Cut over to use affix setting for remote cluster configuration

Instead of `search.remote.seeds.${clustername}` we now specify the seeds as:
`search.remote.${clustername}.seeds` which is a real list setting compared to an unvalidated
group setting before.

core/src/main/java/org/elasticsearch/action/search/RemoteClusterConnection.java

@@ -236,6 +236,10 @@ final class RemoteClusterConnection extends AbstractComponent implements Transpo
         connectHandler.close();
     }
 
+    public boolean isClosed() {
+        return connectHandler.isClosed();
+    }
+
     /**
      * The connect handler manages node discovery and the actual connect to the remote cluster.
      * There is at most one connect job running at any time. If such a connect job is triggered
@@ -350,6 +354,9 @@ final class RemoteClusterConnection extends AbstractComponent implements Transpo
 
         void collectRemoteNodes(Iterator<DiscoveryNode> seedNodes,
                                 final TransportService transportService, ActionListener<Void> listener) {
+            if (Thread.currentThread().isInterrupted()) {
+                listener.onFailure(new InterruptedException("remote connect thread got interrupted"));
+            }
             try {
                 if (seedNodes.hasNext()) {
                     cancellableThreads.executeIO(() -> {
@@ -409,6 +416,10 @@ final class RemoteClusterConnection extends AbstractComponent implements Transpo
             }
         }
 
+        final boolean isClosed() {
+            return closed.get();
+        }
+
         /* This class handles the _state response from the remote cluster when sniffing nodes to connect to */
         private class SniffClusterStateResponseHandler implements TransportResponseHandler<ClusterStateResponse> {
 

core/src/main/java/org/elasticsearch/action/search/RemoteClusterService.java

@@ -59,6 +59,8 @@ import java.util.concurrent.TimeoutException;
 import java.util.concurrent.atomic.AtomicReference;
 import java.util.function.Function;
 import java.util.function.Predicate;
+import java.util.stream.Collectors;
+import java.util.stream.Stream;
 
 /**
  * Basic service for accessing remote clusters via gateway nodes
@@ -68,11 +70,9 @@ public final class RemoteClusterService extends AbstractComponent implements Clo
     /**
      * A list of initial seed nodes to discover eligible nodes from the remote cluster
      */
-    //TODO this should be an affix settings?
-    public static final Setting<Settings> REMOTE_CLUSTERS_SEEDS = Setting.groupSetting("search.remote.seeds.",
-            RemoteClusterService::validateRemoteClustersSeeds,
-            Setting.Property.NodeScope,
-            Setting.Property.Dynamic);
+    public static final Setting.AffixSetting<List<InetSocketAddress>> REMOTE_CLUSTERS_SEEDS = Setting.affixKeySetting("search.remote.",
+        "seeds", (key) -> Setting.listSetting(key, Collections.emptyList(), RemoteClusterService::parseSeedAddress,
+            Setting.Property.NodeScope, Setting.Property.Dynamic));
     /**
      * The maximum number of connections that will be established to a remote cluster. For instance if there is only a single
      * seed node, other nodes will be discovered up to the given number of nodes in this setting. The default is 3.
@@ -109,12 +109,11 @@ public final class RemoteClusterService extends AbstractComponent implements Clo
 
