[Zen2] Best-effort cluster formation if unconfigured (#36215)

In real deployments it is important that clusters are properly configured to
avoid accidentally forming multiple independent clusters at cluster
bootstrapping time. However we also expect to be able to unpack Elasticsearch
and start up one or more nodes without any up-front configuration, and have
them do their best to find each other and form a cluster after a few seconds.

This change adds a delayed automatic bootstrapping process to nodes that start
up with no relevant settings set to support the desired out-of-the-box
experience without compromising safety in properly-configured deployments.

docs/reference/migration/migrate_7_0/cluster.asciidoc

@@ -24,4 +24,13 @@ These shard preferences are removed in favour of the `_prefer_nodes` and `_only_
 Clusters now have soft limits on the total number of open shards in the cluster
 based on the number of nodes and the `cluster.max_shards_per_node` cluster
 setting, to prevent accidental operations that would destabilize the cluster.
-More information can be found in the <<misc-cluster,documentation for that setting>>.
+More information can be found in the <<misc-cluster,documentation for that setting>>.
+
+[float]
+==== Discovery configuration is required in production
+Production deployments of Elasticsearch now require at least one of the following settings
+to be specified in the `elasticsearch.yml` configuration file:
+
+- `discovery.zen.ping.unicast.hosts`
+- `discovery.zen.hosts_provider`
+- `cluster.initial_master_nodes`

docs/reference/setup/bootstrap-checks.asciidoc

@@ -236,3 +236,21 @@ versions of the HotSpot JVM.
 The all permission check ensures that the security policy used during bootstrap
 does not grant the `java.security.AllPermission` to Elasticsearch. Running with
 the all permission granted is equivalent to disabling the security manager.
+
+=== Discovery configuration check
+
+By default, when Elasticsearch first starts up it will try and discover other
+nodes running on the same host. If no elected master can be discovered within a
+few seconds then Elasticsearch will form a cluster that includes any other
+nodes that were discovered. It is useful to be able to form this cluster
+without any extra configuration in development mode, but this is unsuitable for
+production because it's possible to form multiple clusters and lose data as a
+result.
+
+This bootstrap check ensures that discovery is not running with the default
+configuration. It can be satisfied by setting at least one of the following
+properties:
+
+- `discovery.zen.ping.unicast.hosts`
+- `discovery.zen.hosts_provider`
+- `cluster.initial_master_nodes`

qa/unconfigured-node-name/build.gradle

@@ -22,7 +22,7 @@ apply plugin: 'elasticsearch.rest-test'
 
 integTestCluster {
   setting 'node.name', null
-  // TODO: Run this using zen2
+  // TODO: Run this using zen2, with no discovery configuration at all, demonstrating that the node forms a cluster on its own without help
   setting 'discovery.type', 'zen'
 }
 

server/src/main/java/org/elasticsearch/bootstrap/BootstrapChecks.java

@@ -23,8 +23,10 @@ import org.apache.logging.log4j.LogManager;
 import org.apache.logging.log4j.Logger;
 import org.apache.logging.log4j.message.ParameterizedMessage;
 import org.apache.lucene.util.Constants;
+import org.elasticsearch.cluster.coordination.ClusterBootstrapService;
 import org.elasticsearch.common.SuppressForbidden;
 import org.elasticsearch.common.io.PathUtils;
+import org.elasticsearch.common.settings.Setting;
 import org.elasticsearch.common.transport.BoundTransportAddress;
 import org.elasticsearch.common.transport.TransportAddress;
 import org.elasticsearch.discovery.DiscoveryModule;
@@ -46,6 +48,12 @@ import java.util.Locale;
 import java.util.function.Predicate;
 import java.util.regex.Matcher;
 import java.util.regex.Pattern;
+import java.util.stream.Collectors;
+import java.util.stream.Stream;
+
+import static org.elasticsearch.cluster.coordination.ClusterBootstrapService.INITIAL_MASTER_NODES_SETTING;
+import static org.elasticsearch.discovery.DiscoveryModule.DISCOVERY_HOSTS_PROVIDER_SETTING;
+import static org.elasticsearch.discovery.zen.SettingsBasedHostsProvider.DISCOVERY_ZEN_PING_UNICAST_HOSTS_SETTING;
 
 /**
  * We enforce bootstrap checks once a node has the transport protocol bound to a non-loopback interface or if the system property {@code
@@ -207,6 +215,7 @@ final class BootstrapChecks {
         checks.add(new EarlyAccessCheck());
         checks.add(new G1GCCheck());
         checks.add(new AllPermissionCheck());
+        checks.add(new DiscoveryConfiguredCheck());
         return Collections.unmodifiableList(checks);
     }
 
