fetching term information from master using light-weight call before …

…fetch of cluster-state

server/src/main/java/org/opensearch/action/ActionModule.java

@@ -614,6 +614,7 @@ public <Request extends ActionRequest, Response extends ActionResponse> void reg
         actions.register(ClusterAllocationExplainAction.INSTANCE, TransportClusterAllocationExplainAction.class);
         actions.register(ClusterStatsAction.INSTANCE, TransportClusterStatsAction.class);
         actions.register(ClusterStateAction.INSTANCE, TransportClusterStateAction.class);
+        actions.register(org.opensearch.action.admin.cluster.state.term.GetTermVersionAction.INSTANCE, org.opensearch.action.admin.cluster.state.term.TransportGetTermVersionAction.class);
         actions.register(ClusterHealthAction.INSTANCE, TransportClusterHealthAction.class);
         actions.register(ClusterUpdateSettingsAction.INSTANCE, TransportClusterUpdateSettingsAction.class);
         actions.register(ClusterRerouteAction.INSTANCE, TransportClusterRerouteAction.class);

server/src/main/java/org/opensearch/action/admin/cluster/state/TransportClusterStateAction.java

@@ -231,4 +231,8 @@ private ClusterStateResponse buildResponse(final ClusterStateRequest request, fi
         return new ClusterStateResponse(currentState.getClusterName(), builder.build(), false);
     }
 
+    @Override
+    protected boolean checkTermVersion() {
+        return true;
+    }
 }

server/src/main/java/org/opensearch/action/admin/cluster/state/term/GetTermVersionAction.java

@@ -0,0 +1,31 @@
+/*
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * The OpenSearch Contributors require contributions made to
+ * this file be licensed under the Apache-2.0 license or a
+ * compatible open source license.
+ */
+
+/*
+ * Modifications Copyright OpenSearch Contributors. See
+ * GitHub history for details.
+ */
+
+package org.opensearch.action.admin.cluster.state.term;
+
+import org.opensearch.action.ActionType;
+
+/**
+ * Transport action for fetching cluster term
+ *
+ * @opensearch.internal
+ */
+public class GetTermVersionAction extends ActionType<GetTermVersionResponse> {
+
+    public static final GetTermVersionAction INSTANCE = new GetTermVersionAction();
+    public static final String NAME = "cluster:monitor/term";
+
+    private GetTermVersionAction() {
+        super(NAME, GetTermVersionResponse::new);
+    }
+}

server/src/main/java/org/opensearch/action/admin/cluster/state/term/GetTermVersionRequest.java

@@ -0,0 +1,58 @@
+/*
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * The OpenSearch Contributors require contributions made to
+ * this file be licensed under the Apache-2.0 license or a
+ * compatible open source license.
+ */
+
+/*
+ * 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.
+ */
+
+/*
+ * Modifications Copyright OpenSearch Contributors. See
+ * GitHub history for details.
+ */
+
+package org.opensearch.action.admin.cluster.state.term;
+
+import org.opensearch.action.ActionRequestValidationException;
+import org.opensearch.action.support.clustermanager.ClusterManagerNodeReadRequest;
+import org.opensearch.core.common.io.stream.StreamInput;
+
+import java.io.IOException;
+
+/**
+ * Request object to get cluster term
+ *
+ * @opensearch.internal
+ */
+public class GetTermVersionRequest extends ClusterManagerNodeReadRequest<GetTermVersionRequest> {
+
+    public GetTermVersionRequest() {}
+
+    public GetTermVersionRequest(StreamInput in) throws IOException {
+        super(in);
+    }
+
+    @Override
+    public ActionRequestValidationException validate() {
+        return null;
+    }
+}

server/src/main/java/org/opensearch/action/admin/cluster/state/term/GetTermVersionResponse.java