     /**
      * This method updates the list of remote clusters. It's intended to be used as an update consumer on the settings infrastructure
-     * @param seedSettings the group settings returned from {@link #REMOTE_CLUSTERS_SEEDS}
+     * @param seeds a cluster alias to discovery node mapping representing the remote clusters seeds nodes
      * @param connectionListener a listener invoked once every configured cluster has been connected to
      */
-    void updateRemoteClusters(Settings seedSettings, ActionListener<Void> connectionListener) {
+    private synchronized void updateRemoteClusters(Map<String, List<DiscoveryNode>> seeds, ActionListener<Void> connectionListener) {
         Map<String, RemoteClusterConnection> remoteClusters = new HashMap<>();
-        Map<String, List<DiscoveryNode>> seeds = buildRemoteClustersSeeds(seedSettings);
         if (seeds.isEmpty()) {
             connectionListener.onResponse(null);
         } else {
@@ -126,13 +125,27 @@ public final class RemoteClusterService extends AbstractComponent implements Clo
                 String attribute = REMOTE_NODE_ATTRIBUTE.get(settings);
                 nodePredicate = nodePredicate.and((node) -> Boolean.getBoolean(node.getAttributes().getOrDefault(attribute, "false")));
             }
+            remoteClusters.putAll(this.remoteClusters);
             for (Map.Entry<String, List<DiscoveryNode>> entry : seeds.entrySet()) {
                 RemoteClusterConnection remote = this.remoteClusters.get(entry.getKey());
-                if (remote == null) {
+                if (entry.getValue().isEmpty()) { // with no seed nodes we just remove the connection
+                    try {
+                        IOUtils.close(remote);
+                    } catch (IOException e) {
+                        logger.warn("failed to close remote cluster connections for cluster: " + entry.getKey(), e);
+                    }
+                    remoteClusters.remove(entry.getKey());
+                    continue;
+                }
+
+                if (remote == null) { // this is a new cluster we have to add a new representation
                     remote = new RemoteClusterConnection(settings, entry.getKey(), entry.getValue(), transportService, numRemoteConnections,
                         nodePredicate);
                     remoteClusters.put(entry.getKey(), remote);
                 }
+
+                // now update the seed nodes no matter if it's new or already existing
+                RemoteClusterConnection finalRemote = remote;
                 remote.updateSeedNodes(entry.getValue(), ActionListener.wrap(
                     response -> {
                         if (countDown.countDown()) {
@@ -143,14 +156,13 @@ public final class RemoteClusterService extends AbstractComponent implements Clo
                         if (countDown.fastForward()) {
                             connectionListener.onFailure(exception);
                         }
-                        logger.error("failed to update seed list for cluster: " + entry.getKey(), exception);
+                        if (finalRemote.isClosed() == false) {
+                            logger.warn("failed to update seed list for cluster: " + entry.getKey(), exception);
+                        }
                     }));
             }
         }
-        if (remoteClusters.isEmpty() == false) {
-            remoteClusters.putAll(this.remoteClusters);
-            this.remoteClusters = Collections.unmodifiableMap(remoteClusters);
-        }
+        this.remoteClusters = Collections.unmodifiableMap(remoteClusters);
     }
 
     /**
@@ -296,65 +308,55 @@ public final class RemoteClusterService extends AbstractComponent implements Clo
         return connection.getConnection(node);
     }
 
+    public void updateRemoteCluster(String clusterAlias, List<InetSocketAddress> addresses) {
+        updateRemoteClusters(Collections.singletonMap(clusterAlias, addresses.stream().map(address -> {
+                TransportAddress transportAddress = new TransportAddress(address);
+                return new DiscoveryNode(clusterAlias + "#" + transportAddress.toString(),
+                    transportAddress,
+                    Version.CURRENT.minimumCompatibilityVersion());
+            }).collect(Collectors.toList())),
+            ActionListener.wrap((x) -> {}, (x) -> {}) );
+    }
 