@@ -713,4 +722,21 @@ final class BootstrapChecks {
 
     }
 
+    static class DiscoveryConfiguredCheck implements BootstrapCheck {
+        @Override
+        public BootstrapCheckResult check(BootstrapContext context) {
+            if (DiscoveryModule.ZEN2_DISCOVERY_TYPE.equals(DiscoveryModule.DISCOVERY_TYPE_SETTING.get(context.settings)) == false) {
+                return BootstrapCheckResult.success();
+            }
+            if (ClusterBootstrapService.discoveryIsConfigured(context.settings)) {
+                return BootstrapCheckResult.success();
+            }
+
+            return BootstrapCheckResult.failure(String.format(
+                Locale.ROOT,
+                "the default discovery settings are unsuitable for production use; at least one of [%s] must be configured",
+                Stream.of(DISCOVERY_ZEN_PING_UNICAST_HOSTS_SETTING, DISCOVERY_HOSTS_PROVIDER_SETTING, INITIAL_MASTER_NODES_SETTING)
+                    .map(Setting::getKey).collect(Collectors.joining(", "))));
+        }
+    }
 }

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

@@ -29,6 +29,7 @@ import org.elasticsearch.action.admin.cluster.bootstrap.GetDiscoveredNodesAction
 import org.elasticsearch.action.admin.cluster.bootstrap.GetDiscoveredNodesRequest;
 import org.elasticsearch.action.admin.cluster.bootstrap.GetDiscoveredNodesResponse;
 import org.elasticsearch.cluster.node.DiscoveryNode;
+import org.elasticsearch.common.Nullable;
 import org.elasticsearch.common.io.stream.StreamInput;
 import org.elasticsearch.common.settings.Setting;
 import org.elasticsearch.common.settings.Setting.Property;
@@ -44,6 +45,10 @@ import java.io.IOException;
 import java.util.Collections;
 import java.util.List;
 import java.util.function.Function;
+import java.util.stream.Stream;
+
+import static org.elasticsearch.discovery.DiscoveryModule.DISCOVERY_HOSTS_PROVIDER_SETTING;
+import static org.elasticsearch.discovery.zen.SettingsBasedHostsProvider.DISCOVERY_ZEN_PING_UNICAST_HOSTS_SETTING;
 
 public class ClusterBootstrapService {
 
@@ -57,22 +62,82 @@ public class ClusterBootstrapService {
     public static final Setting<List<String>> INITIAL_MASTER_NODES_SETTING =
         Setting.listSetting("cluster.initial_master_nodes", Collections.emptyList(), Function.identity(), Property.NodeScope);
 
+    public static final Setting<TimeValue> UNCONFIGURED_BOOTSTRAP_TIMEOUT_SETTING =
+        Setting.timeSetting("discovery.unconfigured_bootstrap_timeout",
+            TimeValue.timeValueSeconds(3), TimeValue.timeValueMillis(1), Property.NodeScope);
+
     private final int initialMasterNodeCount;
     private final List<String> initialMasterNodes;
+    @Nullable
+    private final TimeValue unconfiguredBootstrapTimeout;
     private final TransportService transportService;
     private volatile boolean running;
 
     public ClusterBootstrapService(Settings settings, TransportService transportService) {
         initialMasterNodeCount = INITIAL_MASTER_NODE_COUNT_SETTING.get(settings);
         initialMasterNodes = INITIAL_MASTER_NODES_SETTING.get(settings);
+        unconfiguredBootstrapTimeout = discoveryIsConfigured(settings) ? null : UNCONFIGURED_BOOTSTRAP_TIMEOUT_SETTING.get(settings);
         this.transportService = transportService;
     }
 
+    public static boolean discoveryIsConfigured(Settings settings) {
+        return Stream.of(DISCOVERY_HOSTS_PROVIDER_SETTING, DISCOVERY_ZEN_PING_UNICAST_HOSTS_SETTING,
+            INITIAL_MASTER_NODE_COUNT_SETTING, INITIAL_MASTER_NODES_SETTING).anyMatch(s -> s.exists(settings));
+    }
+
     public void start() {
         assert running == false;
         running = true;
 
