Move ElectionStrategy from interface to class

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

@@ -23,27 +23,31 @@
 import org.elasticsearch.cluster.node.DiscoveryNode;
 
 /**
- * Allows plugging in a custom election strategy. Note that, in order to guarantee safety of the system, custom election strategies should
- * only be more restrictive than the behavior that's provided by {@link DefaultElectionStrategy}.
+ * Allows plugging in a custom election strategy, restricting the notion of an election quorum.
  */
-public interface ElectionStrategy {
+public class ElectionStrategy {
+
+    public static final ElectionStrategy DEFAULT_INSTANCE = new ElectionStrategy();
+
+    protected ElectionStrategy() {
+
+    }
 
     /**
      * Whether there is an election quorum from the point of view of the given local node under the provided voting configurations
      */
-    boolean isElectionQuorum(DiscoveryNode localNode, long localCurrentTerm, long localAcceptedTerm, long localAcceptedVersion,
-                             VotingConfiguration lastCommittedConfiguration, VotingConfiguration lastAcceptedConfiguration,
-                             VoteCollection joinVotes);
-
-    class DefaultElectionStrategy implements ElectionStrategy {
-
-        public static final ElectionStrategy INSTANCE = new DefaultElectionStrategy();
+    public final boolean isElectionQuorum(DiscoveryNode localNode, long localCurrentTerm, long localAcceptedTerm, long localAcceptedVersion,
+                                          VotingConfiguration lastCommittedConfiguration, VotingConfiguration lastAcceptedConfiguration,
+                                          VoteCollection joinVotes) {
+        return joinVotes.isQuorum(lastCommittedConfiguration) &&
+            joinVotes.isQuorum(lastAcceptedConfiguration) &&
+            isCustomElectionQuorum(localNode, localCurrentTerm, localAcceptedTerm, localAcceptedVersion, lastCommittedConfiguration,
+                lastAcceptedConfiguration, joinVotes);
+    }
 
-        @Override
-        public boolean isElectionQuorum(DiscoveryNode localNode, long localCurrentTerm, long localAcceptedTerm, long localAcceptedVersion,
-                                        VotingConfiguration lastCommittedConfiguration, VotingConfiguration lastAcceptedConfiguration,
-                                        VoteCollection joinVotes) {
-            return joinVotes.isQuorum(lastCommittedConfiguration) && joinVotes.isQuorum(lastAcceptedConfiguration);
-        }
+    protected boolean isCustomElectionQuorum(DiscoveryNode localNode, long localCurrentTerm, long localAcceptedTerm,
+                                             long localAcceptedVersion, VotingConfiguration lastCommittedConfiguration,
+                                             VotingConfiguration lastAcceptedConfiguration, VoteCollection joinVotes) {
+        return true;
     }
 }

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

@@ -93,7 +93,7 @@ public DiscoveryModule(Settings settings, ThreadPool threadPool, TransportServic
         hostProviders.put("settings", () -> new SettingsBasedSeedHostsProvider(settings, transportService));
         hostProviders.put("file", () -> new FileBasedSeedHostsProvider(configFile));
         final Map<String, ElectionStrategy> electionStrategies = new HashMap<>();
-        electionStrategies.put(DEFAULT_ELECTION_STRATEGY, ElectionStrategy.DefaultElectionStrategy.INSTANCE);
+        electionStrategies.put(DEFAULT_ELECTION_STRATEGY, ElectionStrategy.DEFAULT_INSTANCE);
         for (DiscoveryPlugin plugin : plugins) {
             plugin.getSeedHostProviders(transportService, networkService).forEach((key, value) -> {
                 if (hostProviders.put(key, value) != null) {

server/src/main/java/org/elasticsearch/plugins/DiscoveryPlugin.java

@@ -87,6 +87,9 @@ default Map<String, Supplier<SeedHostsProvider>> getSeedHostProviders(TransportS
      */
     default BiConsumer<DiscoveryNode,ClusterState> getJoinValidator() { return null; }
 
+    /**
+     * Allows plugging in election strategies (see {@link ElectionStrategy}) that define a customized notion of an election quorum.
+     */
     default Map<String, ElectionStrategy> getElectionStrategies() {
         return Collections.emptyMap();
     }

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

@@ -49,7 +49,7 @@
 
 public class ClusterFormationFailureHelperTests extends ESTestCase {
 
-    private static final ElectionStrategy electionStrategy = ElectionStrategy.DefaultElectionStrategy.INSTANCE;
+    private static final ElectionStrategy electionStrategy = ElectionStrategy.DEFAULT_INSTANCE;
 
     public void testScheduling() {
         final long expectedDelayMillis;

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

@@ -766,12 +766,12 @@ public void testVoteCollection() {
     public void testSafety() {
         new CoordinationStateTestCluster(IntStream.range(0, randomIntBetween(1, 5))
             .mapToObj(i -> new DiscoveryNode("node_" + i, buildNewFakeTransportAddress(), Version.CURRENT))
-            .collect(Collectors.toList()), ElectionStrategy.DefaultElectionStrategy.INSTANCE)
+            .collect(Collectors.toList()), ElectionStrategy.DEFAULT_INSTANCE)
             .runRandomly();
     }
 
     public static CoordinationState createCoordinationState(PersistedState storage, DiscoveryNode localNode) {
-        return new CoordinationState(localNode, storage, ElectionStrategy.DefaultElectionStrategy.INSTANCE);
+        return new CoordinationState(localNode, storage, ElectionStrategy.DEFAULT_INSTANCE);
     }
 
     public static ClusterState clusterState(long term, long version, DiscoveryNode localNode, VotingConfiguration lastCommittedConfig,