     static Map<String, List<DiscoveryNode>> buildRemoteClustersSeeds(Settings settings) {
-        Map<String, List<DiscoveryNode>> remoteClustersNodes = new HashMap<>();
-        for (String clusterName : settings.names()) {
-            String[] remoteHosts = settings.getAsArray(clusterName);
-            for (String remoteHost : remoteHosts) {
-                int portSeparator = remoteHost.lastIndexOf(':'); // in case we have a IPv6 address ie. [::1]:9300
-                String host = remoteHost.substring(0, portSeparator);
-                InetAddress hostAddress;
-                try {
-                    hostAddress = InetAddress.getByName(host);
-                } catch (UnknownHostException e) {
-                    throw new IllegalArgumentException("unknown host [" + host + "]", e);
-                }
-                int port = Integer.valueOf(remoteHost.substring(portSeparator + 1));
-                DiscoveryNode node = new DiscoveryNode(clusterName + "#" + remoteHost,
-                    new TransportAddress(new InetSocketAddress(hostAddress, port)),
+        Stream<Setting<List<InetSocketAddress>>> allConcreteSettings = REMOTE_CLUSTERS_SEEDS.getAllConcreteSettings(settings);
+        return allConcreteSettings.collect(
+            Collectors.toMap(REMOTE_CLUSTERS_SEEDS::getNamespace,  concreteSetting -> {
+            String clusterName = REMOTE_CLUSTERS_SEEDS.getNamespace(concreteSetting);
+            List<DiscoveryNode> nodes = new ArrayList<>();
+            for (InetSocketAddress address : concreteSetting.get(settings)) {
+                TransportAddress transportAddress = new TransportAddress(address);
+                DiscoveryNode node = new DiscoveryNode(clusterName + "#" + transportAddress.toString(),
+                    transportAddress,
                     Version.CURRENT.minimumCompatibilityVersion());
-                List<DiscoveryNode> nodes = remoteClustersNodes.get(clusterName);
-                if (nodes == null) {
-                    nodes = new ArrayList<>();
-                    remoteClustersNodes.put(clusterName, nodes);
-                }
                 nodes.add(node);
             }
-        }
-        return remoteClustersNodes;
+            return nodes;
+        }));
     }
 
-    static void validateRemoteClustersSeeds(Settings settings) {
-        for (String clusterName : settings.names()) {
-            String[] remoteHosts = settings.getAsArray(clusterName);
-            if (remoteHosts.length == 0) {
-                throw new IllegalArgumentException("no hosts set for remote cluster [" + clusterName + "], at least one host is required");
-            }
-            for (String remoteHost : remoteHosts) {
-                int portSeparator = remoteHost.lastIndexOf(':'); // in case we have a IPv6 address ie. [::1]:9300
-                if (portSeparator == -1 || portSeparator == remoteHost.length()) {
-                    throw new IllegalArgumentException("remote hosts need to be configured as [host:port], found [" + remoteHost + "] " +
-                        "instead for remote cluster [" + clusterName + "]");
-                }
-                String host = remoteHost.substring(0, portSeparator);
-                try {
-                    InetAddress.getByName(host);
-                } catch (UnknownHostException e) {
-                    throw new IllegalArgumentException("unknown host [" + host + "]", e);
-                }
-                String port = remoteHost.substring(portSeparator + 1);
-                try {
-                    Integer portValue = Integer.valueOf(port);
-                    if (portValue <= 0) {
-                        throw new IllegalArgumentException("port number must be > 0 but was: [" + portValue + "]");
-                    }
-                } catch (NumberFormatException e) {
-                    throw new IllegalArgumentException("port must be a number, found [" + port + "] instead for remote cluster [" +
-                        clusterName + "]");
-                }
+    static final InetSocketAddress parseSeedAddress(String remoteHost) {
+        int portSeparator = remoteHost.lastIndexOf(':'); // in case we have a IPv6 address ie. [::1]:9300
+        if (portSeparator == -1 || portSeparator == remoteHost.length()) {
+            throw new IllegalArgumentException("remote hosts need to be configured as [host:port], found [" + remoteHost + "] instead");
+        }
+        String host = remoteHost.substring(0, portSeparator);
+        InetAddress hostAddress;
+        try {
+            hostAddress = InetAddress.getByName(host);
+        } catch (UnknownHostException e) {
+            throw new IllegalArgumentException("unknown host [" + host + "]", e);
+        }
+        try {
+            int port = Integer.valueOf(remoteHost.substring(portSeparator + 1));
+            if (port <= 0) {
+                throw new IllegalArgumentException("port number must be > 0 but was: [" + port + "]");
             }
+            return new InetSocketAddress(hostAddress, port);
+        } catch (NumberFormatException e) {
+            throw new IllegalArgumentException("port must be a number");
         }
+
     }
 
