Add remote cluster infrastructure to fetch discovery nodes. (#25123)

In order to add scroll support for cross cluster search we need
to resolve the nodes encoded in the scroll ID to send requests to the
corresponding nodes. This change adds the low level connection infrastructure
that also ensures that connections are re-established if the cluster is
disconnected due to a network failure or restarts.

Relates to #25094

core/src/main/java/org/elasticsearch/transport/RemoteClusterConnection.java

@@ -61,8 +61,10 @@ import java.util.concurrent.ExecutorService;
 import java.util.concurrent.RejectedExecutionException;
 import java.util.concurrent.Semaphore;
 import java.util.concurrent.atomic.AtomicBoolean;
+import java.util.function.Function;
 import java.util.function.Predicate;
 import java.util.stream.Collectors;
+import java.util.stream.Stream;
 
 /**
  * Represents a connection to a single remote cluster. In contrast to a local cluster a remote cluster is not joined such that the
@@ -206,6 +208,53 @@ final class RemoteClusterConnection extends AbstractComponent implements Transpo
             });
     }
 
+    /**
+     * Collects all nodes on the connected cluster and returns / passes a nodeID to {@link DiscoveryNode} lookup function
+     * that returns <code>null</code> if the node ID is not found.
+     */
+    void collectNodes(ActionListener<Function<String, DiscoveryNode>> listener) {
+        Runnable runnable = () -> {
+            final ClusterStateRequest request = new ClusterStateRequest();
+            request.clear();
+            request.nodes(true);
+            request.local(true); // run this on the node that gets the request it's as good as any other
+            final DiscoveryNode node = nodeSupplier.get();
+            transportService.sendRequest(node, ClusterStateAction.NAME, request, TransportRequestOptions.EMPTY,
+                new TransportResponseHandler<ClusterStateResponse>() {
+                    @Override
+                    public ClusterStateResponse newInstance() {
+                        return new ClusterStateResponse();
+                    }
+
+                    @Override
+                    public void handleResponse(ClusterStateResponse response) {
+                        DiscoveryNodes nodes = response.getState().nodes();
+                        listener.onResponse(nodes::get);
+                    }
+
+                    @Override
+                    public void handleException(TransportException exp) {
+                        listener.onFailure(exp);
+                    }
+
+                    @Override
+                    public String executor() {
+                        return ThreadPool.Names.SAME;
+                    }
+                });
+        };
+        if (connectedNodes.isEmpty()) {
+            // just in case if we are not connected for some reason we try to connect and if we fail we have to notify the listener
+            // this will cause some back pressure on the search end and eventually will cause rejections but that's fine
+            // we can't proceed with a search on a cluster level.
+            // in the future we might want to just skip the remote nodes in such a case but that can already be implemented on the
+            // caller end since they provide the listener.
+            ensureConnected(ActionListener.wrap((x) -> runnable.run(), listener::onFailure));
+        } else {
+            runnable.run();
+        }
+    }
+
     /**
      * Returns a connection to the remote cluster. This connection might be a proxy connection that redirects internally to the
      * given node.

core/src/main/java/org/elasticsearch/transport/RemoteClusterService.java

@@ -18,17 +18,12 @@
  */
 package org.elasticsearch.transport;
 
-import org.apache.logging.log4j.util.Supplier;
 import org.apache.lucene.util.IOUtils;
 import org.elasticsearch.Version;
 import org.elasticsearch.action.ActionListener;
-import org.elasticsearch.action.LatchedActionListener;
 import org.elasticsearch.action.OriginalIndices;
-import org.elasticsearch.action.admin.cluster.shards.ClusterSearchShardsGroup;
 import org.elasticsearch.action.admin.cluster.shards.ClusterSearchShardsRequest;
 import org.elasticsearch.action.admin.cluster.shards.ClusterSearchShardsResponse;
-import org.elasticsearch.action.search.SearchRequest;
-import org.elasticsearch.action.search.SearchShardIterator;
 import org.elasticsearch.action.support.GroupedActionListener;
 import org.elasticsearch.action.support.IndicesOptions;
 import org.elasticsearch.action.support.PlainActionFuture;
@@ -40,15 +35,10 @@ import org.elasticsearch.common.settings.Settings;
 import org.elasticsearch.common.transport.TransportAddress;
 import org.elasticsearch.common.unit.TimeValue;
 import org.elasticsearch.common.util.concurrent.CountDown;
-import org.elasticsearch.index.Index;
-import org.elasticsearch.index.shard.ShardId;
-import org.elasticsearch.search.internal.AliasFilter;
 