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

@@ -174,7 +174,7 @@ transportService, writableRegistry(),
             () -> new InMemoryPersistedState(term, initialState), r -> emptyList(),
             new NoOpClusterApplier(),
             Collections.emptyList(),
-            random, s -> {}, ElectionStrategy.DefaultElectionStrategy.INSTANCE);
+            random, s -> {}, ElectionStrategy.DEFAULT_INSTANCE);
         transportService.start();
         transportService.acceptIncomingRequests();
         transport = capturingTransport;

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

@@ -114,7 +114,7 @@ public String toString() {
             assert electionOccurred == false;
             electionOccurred = true;
         }, l -> {
-        }, ElectionStrategy.DefaultElectionStrategy.INSTANCE); // TODO need tests that check that the max term seen is updated
+        }, ElectionStrategy.DEFAULT_INSTANCE); // TODO need tests that check that the max term seen is updated
         preVoteCollector.update(getLocalPreVoteResponse(), null);
     }
 
@@ -234,7 +234,7 @@ public void testPrevotingIndicatesElectionSuccess() {
         startAndRunCollector(votingNodes);
 
         final CoordinationState coordinationState = new CoordinationState(localNode,
-            new InMemoryPersistedState(currentTerm, makeClusterState(votingNodes)), ElectionStrategy.DefaultElectionStrategy.INSTANCE);
+            new InMemoryPersistedState(currentTerm, makeClusterState(votingNodes)), ElectionStrategy.DEFAULT_INSTANCE);
 
         final long newTerm = randomLongBetween(currentTerm + 1, Long.MAX_VALUE);
 

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

@@ -73,7 +73,7 @@ class MockNode {
             ClusterState initialState = CoordinationStateTests.clusterState(0L, 0L, localNode,
                 CoordinationMetaData.VotingConfiguration.EMPTY_CONFIG, CoordinationMetaData.VotingConfiguration.EMPTY_CONFIG, 0L);
             coordinationState = new CoordinationState(localNode, new InMemoryPersistedState(0L, initialState),
-                ElectionStrategy.DefaultElectionStrategy.INSTANCE);
+                ElectionStrategy.DEFAULT_INSTANCE);
         }
 
         final DiscoveryNode localNode;

server/src/test/java/org/elasticsearch/snapshots/SnapshotResiliencyTests.java

@@ -1247,7 +1247,7 @@ public void start(ClusterState initialState) {
                 hostsResolver -> testClusterNodes.nodes.values().stream().filter(n -> n.node.isMasterNode())
                     .map(n -> n.node.getAddress()).collect(Collectors.toList()),
                 clusterService.getClusterApplierService(), Collections.emptyList(), random(),
-                new RoutingService(clusterService, allocationService)::reroute, ElectionStrategy.DefaultElectionStrategy.INSTANCE);
+                new RoutingService(clusterService, allocationService)::reroute, ElectionStrategy.DEFAULT_INSTANCE);
             masterService.setClusterStatePublisher(coordinator);
             coordinator.start();
             masterService.start();

test/framework/src/main/java/org/elasticsearch/cluster/coordination/AbstractCoordinatorTestCase.java

@@ -839,7 +839,7 @@ protected Optional<DisruptableMockTransport> getDisruptableMockTransport(Transpo
                 coordinator = new Coordinator("test_node", settings, clusterSettings, transportService, writableRegistry(),
                     allocationService, masterService, this::getPersistedState,
                     Cluster.this::provideSeedHosts, clusterApplierService, onJoinValidators, Randomness.get(), s -> {},
-                    ElectionStrategy.DefaultElectionStrategy.INSTANCE);
+                    ElectionStrategy.DEFAULT_INSTANCE);
                 masterService.setClusterStatePublisher(coordinator);
                 final GatewayService gatewayService = new GatewayService(settings, allocationService, clusterService,
                     deterministicTaskQueue.getThreadPool(this::onNode), null, coordinator);

x-pack/plugin/voting-only-node/src/main/java/org/elasticsearch/cluster/coordination/VotingOnlyNodePlugin.java

@@ -157,12 +157,12 @@ public Settings additionalSettings() {
         return Settings.builder().put(DiscoveryModule.ELECTION_STRATEGY_SETTING.getKey(), VOTING_ONLY_ELECTION_STRATEGY).build();
     }
 
-    static class VotingOnlyNodeElectionStrategy extends ElectionStrategy.DefaultElectionStrategy {
+    static class VotingOnlyNodeElectionStrategy extends ElectionStrategy {
 
         @Override
-        public boolean isElectionQuorum(DiscoveryNode localNode, long localCurrentTerm, long localAcceptedTerm, long localAcceptedVersion,
-                                        VotingConfiguration lastCommittedConfiguration, VotingConfiguration lastAcceptedConfiguration,
-                                        VoteCollection joinVotes) {
+        public boolean isCustomElectionQuorum(DiscoveryNode localNode, long localCurrentTerm, long localAcceptedTerm,
+                                              long localAcceptedVersion, VotingConfiguration lastCommittedConfiguration,
+                                              VotingConfiguration lastAcceptedConfiguration, VoteCollection joinVotes) {
             // if local node is voting only, have additional checks on election quorum definition
             if (isVotingOnlyNode(localNode)) {
                 // if all votes are from voting only nodes, do not elect as master (no need to transfer state)
@@ -177,9 +177,7 @@ public boolean isElectionQuorum(DiscoveryNode localNode, long localCurrentTerm,
                     return false;
                 }
             }
-            // fall back to default election quorum definition
-            return super.isElectionQuorum(localNode, localCurrentTerm, localAcceptedTerm, localAcceptedVersion,
-                lastCommittedConfiguration, lastAcceptedConfiguration, joinVotes);
+            return true;
         }
     }