     /**
@@ -364,7 +366,8 @@ public final class RemoteClusterService extends AbstractComponent implements Clo
     void initializeRemoteClusters() {
         final TimeValue timeValue = REMOTE_INITIAL_CONNECTION_TIMEOUT_SETTING.get(settings);
         final PlainActionFuture<Void> future = new PlainActionFuture<>();
-        updateRemoteClusters(REMOTE_CLUSTERS_SEEDS.get(settings), future);
+        Map<String, List<DiscoveryNode>> seeds = buildRemoteClustersSeeds(settings);
+        updateRemoteClusters(seeds, future);
         try {
             future.get(timeValue.millis(), TimeUnit.MILLISECONDS);
         } catch (InterruptedException e) {

core/src/main/java/org/elasticsearch/action/search/SearchTransportService.java

@@ -83,10 +83,8 @@ public class SearchTransportService extends AbstractLifecycleComponent {
         super(settings);
         this.transportService = transportService;
         this.remoteClusterService = new RemoteClusterService(settings, transportService);
-        final Consumer<Settings> clusterUpdateConsumer = (s) -> remoteClusterService.updateRemoteClusters(s,
-            ActionListener.wrap((x) -> {}, (x) -> {}));
-        clusterSettings.addSettingsUpdateConsumer(RemoteClusterService.REMOTE_CLUSTERS_SEEDS, clusterUpdateConsumer,
-            RemoteClusterService::validateRemoteClustersSeeds);
+        clusterSettings.addAffixUpdateConsumer(RemoteClusterService.REMOTE_CLUSTERS_SEEDS, remoteClusterService::updateRemoteCluster,
+            (namespace, value) -> {});
     }
 
     public void sendFreeContext(Transport.Connection connection, final long contextId, SearchRequest request) {

core/src/main/java/org/elasticsearch/common/settings/Setting.java

@@ -42,6 +42,7 @@ import java.io.IOException;
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.EnumSet;
+import java.util.HashMap;
 import java.util.IdentityHashMap;
 import java.util.List;
 import java.util.Map;
@@ -480,7 +481,7 @@ public class Setting<T> extends ToXContentToBytes {
                 public Map<AbstractScopedSettings.SettingUpdater<T>, T> getValue(Settings current, Settings previous) {
                     // we collect all concrete keys and then delegate to the actual setting for validation and settings extraction
                     final Map<AbstractScopedSettings.SettingUpdater<T>, T> result = new IdentityHashMap<>();
-                    Stream.concat(matchStream(current), matchStream(previous)).forEach(aKey -> {
+                    Stream.concat(matchStream(current), matchStream(previous)).distinct().forEach(aKey -> {
                         String namespace = key.getNamespace(aKey);
                         AbstractScopedSettings.SettingUpdater<T> updater =
                             getConcreteSetting(aKey).newUpdater((v) -> consumer.accept(namespace, v), logger,
@@ -504,6 +505,18 @@ public class Setting<T> extends ToXContentToBytes {
             };
         }
 
+        @Override
+        public T get(Settings settings) {
+            throw new UnsupportedOperationException("affix settings can't return values" +
+                " use #getConcreteSetting to obtain a concrete setting");
+        }
+
+        @Override
+        public String getRaw(Settings settings) {
+            throw new UnsupportedOperationException("affix settings can't return values" +
+                " use #getConcreteSetting to obtain a concrete setting");
+        }
+
         @Override
         public Setting<T> getConcreteSetting(String key) {
             if (match(key)) {
@@ -517,6 +530,22 @@ public class Setting<T> extends ToXContentToBytes {
         public void diff(Settings.Builder builder, Settings source, Settings defaultSettings) {
             matchStream(defaultSettings).forEach((key) -> getConcreteSetting(key).diff(builder, source, defaultSettings));
         }
+
+        /**
+         * Returns the namespace for a concrete settting. Ie. an affix setting with prefix: <tt>search.</tt> and suffix: <tt>username</tt>
+         * will return <tt>remote</tt> as a namespace for the setting <tt>search.remote.username</tt>
+         */
+        public String getNamespace(Setting<T> concreteSetting) {
+            return key.getNamespace(concreteSetting.getKey());
+        }
+
+        /**
+         * Returns a stream of all concrete setting instances for the given settings. AffixSetting is only a specification, concrete
+         * settings depend on an actual set of setting keys.
+         */
+        public Stream<Setting<T>> getAllConcreteSettings(Settings settings) {
+            return matchStream(settings).distinct().map(this::getConcreteSetting);
+        }
     }
 