 import java.io.Closeable;
 import java.io.IOException;
 import java.net.InetSocketAddress;
-import java.util.ArrayList;
-import java.util.Arrays;
 import java.util.Collection;
 import java.util.Collections;
 import java.util.HashMap;
@@ -59,6 +49,7 @@ import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.TimeUnit;
 import java.util.concurrent.TimeoutException;
 import java.util.concurrent.atomic.AtomicReference;
+import java.util.function.BiFunction;
 import java.util.function.Function;
 import java.util.function.Predicate;
 import java.util.stream.Collectors;
@@ -346,4 +337,44 @@ public final class RemoteClusterService extends RemoteClusterAware implements Cl
             }
         }
     }
+
+    /**
+     * Collects all nodes of the given clusters and returns / passes a (clusterAlias, nodeId) to {@link DiscoveryNode}
+     * function on success.
+     */
+    public void collectNodes(Set<String> clusters, ActionListener<BiFunction<String, String, DiscoveryNode>> listener) {
+        Map<String, RemoteClusterConnection> remoteClusters = this.remoteClusters;
+        for (String cluster : clusters) {
+            if (remoteClusters.containsKey(cluster) == false) {
+                listener.onFailure(new IllegalArgumentException("no such remote cluster: [" + cluster + "]"));
+                return;
+            }
+        }
+
+        final Map<String, Function<String, DiscoveryNode>> clusterMap = new HashMap<>();
+        CountDown countDown = new CountDown(clusters.size());
+        Function<String, DiscoveryNode> nullFunction = s -> null;
+        for (final String cluster : clusters) {
+            RemoteClusterConnection connection = remoteClusters.get(cluster);
+            connection.collectNodes(new ActionListener<Function<String, DiscoveryNode>>() {
+                @Override
+                public void onResponse(Function<String, DiscoveryNode> nodeLookup) {
+                    synchronized (clusterMap) {
+                        clusterMap.put(cluster, nodeLookup);
+                    }
+                    if (countDown.countDown()) {
+                        listener.onResponse((clusterAlias, nodeId)
+                            -> clusterMap.getOrDefault(clusterAlias, nullFunction).apply(nodeId));
+                    }
+                }
+
+                @Override
+                public void onFailure(Exception e) {
+                    if (countDown.fastForward()) { // we need to check if it's true since we could have multiple failures
+                        listener.onFailure(e);
+                    }
+                }
+            });
+        }
+    }
 }

core/src/test/java/org/elasticsearch/transport/RemoteClusterConnectionTests.java

@@ -55,11 +55,6 @@ import org.elasticsearch.test.ESTestCase;
 import org.elasticsearch.test.transport.MockTransportService;
 import org.elasticsearch.threadpool.TestThreadPool;
 import org.elasticsearch.threadpool.ThreadPool;
-import org.elasticsearch.transport.RemoteClusterConnection;
-import org.elasticsearch.transport.RemoteConnectionInfo;
-import org.elasticsearch.transport.RemoteTransportException;
-import org.elasticsearch.transport.TransportConnectionListener;
-import org.elasticsearch.transport.TransportService;
 
 import java.io.IOException;
 import java.net.InetAddress;
@@ -78,6 +73,7 @@ import java.util.concurrent.RejectedExecutionException;
 import java.util.concurrent.TimeUnit;
 import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.concurrent.atomic.AtomicReference;
+import java.util.function.Function;
 
 import static java.util.Collections.emptyMap;
 import static java.util.Collections.emptySet;
@@ -357,7 +353,6 @@ public class RemoteClusterConnectionTests extends ESTestCase {
 