@@ -0,0 +1,77 @@
+/*
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * The OpenSearch Contributors require contributions made to
+ * this file be licensed under the Apache-2.0 license or a
+ * compatible open source license.
+ */
+
+package org.opensearch.action.admin.cluster.state.term;
+
+import org.opensearch.cluster.ClusterName;
+import org.opensearch.cluster.ClusterState;
+import org.opensearch.core.action.ActionResponse;
+import org.opensearch.core.common.io.stream.StreamInput;
+import org.opensearch.core.common.io.stream.StreamOutput;
+
+import java.io.IOException;
+
+/**
+ * Response object of cluster term
+ *
+ * @opensearch.internal
+ */
+public class GetTermVersionResponse extends ActionResponse {
+
+    private final ClusterName clusterName;
+    private final String stateUUID;
+    private final long term;
+    private final long version;
+
+    public GetTermVersionResponse(ClusterName clusterName, String stateUUID, long term, long version) {
+        this.clusterName = clusterName;
+        this.stateUUID = stateUUID;
+        this.term = term;
+        this.version = version;
+    }
+
+    public GetTermVersionResponse(StreamInput in) throws IOException {
+        super(in);
+        this.clusterName = new ClusterName(in);
+        this.stateUUID = in.readString();
+        this.term = in.readLong();
+        this.version = in.readLong();
+    }
+
+    @Override
+    public void writeTo(StreamOutput out) throws IOException {
+        clusterName.writeTo(out);
+        out.writeString(stateUUID);
+        out.writeLong(term);
+        out.writeLong(version);
+    }
+
+    public long getTerm() {
+        return term;
+    }
+
+    public long getVersion() {
+        return version;
+    }
+
+    public ClusterName getClusterName() {
+        return clusterName;
+    }
+
+    public String getStateUUID() {
+        return stateUUID;
+    }
+
+    public boolean matches(ClusterState clusterState) {
+        return clusterName.equals(clusterState.getClusterName())
+            && stateUUID.equals(clusterState.stateUUID())
+            && term == clusterState.term()
+            && version == clusterState.version();
+    }
+
+}

server/src/main/java/org/opensearch/action/admin/cluster/state/term/TransportGetTermVersionAction.java

@@ -0,0 +1,90 @@
+/*
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * The OpenSearch Contributors require contributions made to
+ * this file be licensed under the Apache-2.0 license or a
+ * compatible open source license.
+ */
+
+/*
+ * Modifications Copyright OpenSearch Contributors. See
+ * GitHub history for details.
+ */
+
+package org.opensearch.action.admin.cluster.state.term;
+
+import org.apache.logging.log4j.LogManager;
+import org.apache.logging.log4j.Logger;
+import org.opensearch.action.support.ActionFilters;
+import org.opensearch.action.support.clustermanager.TransportClusterManagerNodeReadAction;
+import org.opensearch.cluster.ClusterState;
+import org.opensearch.cluster.block.ClusterBlockException;
+import org.opensearch.cluster.metadata.IndexNameExpressionResolver;
+import org.opensearch.cluster.service.ClusterService;
+import org.opensearch.common.inject.Inject;
+import org.opensearch.core.action.ActionListener;
+import org.opensearch.core.common.io.stream.StreamInput;
+import org.opensearch.threadpool.ThreadPool;
+import org.opensearch.transport.TransportService;
+
+import java.io.IOException;
+
+/**
+ * Transport action for obtaining cluster term and version from cluster-manager
+ *
+ * @opensearch.internal
+ */
+public class TransportGetTermVersionAction extends TransportClusterManagerNodeReadAction<GetTermVersionRequest, GetTermVersionResponse> {
+
+    private final Logger logger = LogManager.getLogger(getClass());
+
+    @Inject
+    public TransportGetTermVersionAction(
+        TransportService transportService,
+        ClusterService clusterService,
+        ThreadPool threadPool,
+        ActionFilters actionFilters,
+        IndexNameExpressionResolver indexNameExpressionResolver
+    ) {
+        super(
+            GetTermVersionAction.NAME,
+            false,
+            transportService,
+            clusterService,
+            threadPool,
+            actionFilters,
+            GetTermVersionRequest::new,
+            indexNameExpressionResolver
+        );
+    }
+
+    @Override
+    protected String executor() {
+        return ThreadPool.Names.SAME;
+    }
+
+    @Override
+    public GetTermVersionResponse read(StreamInput in) throws IOException {
+        return new GetTermVersionResponse(in);
+    }
+
+    @Override
+    protected ClusterBlockException checkBlock(GetTermVersionRequest request, ClusterState state) {
+        // cluster state term and version needs to be retrieved even on a fully blocked cluster
+        return null;
+    }
+
+    @Override
+    protected void clusterManagerOperation(
+        GetTermVersionRequest request,
+        ClusterState state,
+        ActionListener<GetTermVersionResponse> listener
+    ) throws Exception {
+        ActionListener.completeWith(listener, () -> buildResponse(request, state));
+    }
+
+    private GetTermVersionResponse buildResponse(GetTermVersionRequest request, ClusterState state) {
+        logger.trace("Serving cluster term version request using term {} and version {}", state.term(), state.version());
+        return new GetTermVersionResponse(state.getClusterName(), state.stateUUID(), state.term(), state.getVersion());
+    }
+}

server/src/main/java/org/opensearch/action/admin/cluster/state/term/package-info.java

@@ -0,0 +1,10 @@
+/*
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * The OpenSearch Contributors require contributions made to
+ * this file be licensed under the Apache-2.0 license or a
+ * compatible open source license.
+ */
+
+/** Cluster Term transport handler. */
+package org.opensearch.action.admin.cluster.state.term;