 

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

@@ -477,8 +477,10 @@ public abstract class TcpTransport<Channel> extends AbstractLifecycleComponent i
 
     @Override
     public final NodeChannels openConnection(DiscoveryNode node, ConnectionProfile connectionProfile) throws IOException {
+        boolean success = false;
+        NodeChannels nodeChannels = null;
         try {
-            NodeChannels nodeChannels = connectToChannels(node, connectionProfile);
+            nodeChannels = connectToChannels(node, connectionProfile);
             final Channel channel = nodeChannels.getChannels().get(0); // one channel is guaranteed by the connection profile
             final TimeValue connectTimeout = connectionProfile.getConnectTimeout() == null ?
                 defaultConnectionProfile.getConnectTimeout() :
@@ -487,13 +489,19 @@ public abstract class TcpTransport<Channel> extends AbstractLifecycleComponent i
                 connectTimeout : connectionProfile.getHandshakeTimeout();
             final Version version = executeHandshake(node, channel, handshakeTimeout);
             transportServiceAdapter.onConnectionOpened(node);
-            return new NodeChannels(nodeChannels, version); // clone the channels - we now have the correct version
+            nodeChannels = new NodeChannels(nodeChannels, version);// clone the channels - we now have the correct version
+            success = true;
+            return nodeChannels;
         } catch (ConnectTransportException e) {
             throw e;
         } catch (Exception e) {
             // ConnectTransportExceptions are handled specifically on the caller end - we wrap the actual exception to ensure
             // only relevant exceptions are logged on the caller end.. this is the same as in connectToNode
             throw new ConnectTransportException(node, "general node connection failure", e);
+        } finally {
+            if (success == false) {
+                IOUtils.closeWhileHandlingException(nodeChannels);
+            }
         }
     }
 

core/src/test/java/org/elasticsearch/action/search/RemoteClusterServiceTests.java

@@ -85,20 +85,20 @@ public class RemoteClusterServiceTests extends ESTestCase {
 
     public void testRemoteClusterSeedSetting() {
         // simple validation
-        RemoteClusterService.REMOTE_CLUSTERS_SEEDS.get(Settings.builder()
-            .put("search.remote.seeds.foo", "192.168.0.1:8080")
-            .put("search.remote.seeds.bar", "[::1]:9090").build());
+        Settings settings = Settings.builder()
+            .put("search.remote.foo.seeds", "192.168.0.1:8080")
+            .put("search.remote.bar.seed", "[::1]:9090").build();
+        RemoteClusterService.REMOTE_CLUSTERS_SEEDS.getAllConcreteSettings(settings).forEach(setting -> setting.get(settings));
 
+        Settings brokenSettings = Settings.builder()
+            .put("search.remote.foo.seeds", "192.168.0.1").build();
         expectThrows(IllegalArgumentException.class, () ->
-        RemoteClusterService.REMOTE_CLUSTERS_SEEDS.get(Settings.builder()
-            .put("search.remote.seeds.foo", "192.168.0.1").build()));
+        RemoteClusterService.REMOTE_CLUSTERS_SEEDS.getAllConcreteSettings(brokenSettings).forEach(setting -> setting.get(brokenSettings)));
     }
 