     public void testFetchShards() throws Exception {
         List<DiscoveryNode> knownNodes = new CopyOnWriteArrayList<>();
-
         try (MockTransportService seedTransport = startTransport("seed_node", knownNodes, Version.CURRENT);
              MockTransportService discoverableTransport = startTransport("discoverable_node", knownNodes, Version.CURRENT)) {
             DiscoveryNode seedNode = seedTransport.getLocalDiscoNode();
@@ -785,4 +780,42 @@ public class RemoteClusterConnectionTests extends ESTestCase {
             }
         }
     }
+
+    public void testCollectNodes() throws Exception {
+        List<DiscoveryNode> knownNodes = new CopyOnWriteArrayList<>();
+        try (MockTransportService seedTransport = startTransport("seed_node", knownNodes, Version.CURRENT)) {
+            DiscoveryNode seedNode = seedTransport.getLocalDiscoNode();
+            knownNodes.add(seedTransport.getLocalDiscoNode());
+            try (MockTransportService service = MockTransportService.createNewService(Settings.EMPTY, Version.CURRENT, threadPool, null)) {
+                service.start();
+                service.acceptIncomingRequests();
+                try (RemoteClusterConnection connection = new RemoteClusterConnection(Settings.EMPTY, "test-cluster",
+                    Arrays.asList(seedNode), service, Integer.MAX_VALUE, n -> true)) {
+                    if (randomBoolean()) {
+                        updateSeedNodes(connection, Arrays.asList(seedNode));
+                    }
+                    CountDownLatch responseLatch = new CountDownLatch(1);
+                    AtomicReference<Function<String, DiscoveryNode>> reference = new AtomicReference<>();
+                    AtomicReference<Exception> failReference = new AtomicReference<>();
+                    ActionListener<Function<String, DiscoveryNode>> shardsListener = ActionListener.wrap(
+                        x -> {
+                            reference.set(x);
+                            responseLatch.countDown();
+                        },
+                        x -> {
+                            failReference.set(x);
+                            responseLatch.countDown();
+                        });
+                    connection.collectNodes(shardsListener);
+                    responseLatch.await();
+                    assertNull(failReference.get());
+                    assertNotNull(reference.get());
+                    Function<String, DiscoveryNode> function = reference.get();
+                    assertEquals(seedNode, function.apply(seedNode.getId()));
+                    assertNull(function.apply(seedNode.getId() + "foo"));
+                    assertTrue(connection.assertNoRunningConnections());
+                }
+            }
+        }
+    }
 }

core/src/test/java/org/elasticsearch/transport/RemoteClusterServiceTests.java

@@ -18,6 +18,7 @@
  */
 package org.elasticsearch.transport;
 
+import org.apache.lucene.util.IOUtils;
 import org.elasticsearch.Version;
 import org.elasticsearch.action.ActionListener;
 import org.elasticsearch.cluster.node.DiscoveryNode;
@@ -34,11 +35,14 @@ import java.net.InetAddress;
 import java.net.InetSocketAddress;
 import java.util.Arrays;
 import java.util.Collections;
+import java.util.HashSet;
 import java.util.List;
 import java.util.Map;
 import java.util.concurrent.CopyOnWriteArrayList;
 import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicReference;
+import java.util.function.BiFunction;
 
 public class RemoteClusterServiceTests extends ESTestCase {
 
@@ -265,4 +269,151 @@ public class RemoteClusterServiceTests extends ESTestCase {
         return ActionListener.wrap(x -> latch.countDown(), x -> fail());
     }
 