-        if ((initialMasterNodeCount > 0 || initialMasterNodes.isEmpty() == false) && transportService.getLocalNode().isMasterNode()) {
+        if (transportService.getLocalNode().isMasterNode() == false) {
+            return;
+        }
+
+        if (unconfiguredBootstrapTimeout != null) {
+            logger.info("no discovery configuration found, will perform best-effort cluster bootstrapping after [{}] " +
+                    "unless existing master is discovered", unconfiguredBootstrapTimeout);
+            final ThreadContext threadContext = transportService.getThreadPool().getThreadContext();
+            try (ThreadContext.StoredContext ignore = threadContext.stashContext()) {
+                threadContext.markAsSystemContext();
+
+                transportService.getThreadPool().scheduleUnlessShuttingDown(unconfiguredBootstrapTimeout, Names.SAME, new Runnable() {
+                    @Override
+                    public void run() {
+                        final GetDiscoveredNodesRequest request = new GetDiscoveredNodesRequest();
+                        logger.trace("sending {}", request);
+                        transportService.sendRequest(transportService.getLocalNode(), GetDiscoveredNodesAction.NAME, request,
+                            new TransportResponseHandler<GetDiscoveredNodesResponse>() {
+                                @Override
+                                public void handleResponse(GetDiscoveredNodesResponse response) {
+                                    logger.debug("discovered {}, starting to bootstrap", response.getNodes());
+                                    awaitBootstrap(response.getBootstrapConfiguration());
+                                }
+
+                                @Override
+                                public void handleException(TransportException exp) {
+                                    logger.warn("discovery attempt failed", exp);
+                                }
+
+                                @Override
+                                public String executor() {
+                                    return Names.SAME;
+                                }
+
+                                @Override
+                                public GetDiscoveredNodesResponse read(StreamInput in) throws IOException {
+                                    return new GetDiscoveredNodesResponse(in);
+                                }
+                            });
+                    }
+
+                    @Override
+                    public String toString() {
+                        return "unconfigured-discovery delayed bootstrap";
+                    }
+                });
+
+            }
+        } else if (initialMasterNodeCount > 0 || initialMasterNodes.isEmpty() == false) {
             logger.debug("unsafely waiting for discovery of [{}] master-eligible nodes", initialMasterNodeCount);
 
             final ThreadContext threadContext = transportService.getThreadPool().getThreadContext();
@@ -116,7 +181,6 @@ public class ClusterBootstrapService {
     }
 
     public void stop() {
-        assert running == true;
         running = false;
     }
 

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

@@ -501,7 +501,9 @@ public class Coordinator extends AbstractLifecycleComponent implements Discovery
             becomeCandidate("startInitialJoin");
         }
 
-        clusterBootstrapService.start();
+        if (isInitialConfigurationSet() == false) {
+            clusterBootstrapService.start();
+        }
     }
 
     @Override

server/src/main/java/org/elasticsearch/common/settings/ClusterSettings.java

@@ -473,6 +473,7 @@ public final class ClusterSettings extends AbstractScopedSettings {
                     TransportAddVotingConfigExclusionsAction.MAXIMUM_VOTING_CONFIG_EXCLUSIONS_SETTING,
                     ClusterBootstrapService.INITIAL_MASTER_NODES_SETTING,
                     ClusterBootstrapService.INITIAL_MASTER_NODE_COUNT_SETTING,
+                    ClusterBootstrapService.UNCONFIGURED_BOOTSTRAP_TIMEOUT_SETTING,
                     LagDetector.CLUSTER_FOLLOWER_LAG_TIMEOUT_SETTING
             )));
 

server/src/test/java/org/elasticsearch/bootstrap/BootstrapChecksTests.java

@@ -21,10 +21,14 @@ package org.elasticsearch.bootstrap;
 
 import org.apache.logging.log4j.Logger;
 import org.apache.lucene.util.Constants;
+import org.elasticsearch.cluster.coordination.ClusterBootstrapService;
 import org.elasticsearch.cluster.metadata.MetaData;
+import org.elasticsearch.common.CheckedConsumer;
 import org.elasticsearch.common.settings.Settings;
 import org.elasticsearch.common.transport.BoundTransportAddress;
 import org.elasticsearch.common.transport.TransportAddress;
+import org.elasticsearch.discovery.DiscoveryModule;
+import org.elasticsearch.discovery.zen.SettingsBasedHostsProvider;
 import org.elasticsearch.monitor.jvm.JvmInfo;
 import org.elasticsearch.node.NodeValidationException;
 import org.elasticsearch.test.ESTestCase;
@@ -700,4 +704,34 @@ public class BootstrapChecksTests extends ESTestCase {
         assertThat(alwaysEnforced, hasToString(containsString("error")));
     }
 