     public void testBuiltRemoteClustersSeeds() throws Exception {
         Map<String, List<DiscoveryNode>> map = RemoteClusterService.buildRemoteClustersSeeds(
-            RemoteClusterService.REMOTE_CLUSTERS_SEEDS.get(Settings.builder()
-            .put("search.remote.seeds.foo", "192.168.0.1:8080")
-            .put("search.remote.seeds.bar", "[::1]:9090").build()));
+            Settings.builder().put("search.remote.foo.seeds", "192.168.0.1:8080").put("search.remote.bar.seeds", "[::1]:9090").build());
         assertEquals(2, map.size());
         assertTrue(map.containsKey("foo"));
         assertTrue(map.containsKey("bar"));
@@ -133,8 +133,8 @@ public class RemoteClusterServiceTests extends ESTestCase {
                 transportService.start();
                 transportService.acceptIncomingRequests();
                 Settings.Builder builder = Settings.builder();
-                builder.putArray("search.remote.seeds.cluster_1", seedNode.getAddress().toString());
-                builder.putArray("search.remote.seeds.cluster_2", otherSeedNode.getAddress().toString());
+                builder.putArray("search.remote.cluster_1.seeds", seedNode.getAddress().toString());
+                builder.putArray("search.remote.cluster_2.seeds", otherSeedNode.getAddress().toString());
                 try (RemoteClusterService service = new RemoteClusterService(builder.build(), transportService)) {
                     assertFalse(service.isCrossClusterSearchEnabled());
                     service.initializeRemoteClusters();
@@ -154,6 +154,41 @@ public class RemoteClusterServiceTests extends ESTestCase {
         }
     }
 
+    public void testIncrementallyAddClusters() throws IOException {
+        List<DiscoveryNode> knownNodes = new CopyOnWriteArrayList<>();
+        try (MockTransportService seedTransport = startTransport("cluster_1_node", knownNodes, Version.CURRENT);
+             MockTransportService otherSeedTransport = startTransport("cluster_2_node", knownNodes, Version.CURRENT)) {
+            DiscoveryNode seedNode = seedTransport.getLocalDiscoNode();
+            DiscoveryNode otherSeedNode = otherSeedTransport.getLocalDiscoNode();
+            knownNodes.add(seedTransport.getLocalDiscoNode());
+            knownNodes.add(otherSeedTransport.getLocalDiscoNode());
+            Collections.shuffle(knownNodes, random());
+
+            try (MockTransportService transportService = MockTransportService.createNewService(Settings.EMPTY, Version.CURRENT, threadPool,
+                null)) {
+                transportService.start();
+                transportService.acceptIncomingRequests();
+                Settings.Builder builder = Settings.builder();
+                builder.putArray("search.remote.cluster_1.seeds", seedNode.getAddress().toString());
+                builder.putArray("search.remote.cluster_2.seeds", otherSeedNode.getAddress().toString());
+                try (RemoteClusterService service = new RemoteClusterService(Settings.EMPTY, transportService)) {
+                    assertFalse(service.isCrossClusterSearchEnabled());
+                    service.initializeRemoteClusters();
+                    assertFalse(service.isCrossClusterSearchEnabled());
+                    service.updateRemoteCluster("cluster_1", Collections.singletonList(seedNode.getAddress().address()));
+                    assertTrue(service.isCrossClusterSearchEnabled());
+                    assertTrue(service.isRemoteClusterRegistered("cluster_1"));
+                    service.updateRemoteCluster("cluster_2", Collections.singletonList(otherSeedNode.getAddress().address()));
+                    assertTrue(service.isCrossClusterSearchEnabled());
+                    assertTrue(service.isRemoteClusterRegistered("cluster_1"));
+                    assertTrue(service.isRemoteClusterRegistered("cluster_2"));
+                    service.updateRemoteCluster("cluster_2", Collections.emptyList());
+                    assertFalse(service.isRemoteClusterRegistered("cluster_2"));
+                }
+            }
+        }
+    }
+
     public void testProcessRemoteShards() throws IOException {
         try (RemoteClusterService service = new RemoteClusterService(Settings.EMPTY, null)) {
             assertFalse(service.isCrossClusterSearchEnabled());

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

@@ -31,6 +31,8 @@ import java.util.List;
 import java.util.Map;
 import java.util.concurrent.atomic.AtomicReference;
 import java.util.function.Function;
+import java.util.stream.Collectors;
+import java.util.stream.Stream;
 
 import static org.hamcrest.Matchers.containsString;
 import static org.hamcrest.Matchers.equalTo;
@@ -475,6 +477,33 @@ public class SettingTests extends ESTestCase {
         assertFalse(listAffixSetting.match("foo"));
     }
 