+
+    public void testCollectNodes() throws InterruptedException, IOException {
+        final Settings settings = Settings.EMPTY;
+        final List<DiscoveryNode> knownNodes_c1 = new CopyOnWriteArrayList<>();
+        final List<DiscoveryNode> knownNodes_c2 = new CopyOnWriteArrayList<>();
+
+        try (MockTransportService c1N1 =
+                 startTransport("cluster_1_node_1", knownNodes_c1, Version.CURRENT);
+             MockTransportService c1N2 =
+                 startTransport("cluster_1_node_2", knownNodes_c1, Version.CURRENT);
+             MockTransportService c2N1 =
+                 startTransport("cluster_2_node_1", knownNodes_c2, Version.CURRENT);
+             MockTransportService c2N2 =
+                 startTransport("cluster_2_node_2", knownNodes_c2, Version.CURRENT)) {
+            final DiscoveryNode c1N1Node = c1N1.getLocalDiscoNode();
+            final DiscoveryNode c1N2Node = c1N2.getLocalDiscoNode();
+            final DiscoveryNode c2N1Node = c2N1.getLocalDiscoNode();
+            final DiscoveryNode c2N2Node = c2N2.getLocalDiscoNode();
+            knownNodes_c1.add(c1N1Node);
+            knownNodes_c1.add(c1N2Node);
+            knownNodes_c2.add(c2N1Node);
+            knownNodes_c2.add(c2N2Node);
+            Collections.shuffle(knownNodes_c1, random());
+            Collections.shuffle(knownNodes_c2, random());
+
+            try (MockTransportService transportService = MockTransportService.createNewService(
+                settings,
+                Version.CURRENT,
+                threadPool,
+                null)) {
+                transportService.start();
+                transportService.acceptIncomingRequests();
+                final Settings.Builder builder = Settings.builder();
+                builder.putArray(
+                    "search.remote.cluster_1.seeds", c1N1Node.getAddress().toString());
+                builder.putArray(
+                    "search.remote.cluster_2.seeds", c2N1Node.getAddress().toString());
+                try (RemoteClusterService service =
+                         new RemoteClusterService(settings, transportService)) {
+                    assertFalse(service.isCrossClusterSearchEnabled());
+                    service.initializeRemoteClusters();
+                    assertFalse(service.isCrossClusterSearchEnabled());
+
+                    final InetSocketAddress c1N1Address = c1N1Node.getAddress().address();
+                    final InetSocketAddress c1N2Address = c1N2Node.getAddress().address();
+                    final InetSocketAddress c2N1Address = c2N1Node.getAddress().address();
+                    final InetSocketAddress c2N2Address = c2N2Node.getAddress().address();
+
+                    final CountDownLatch firstLatch = new CountDownLatch(1);
+                    service.updateRemoteCluster(
+                        "cluster_1",
+                        Arrays.asList(c1N1Address, c1N2Address),
+                        connectionListener(firstLatch));
+                    firstLatch.await();
+
+                    final CountDownLatch secondLatch = new CountDownLatch(1);
+                    service.updateRemoteCluster(
+                        "cluster_2",
+                        Arrays.asList(c2N1Address, c2N2Address),
+                        connectionListener(secondLatch));
+                    secondLatch.await();
+                    CountDownLatch latch = new CountDownLatch(1);
+                    service.collectNodes(new HashSet<>(Arrays.asList("cluster_1", "cluster_2")),
+                        new ActionListener<BiFunction<String, String, DiscoveryNode>>() {
+                        @Override
+                        public void onResponse(BiFunction<String, String, DiscoveryNode> func) {
+                            try {
+                                assertEquals(c1N1Node, func.apply("cluster_1", c1N1Node.getId()));
+                                assertEquals(c1N2Node, func.apply("cluster_1", c1N2Node.getId()));
+                                assertEquals(c2N1Node, func.apply("cluster_2", c2N1Node.getId()));
+                                assertEquals(c2N2Node, func.apply("cluster_2", c2N2Node.getId()));
+                            } finally {
+                                latch.countDown();
+                            }
+                        }
+
+                        @Override
+                        public void onFailure(Exception e) {
+                            try {
+                                throw new AssertionError(e);
+                            } finally {
+                                latch.countDown();
+                            }
+                        }
+                    });
+                    latch.await();
+                    {
+                        CountDownLatch failLatch = new CountDownLatch(1);
+                        AtomicReference<Exception> ex = new AtomicReference<>();
+                        service.collectNodes(new HashSet<>(Arrays.asList("cluster_1", "cluster_2", "no such cluster")),
+                            new ActionListener<BiFunction<String, String, DiscoveryNode>>() {
+                                @Override
+                                public void onResponse(BiFunction<String, String, DiscoveryNode> stringStringDiscoveryNodeBiFunction) {
+                                    try {
+                                        fail("should not be called");
+                                    } finally {
+                                        failLatch.countDown();
+                                    }
+                                }
+
+                                @Override
+                                public void onFailure(Exception e) {
+                                    try {
+                                        ex.set(e);
+                                    } finally {
+                                        failLatch.countDown();
+                                    }
+                                }
+                            });
+                        failLatch.await();
+                        assertNotNull(ex.get());
+                        assertTrue(ex.get() instanceof IllegalArgumentException);
+                        assertEquals("no such remote cluster: [no such cluster]", ex.get().getMessage());
+                    }
+                    {
+                        // close all targets and check for the transport level failure path
+                        IOUtils.close(c1N1, c1N2, c2N1, c2N2);
+                        CountDownLatch failLatch = new CountDownLatch(1);
+                        AtomicReference<Exception> ex = new AtomicReference<>();
+                        service.collectNodes(new HashSet<>(Arrays.asList("cluster_1", "cluster_2")),
+                            new ActionListener<BiFunction<String, String, DiscoveryNode>>() {
+                                @Override
+                                public void onResponse(BiFunction<String, String, DiscoveryNode> stringStringDiscoveryNodeBiFunction) {
+                                    try {
+                                        fail("should not be called");
+                                    } finally {
+                                        failLatch.countDown();
+                                    }
+                                }
+
+                                @Override
+                                public void onFailure(Exception e) {
+                                    try {
+                                        ex.set(e);
+                                    } finally {
+                                        failLatch.countDown();
+                                    }
+                                }
+                            });
+                        failLatch.await();
+                        assertNotNull(ex.get());
+                        assertTrue(ex.get().getClass().toString(), ex.get() instanceof TransportException);
+                    }
+                }
+            }
+        }
+    }
 }