+    public void testDiscoveryConfiguredCheck() throws NodeValidationException {
+        final List<BootstrapCheck> checks = Collections.singletonList(new BootstrapChecks.DiscoveryConfiguredCheck());
+
+        final BootstrapContext zen2Context = new BootstrapContext(Settings.builder()
+            .put(DiscoveryModule.DISCOVERY_TYPE_SETTING.getKey(), ZEN2_DISCOVERY_TYPE).build(), MetaData.EMPTY_META_DATA);
+
+        // not always enforced
+        BootstrapChecks.check(zen2Context, false, checks);
+
+        // not enforced for non-zen2 discovery
+        BootstrapChecks.check(new BootstrapContext(Settings.builder().put(DiscoveryModule.DISCOVERY_TYPE_SETTING.getKey(),
+            randomFrom(ZEN_DISCOVERY_TYPE, "single-node", randomAlphaOfLength(5))).build(), MetaData.EMPTY_META_DATA), true, checks);
+
+        final NodeValidationException e = expectThrows(NodeValidationException.class,
+            () -> BootstrapChecks.check(zen2Context, true, checks));
+        assertThat(e, hasToString(containsString("the default discovery settings are unsuitable for production use; at least one " +
+            "of [discovery.zen.ping.unicast.hosts, discovery.zen.hosts_provider, cluster.initial_master_nodes] must be configured")));
+
+        CheckedConsumer<Settings.Builder, NodeValidationException> ensureChecksPass = b ->
+        {
+            final BootstrapContext context = new BootstrapContext(b
+                .put(DiscoveryModule.DISCOVERY_TYPE_SETTING.getKey(), ZEN2_DISCOVERY_TYPE).build(), MetaData.EMPTY_META_DATA);
+            BootstrapChecks.check(context, true, checks);
+        };
+
+        ensureChecksPass.accept(Settings.builder().putList(DiscoveryModule.DISCOVERY_HOSTS_PROVIDER_SETTING.getKey()));
+        ensureChecksPass.accept(Settings.builder().putList(SettingsBasedHostsProvider.DISCOVERY_ZEN_PING_UNICAST_HOSTS_SETTING.getKey()));
+        ensureChecksPass.accept(Settings.builder().put(ClusterBootstrapService.INITIAL_MASTER_NODE_COUNT_SETTING.getKey(), 0));
+        ensureChecksPass.accept(Settings.builder().putList(ClusterBootstrapService.INITIAL_MASTER_NODES_SETTING.getKey()));
+    }
 }

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

@@ -23,12 +23,14 @@ import org.elasticsearch.Version;
 import org.elasticsearch.action.admin.cluster.bootstrap.BootstrapClusterAction;
 import org.elasticsearch.action.admin.cluster.bootstrap.BootstrapClusterRequest;
 import org.elasticsearch.action.admin.cluster.bootstrap.BootstrapClusterResponse;
+import org.elasticsearch.action.admin.cluster.bootstrap.BootstrapConfiguration.NodeDescription;
 import org.elasticsearch.action.admin.cluster.bootstrap.GetDiscoveredNodesAction;
 import org.elasticsearch.action.admin.cluster.bootstrap.GetDiscoveredNodesRequest;
 import org.elasticsearch.action.admin.cluster.bootstrap.GetDiscoveredNodesResponse;
 import org.elasticsearch.cluster.node.DiscoveryNode;
 import org.elasticsearch.cluster.node.DiscoveryNode.Role;
 import org.elasticsearch.common.settings.Settings;
+import org.elasticsearch.common.settings.Settings.Builder;
 import org.elasticsearch.tasks.Task;
 import org.elasticsearch.test.ESTestCase;
 import org.elasticsearch.test.transport.MockTransport;
@@ -50,7 +52,11 @@ import static java.util.Collections.emptySet;
 import static java.util.Collections.singleton;
 import static org.elasticsearch.cluster.coordination.ClusterBootstrapService.INITIAL_MASTER_NODES_SETTING;
 import static org.elasticsearch.cluster.coordination.ClusterBootstrapService.INITIAL_MASTER_NODE_COUNT_SETTING;
+import static org.elasticsearch.common.settings.Settings.builder;
+import static org.elasticsearch.discovery.DiscoveryModule.DISCOVERY_HOSTS_PROVIDER_SETTING;
+import static org.elasticsearch.discovery.zen.SettingsBasedHostsProvider.DISCOVERY_ZEN_PING_UNICAST_HOSTS_SETTING;
 import static org.elasticsearch.node.Node.NODE_NAME_SETTING;
+import static org.hamcrest.Matchers.equalTo;
 
 public class ClusterBootstrapServiceTests extends ESTestCase {
 
@@ -65,7 +71,7 @@ public class ClusterBootstrapServiceTests extends ESTestCase {
         otherNode1 = newDiscoveryNode("other1");
         otherNode2 = newDiscoveryNode("other2");
 
-        deterministicTaskQueue = new DeterministicTaskQueue(Settings.builder().put(NODE_NAME_SETTING.getKey(), "node").build(), random());
+        deterministicTaskQueue = new DeterministicTaskQueue(builder().put(NODE_NAME_SETTING.getKey(), "node").build(), random());
 
         final MockTransport transport = new MockTransport() {
             @Override
@@ -77,6 +83,9 @@ public class ClusterBootstrapServiceTests extends ESTestCase {
         transportService = transport.createTransportService(Settings.EMPTY, deterministicTaskQueue.getThreadPool(),
             TransportService.NOOP_TRANSPORT_INTERCEPTOR, boundTransportAddress -> localNode, null, emptySet());
 
+        clusterBootstrapService = new ClusterBootstrapService(builder().put(INITIAL_MASTER_NODE_COUNT_SETTING.getKey(), 3).build(),
+            transportService);
+
         final Settings settings;
         if (randomBoolean()) {
             settings = Settings.builder().put(INITIAL_MASTER_NODE_COUNT_SETTING.getKey(), 3).build();
@@ -109,8 +118,24 @@ public class ClusterBootstrapServiceTests extends ESTestCase {
         deterministicTaskQueue.runAllTasks();
     }
 
-    public void testDoesNothingIfSettingIsUnset() {
-        clusterBootstrapService = new ClusterBootstrapService(Settings.EMPTY, transportService);
+    public void testDoesNothingByDefaultIfHostsProviderConfigured() {
+        testConfiguredIfSettingSet(builder().putList(DISCOVERY_HOSTS_PROVIDER_SETTING.getKey()));
+    }
+
+    public void testDoesNothingByDefaultIfUnicastHostsConfigured() {
+        testConfiguredIfSettingSet(builder().putList(DISCOVERY_ZEN_PING_UNICAST_HOSTS_SETTING.getKey()));
+    }
+
+    public void testDoesNothingByDefaultIfMasterNodeCountConfigured() {
+        testConfiguredIfSettingSet(builder().put(INITIAL_MASTER_NODE_COUNT_SETTING.getKey(), 0));
+    }
+
+    public void testDoesNothingByDefaultIfMasterNodesConfigured() {
+        testConfiguredIfSettingSet(builder().putList(INITIAL_MASTER_NODES_SETTING.getKey()));
+    }
+
+    private void testConfiguredIfSettingSet(Builder builder) {
+        clusterBootstrapService = new ClusterBootstrapService(builder.build(), transportService);
         transportService.registerRequestHandler(GetDiscoveredNodesAction.NAME, Names.SAME, GetDiscoveredNodesRequest::new,
             (request, channel, task) -> {
                 throw new AssertionError("should not make a discovery request");
@@ -119,6 +144,30 @@ public class ClusterBootstrapServiceTests extends ESTestCase {
         deterministicTaskQueue.runAllTasks();
     }
 
+    public void testBootstrapsAutomaticallyWithDefaultConfiguration() {
+        clusterBootstrapService = new ClusterBootstrapService(Settings.EMPTY, transportService);
+
+        final Set<DiscoveryNode> discoveredNodes = Stream.of(localNode, otherNode1, otherNode2).collect(Collectors.toSet());
+        transportService.registerRequestHandler(GetDiscoveredNodesAction.NAME, Names.SAME, GetDiscoveredNodesRequest::new,
+            (request, channel, task) -> channel.sendResponse(new GetDiscoveredNodesResponse(discoveredNodes)));
+
+        final AtomicBoolean bootstrapped = new AtomicBoolean();
+        transportService.registerRequestHandler(BootstrapClusterAction.NAME, Names.SAME, BootstrapClusterRequest::new,
+            (request, channel, task) -> {
+                assertThat(request.getBootstrapConfiguration().getNodeDescriptions().stream()
+                        .map(NodeDescription::getId).collect(Collectors.toSet()),
+                    equalTo(discoveredNodes.stream().map(DiscoveryNode::getId).collect(Collectors.toSet())));
+
+                channel.sendResponse(new BootstrapClusterResponse(randomBoolean()));
+                assertTrue(bootstrapped.compareAndSet(false, true));
+            });
+
+        startServices();
+        deterministicTaskQueue.runAllTasks();
+
+        assertTrue(bootstrapped.get());
+    }
+
     public void testDoesNotRetryOnDiscoveryFailure() {
         transportService.registerRequestHandler(GetDiscoveredNodesAction.NAME, Names.SAME, GetDiscoveredNodesRequest::new,
             new TransportRequestHandler<GetDiscoveredNodesRequest>() {