+    public void testGetAllConcreteSettings() {
+        Setting.AffixSetting<List<String>> listAffixSetting = Setting.affixKeySetting("foo.", "bar",
+            (key) -> Setting.listSetting(key, Collections.emptyList(), Function.identity(), Property.NodeScope));
+
+        Settings settings = Settings.builder()
+            .putArray("foo.1.bar", "1", "2")
+            .putArray("foo.2.bar", "3", "4", "5")
+            .putArray("foo.bar", "6")
+            .putArray("some.other", "6")
+            .putArray("foo.3.bar", "6")
+            .build();
+        Stream<Setting<List<String>>> allConcreteSettings = listAffixSetting.getAllConcreteSettings(settings);
+        Map<String, List<String>> collect = allConcreteSettings.collect(Collectors.toMap(Setting::getKey, (s) -> s.get(settings)));
+        assertEquals(3, collect.size());
+        assertEquals(Arrays.asList("1", "2"), collect.get("foo.1.bar"));
+        assertEquals(Arrays.asList("3", "4", "5"), collect.get("foo.2.bar"));
+        assertEquals(Arrays.asList("6"), collect.get("foo.3.bar"));
+    }
+
+    public void testAffixSettingsFailOnGet() {
+        Setting.AffixSetting<List<String>> listAffixSetting = Setting.affixKeySetting("foo.", "bar",
+            (key) -> Setting.listSetting(key, Collections.singletonList("testelement"), Function.identity(), Property.NodeScope));
+        expectThrows(UnsupportedOperationException.class, () -> listAffixSetting.get(Settings.EMPTY));
+        expectThrows(UnsupportedOperationException.class, () -> listAffixSetting.getRaw(Settings.EMPTY));
+        assertEquals(Collections.singletonList("testelement"), listAffixSetting.getDefault(Settings.EMPTY));
+        assertEquals("[\"testelement\"]", listAffixSetting.getDefaultRaw(Settings.EMPTY));
+    }
 
     public void testMinMaxInt() {
         Setting<Integer> integerSetting = Setting.intSetting("foo.bar", 1, 0, 10, Property.NodeScope);

docs/reference/modules/cross-cluster-search.asciidoc

@@ -24,9 +24,10 @@ remote clusters that should be connected to, for instance:
 --------------------------------
 search:
     remote:
-        seeds:
-            cluster_one: 127.0.0.1:9300 <1>
-            cluster_two: 127.0.0.1:9301 <1>
+        cluster_one: <1>
+            seeds: 127.0.0.1:9300
+        cluster_two: <1>
+            seeds: 127.0.0.1:9301
 
 --------------------------------
 <1> `cluster_one` and `cluster_two` are arbitrary names representing the connection to each cluster. These names are subsequently used to distinguish between local and remote indices.

qa/multi-cluster-search/build.gradle

@@ -35,7 +35,7 @@ task mixedClusterTest(type: RestIntegTestTask) {
   dependsOn(remoteClusterTest)
   cluster {
     distribution = 'zip'
-    setting 'search.remote.seeds.my_remote_cluster', "\"${-> remoteClusterTest.nodes.get(0).transportUri()}\""
+    setting 'search.remote.my_remote_cluster.seeds', "\"${-> remoteClusterTest.nodes.get(0).transportUri()}\""
     setting 'search.remote.connections_per_cluster', 1
 
   }

qa/multi-cluster-search/src/test/resources/rest-api-spec/test/multi_cluster/10_basic.yaml

@@ -99,16 +99,16 @@
     cluster.get_settings:
       include_defaults: true
 
-  - set: { defaults.search.remote.seeds.my_remote_cluster: remote_ip }
+  - set: { defaults.search.remote.my_remote_cluster.seeds.0: remote_ip }
 
   - do:
     cluster.put_settings:
       flat_settings: true
       body:
         transient:
-          search.remote.seeds.test_remote_cluster: $remote_ip
+          search.remote.test_remote_cluster.seeds: $remote_ip
 
-  - match: {transient: {search.remote.seeds.test_remote_cluster: $remote_ip}}
+  - match: {transient: {search.remote.test_remote_cluster.seeds: $remote_ip}}
 
   - do:
     search: