ? " (in native code)" : "");
         b.append("\n");
-        
+
         final StackTraceElement[] stack = ti.getStackTrace();
         final LockInfo lockInfo = ti.getLockInfo();
         final MonitorInfo [] monitorInfos = ti.getLockedMonitors();
@@ -559,7 +562,7 @@ public class MonitoringIT extends ESSingleNodeTestCase {
              .append(lockInfo)
              .append("\n");
           }
-          
+
           for (MonitorInfo mi : monitorInfos) {
             if (mi.getLockedStackDepth() == i) {
               b.append("\t- locked ").append(mi).append("\n");