[CCR] Changed AutoFollowCoordinator to keep track of certain statistics (#33684)

The following stats are being kept track of:
1) The total number of times that auto following a leader index succeed.
2) The total number of times that auto following a leader index failed.
3) The total number of times that fetching a remote cluster state failed.
4) The most recent 256 auto follow failures per auto leader index
   (e.g. create_and_follow api call fails) or cluster alias
   (e.g. fetching remote cluster state fails).

Each auto follow run now produces a result that is being used to update
the stats being kept track of in AutoFollowCoordinator.

Relates to #33007

x-pack/plugin/ccr/qa/multi-cluster-with-non-compliant-license/src/test/java/org/elasticsearch/xpack/ccr/CcrMultiClusterLicenseIT.java

@@ -64,7 +64,7 @@ public class CcrMultiClusterLicenseIT extends ESRestTestCase {
                 while (it.hasNext()) {
                     final String line = it.next();
                     if (line.matches(".*\\[WARN\\s*\\]\\[o\\.e\\.x\\.c\\.a\\.AutoFollowCoordinator\\s*\\] \\[node-0\\] " +
-                            "failure occurred during auto-follower coordination")) {
+                            "failure occurred while fetching cluster state in leader cluster \\[leader_cluster\\]")) {
                         warn = true;
                         break;
                     }

x-pack/plugin/ccr/src/main/java/org/elasticsearch/xpack/ccr/CcrLicenseChecker.java

@@ -64,7 +64,7 @@ public final class CcrLicenseChecker {
      *
      * @param isCcrAllowed a boolean supplier that should return true if CCR is allowed and false otherwise
      */
-    CcrLicenseChecker(final BooleanSupplier isCcrAllowed) {
+    public CcrLicenseChecker(final BooleanSupplier isCcrAllowed) {
         this.isCcrAllowed = Objects.requireNonNull(isCcrAllowed);
     }
 

x-pack/plugin/ccr/src/main/java/org/elasticsearch/xpack/ccr/action/AutoFollowCoordinator.java

@@ -7,6 +7,8 @@ package org.elasticsearch.xpack.ccr.action;
 
 import org.apache.logging.log4j.LogManager;
 import org.apache.logging.log4j.Logger;
+import org.apache.logging.log4j.message.ParameterizedMessage;
+import org.elasticsearch.ElasticsearchException;
 import org.elasticsearch.action.ActionListener;
 import org.elasticsearch.action.admin.cluster.state.ClusterStateRequest;
 import org.elasticsearch.client.Client;
@@ -17,8 +19,10 @@ import org.elasticsearch.cluster.ClusterStateUpdateTask;
 import org.elasticsearch.cluster.metadata.IndexMetaData;
 import org.elasticsearch.cluster.metadata.MetaData;
 import org.elasticsearch.cluster.service.ClusterService;
+import org.elasticsearch.common.collect.Tuple;
 import org.elasticsearch.common.settings.Settings;
 import org.elasticsearch.common.unit.TimeValue;
+import org.elasticsearch.common.util.concurrent.AtomicArray;
 import org.elasticsearch.common.util.concurrent.CountDown;
 import org.elasticsearch.index.Index;
 import org.elasticsearch.license.LicenseUtils;
@@ -27,15 +31,18 @@ import org.elasticsearch.xpack.ccr.CcrLicenseChecker;
 import org.elasticsearch.xpack.ccr.CcrSettings;
 import org.elasticsearch.xpack.core.ccr.AutoFollowMetadata;
 import org.elasticsearch.xpack.core.ccr.AutoFollowMetadata.AutoFollowPattern;
+import org.elasticsearch.xpack.core.ccr.AutoFollowStats;
 import org.elasticsearch.xpack.core.ccr.action.CreateAndFollowIndexAction;
 import org.elasticsearch.xpack.core.ccr.action.FollowIndexAction;
 
 import java.util.ArrayList;
+import java.util.Collections;
 import java.util.HashMap;
+import java.util.LinkedHashMap;
 import java.util.List;
 import java.util.Map;
 import java.util.Objects;
-import java.util.concurrent.atomic.AtomicReference;
+import java.util.TreeMap;
 import java.util.function.BiConsumer;
 import java.util.function.Consumer;
 import java.util.function.Function;
@@ -47,6 +54,7 @@ import java.util.function.Function;
 public class AutoFollowCoordinator implements ClusterStateApplier {
 
     private static final Logger LOGGER = LogManager.getLogger(AutoFollowCoordinator.class);
+    private static final int MAX_AUTO_FOLLOW_ERRORS = 256;
 
     private final Client client;
     private final TimeValue pollInterval;
@@ -56,6 +64,12 @@ public class AutoFollowCoordinator implements ClusterStateApplier {
 
     private volatile boolean localNodeMaster = false;
 
+    // The following fields are read and updated under a lock:
+    private long numberOfSuccessfulIndicesAutoFollowed = 0;
+    private long numberOfFailedIndicesAutoFollowed = 0;
+    private long numberOfFailedRemoteClusterStateRequests = 0;
+    private final LinkedHashMap<String, ElasticsearchException> recentAutoFollowErrors;
+
     public AutoFollowCoordinator(
             Settings settings,
             Client client,
@@ -69,6 +83,47 @@ public class AutoFollowCoordinator implements ClusterStateApplier {
 
         this.pollInterval = CcrSettings.CCR_AUTO_FOLLOW_POLL_INTERVAL.get(settings);
         clusterService.addStateApplier(this);
+
+        this.recentAutoFollowErrors = new LinkedHashMap<String, ElasticsearchException>() {
+            @Override
+            protected boolean removeEldestEntry(final Map.Entry<String, ElasticsearchException> eldest) {
+                return size() > MAX_AUTO_FOLLOW_ERRORS;
+            }
+        };
+    }
+
+    public synchronized AutoFollowStats getStats() {
+        return new AutoFollowStats(
+            numberOfFailedIndicesAutoFollowed,
+            numberOfFailedRemoteClusterStateRequests,
+            numberOfSuccessfulIndicesAutoFollowed,
+            new TreeMap<>(recentAutoFollowErrors)
+        );
+    }
+
+    synchronized void updateStats(List<AutoFollowResult> results) {
+        for (AutoFollowResult result : results) {
+            if (result.clusterStateFetchException != null) {
+                recentAutoFollowErrors.put(result.clusterAlias,
+                    new ElasticsearchException(result.clusterStateFetchException));
+                numberOfFailedRemoteClusterStateRequests++;
+                LOGGER.warn(new ParameterizedMessage("failure occurred while fetching cluster state in leader cluster [{}]",
+                    result.clusterAlias), result.clusterStateFetchException);
+            } else {
+                for (Map.Entry<Index, Exception> entry : result.autoFollowExecutionResults.entrySet()) {
+                    if (entry.getValue() != null) {
+                        numberOfFailedIndicesAutoFollowed++;
+                        recentAutoFollowErrors.put(result.clusterAlias + ":" + entry.getKey().getName(),
+                            new ElasticsearchException(entry.getValue()));
+                        LOGGER.warn(new ParameterizedMessage("failure occurred while auto following index [{}] in leader cluster [{}]",
+                            entry.getKey(), result.clusterAlias), entry.getValue());
+                    } else {
+                        numberOfSuccessfulIndicesAutoFollowed++;
+                    }
+                }
+            }
+
+        }
     }
 
     private void doAutoFollow() {
@@ -94,10 +149,8 @@ public class AutoFollowCoordinator implements ClusterStateApplier {
             return;
         }
 
-        Consumer<Exception> handler = e -> {
-            if (e != null) {
-                LOGGER.warn("failure occurred during auto-follower coordination", e);
-            }
+        Consumer<List<AutoFollowResult>> handler = results -> {
+            updateStats(results);
             threadPool.schedule(pollInterval, ThreadPool.Names.SAME, this::doAutoFollow);
         };
         AutoFollower operation = new AutoFollower(handler, followerClusterState) {
@@ -178,101 +231,97 @@ public class AutoFollowCoordinator implements ClusterStateApplier {
 
     abstract static class AutoFollower {
 
-        private final Consumer<Exception> handler;
+        private final Consumer<List<AutoFollowResult>> handler;
         private final ClusterState followerClusterState;
         private final AutoFollowMetadata autoFollowMetadata;
 
         private final CountDown autoFollowPatternsCountDown;
-        private final AtomicReference<Exception> autoFollowPatternsErrorHolder = new AtomicReference<>();
+        private final AtomicArray<AutoFollowResult> autoFollowResults;
 
-        AutoFollower(final Consumer<Exception> handler, final ClusterState followerClusterState) {
+        AutoFollower(final Consumer<List<AutoFollowResult>> handler, final ClusterState followerClusterState) {
             this.handler = handler;
             this.followerClusterState = followerClusterState;
             this.autoFollowMetadata = followerClusterState.getMetaData().custom(AutoFollowMetadata.TYPE);
             this.autoFollowPatternsCountDown = new CountDown(autoFollowMetadata.getPatterns().size());
+            this.autoFollowResults = new AtomicArray<>(autoFollowMetadata.getPatterns().size());
         }
 
         void autoFollowIndices() {
+            int i = 0;
             for (Map.Entry<String, AutoFollowPattern> entry : autoFollowMetadata.getPatterns().entrySet()) {
-                String clusterAlias = entry.getKey();
-                AutoFollowPattern autoFollowPattern = entry.getValue();
-                List<String> followedIndices = autoFollowMetadata.getFollowedLeaderIndexUUIDs().get(clusterAlias);
+                final int slot = i;
+                final String clusterAlias = entry.getKey();
+                final AutoFollowPattern autoFollowPattern = entry.getValue();
 
                 getLeaderClusterState(autoFollowPattern.getHeaders(), clusterAlias, (leaderClusterState, e) -> {
                     if (leaderClusterState != null) {
                         assert e == null;
-                        handleClusterAlias(clusterAlias, autoFollowPattern, followedIndices, leaderClusterState);
+                        final List<String> followedIndices = autoFollowMetadata.getFollowedLeaderIndexUUIDs().get(clusterAlias);
+                        final List<Index> leaderIndicesToFollow =
+                            getLeaderIndicesToFollow(autoFollowPattern, leaderClusterState, followerClusterState, followedIndices);
+                        if (leaderIndicesToFollow.isEmpty()) {
+                            finalise(slot, new AutoFollowResult(clusterAlias));
+                        } else {
+                            Consumer<AutoFollowResult> resultHandler = result -> finalise(slot, result);
+                            checkAutoFollowPattern(clusterAlias, autoFollowPattern, leaderIndicesToFollow, resultHandler);
+                        }
                     } else {
-                        finalise(e);
+                        finalise(slot, new AutoFollowResult(clusterAlias, e));
                     }
                 });
+                i++;
             }
         }
 
-        private void handleClusterAlias(String clusterAlias, AutoFollowPattern autoFollowPattern,
-                                List<String> followedIndexUUIDs, ClusterState leaderClusterState) {
-            final List<Index> leaderIndicesToFollow =
-                getLeaderIndicesToFollow(autoFollowPattern, leaderClusterState, followerClusterState, followedIndexUUIDs);
-            if (leaderIndicesToFollow.isEmpty()) {
-                finalise(null);
-            } else {
-                final CountDown leaderIndicesCountDown = new CountDown(leaderIndicesToFollow.size());
-                final AtomicReference<Exception> leaderIndicesErrorHolder = new AtomicReference<>();
-                for (Index indexToFollow : leaderIndicesToFollow) {
-                    final String leaderIndexName = indexToFollow.getName();
-                    final String followIndexName = getFollowerIndexName(autoFollowPattern, leaderIndexName);
-
-                    String leaderIndexNameWithClusterAliasPrefix = clusterAlias.equals("_local_") ? leaderIndexName :
-                        clusterAlias + ":" + leaderIndexName;
-                    FollowIndexAction.Request followRequest =
-                        new FollowIndexAction.Request(leaderIndexNameWithClusterAliasPrefix, followIndexName,
-                            autoFollowPattern.getMaxBatchOperationCount(), autoFollowPattern.getMaxConcurrentReadBatches(),
-                            autoFollowPattern.getMaxOperationSizeInBytes(), autoFollowPattern.getMaxConcurrentWriteBatches(),
-                            autoFollowPattern.getMaxWriteBufferSize(), autoFollowPattern.getMaxRetryDelay(),
-                            autoFollowPattern.getIdleShardRetryDelay());
-
-                    // Execute if the create and follow api call succeeds:
-                    Runnable successHandler = () -> {
-                        LOGGER.info("Auto followed leader index [{}] as follow index [{}]", leaderIndexName, followIndexName);
-
-                        // This function updates the auto follow metadata in the cluster to record that the leader index has been followed:
-                        // (so that we do not try to follow it in subsequent auto follow runs)
-                        Function<ClusterState, ClusterState> function = recordLeaderIndexAsFollowFunction(clusterAlias, indexToFollow);
-                        // The coordinator always runs on the elected master node, so we can update cluster state here:
-                        updateAutoFollowMetadata(function, updateError -> {
-                            if (updateError != null) {
-                                LOGGER.error("Failed to mark leader index [" + leaderIndexName + "] as auto followed", updateError);
-                                if (leaderIndicesErrorHolder.compareAndSet(null, updateError) == false) {
-                                    leaderIndicesErrorHolder.get().addSuppressed(updateError);
-                                }
-                            } else {
-                                LOGGER.debug("Successfully marked leader index [{}] as auto followed", leaderIndexName);
-                            }
-                            if (leaderIndicesCountDown.countDown()) {
-                                finalise(leaderIndicesErrorHolder.get());
-                            }
-                        });
-                    };
-                    // Execute if the create and follow apu call fails:
-                    Consumer<Exception> failureHandler = followError -> {
-                        assert followError != null;
-                        LOGGER.warn("Failed to auto follow leader index [" + leaderIndexName + "]", followError);
-                        if (leaderIndicesCountDown.countDown()) {
-                            finalise(followError);
-                        }
-                    };
-                    createAndFollow(autoFollowPattern.getHeaders(), followRequest, successHandler, failureHandler);
-                }
+        private void checkAutoFollowPattern(String clusterAlias, AutoFollowPattern autoFollowPattern,
+                                            List<Index> leaderIndicesToFollow, Consumer<AutoFollowResult> resultHandler) {
+
+            final CountDown leaderIndicesCountDown = new CountDown(leaderIndicesToFollow.size());
+            final AtomicArray<Tuple<Index, Exception>> results = new AtomicArray<>(leaderIndicesToFollow.size());
+            for (int i = 0; i < leaderIndicesToFollow.size(); i++) {
+                final Index indexToFollow = leaderIndicesToFollow.get(i);
+                final int slot = i;
+                followLeaderIndex(clusterAlias, indexToFollow, autoFollowPattern, error -> {
+                    results.set(slot, new Tuple<>(indexToFollow, error));
+                    if (leaderIndicesCountDown.countDown()) {
+                        resultHandler.accept(new AutoFollowResult(clusterAlias, results.asList()));
+                    }
+                });
             }
         }
 
-        private void finalise(Exception failure) {
-            if (autoFollowPatternsErrorHolder.compareAndSet(null, failure) == false) {
-                autoFollowPatternsErrorHolder.get().addSuppressed(failure);
-            }
+        private void followLeaderIndex(String clusterAlias, Index indexToFollow,
+                                       AutoFollowPattern pattern, Consumer<Exception> onResult) {
+            final String leaderIndexName = indexToFollow.getName();
+            final String followIndexName = getFollowerIndexName(pattern, leaderIndexName);
+
+            String leaderIndexNameWithClusterAliasPrefix = clusterAlias.equals("_local_") ? leaderIndexName :
+                clusterAlias + ":" + leaderIndexName;
+            FollowIndexAction.Request request =
+                new FollowIndexAction.Request(leaderIndexNameWithClusterAliasPrefix, followIndexName,
+                    pattern.getMaxBatchOperationCount(), pattern.getMaxConcurrentReadBatches(),
+                    pattern.getMaxOperationSizeInBytes(), pattern.getMaxConcurrentWriteBatches(),
+                    pattern.getMaxWriteBufferSize(), pattern.getMaxRetryDelay(),
+                    pattern.getIdleShardRetryDelay());
+
+            // Execute if the create and follow api call succeeds:
+            Runnable successHandler = () -> {
+                LOGGER.info("Auto followed leader index [{}] as follow index [{}]", leaderIndexName, followIndexName);
+
+                // This function updates the auto follow metadata in the cluster to record that the leader index has been followed:
+                // (so that we do not try to follow it in subsequent auto follow runs)
+                Function<ClusterState, ClusterState> function = recordLeaderIndexAsFollowFunction(clusterAlias, indexToFollow);
+                // The coordinator always runs on the elected master node, so we can update cluster state here:
+                updateAutoFollowMetadata(function, onResult);
+            };
+            createAndFollow(pattern.getHeaders(), request, successHandler, onResult);
+        }
 
+        private void finalise(int slot, AutoFollowResult result) {
+            assert autoFollowResults.get(slot) == null;
+            autoFollowResults.set(slot, result);
             if (autoFollowPatternsCountDown.countDown()) {
-                handler.accept(autoFollowPatternsErrorHolder.get());
+                handler.accept(autoFollowResults.asList());
             }
         }
 
@@ -347,4 +396,33 @@ public class AutoFollowCoordinator implements ClusterStateApplier {
         );
 
     }
+
+    static class AutoFollowResult {
+
+        final String clusterAlias;
+        final Exception clusterStateFetchException;
+        final Map<Index, Exception> autoFollowExecutionResults;
+
+        AutoFollowResult(String clusterAlias, List<Tuple<Index, Exception>> results) {
+            this.clusterAlias = clusterAlias;
+
+            Map<Index, Exception> autoFollowExecutionResults = new HashMap<>();
+            for (Tuple<Index, Exception> result : results) {
+                autoFollowExecutionResults.put(result.v1(), result.v2());
+            }
+
+            this.clusterStateFetchException = null;
+            this.autoFollowExecutionResults = Collections.unmodifiableMap(autoFollowExecutionResults);
+        }
+
+        AutoFollowResult(String clusterAlias, Exception e) {
+            this.clusterAlias = clusterAlias;
+            this.clusterStateFetchException = e;
+            this.autoFollowExecutionResults = Collections.emptyMap();
+        }
+
+        AutoFollowResult(String clusterAlias) {
+            this(clusterAlias, (Exception) null);
+        }
+    }
 }

x-pack/plugin/ccr/src/test/java/org/elasticsearch/xpack/ccr/action/AutoFollowCoordinatorTests.java

@@ -11,15 +11,20 @@ import org.elasticsearch.cluster.ClusterName;
 import org.elasticsearch.cluster.ClusterState;
 import org.elasticsearch.cluster.metadata.IndexMetaData;
 import org.elasticsearch.cluster.metadata.MetaData;
+import org.elasticsearch.cluster.service.ClusterService;
+import org.elasticsearch.common.collect.Tuple;
 import org.elasticsearch.common.settings.Settings;
 import org.elasticsearch.index.Index;
 import org.elasticsearch.test.ESTestCase;
+import org.elasticsearch.xpack.ccr.CcrLicenseChecker;
 import org.elasticsearch.xpack.ccr.action.AutoFollowCoordinator.AutoFollower;
 import org.elasticsearch.xpack.core.ccr.AutoFollowMetadata;
 import org.elasticsearch.xpack.core.ccr.AutoFollowMetadata.AutoFollowPattern;
+import org.elasticsearch.xpack.core.ccr.AutoFollowStats;
 import org.elasticsearch.xpack.core.ccr.action.FollowIndexAction;
 
 import java.util.ArrayList;
+import java.util.Arrays;
 import java.util.Collections;
 import java.util.Comparator;
 import java.util.HashMap;
@@ -63,9 +68,15 @@ public class AutoFollowCoordinatorTests extends ESTestCase {
             .build();
 
         boolean[] invoked = new boolean[]{false};
-        Consumer<Exception> handler = e -> {
+        Consumer<List<AutoFollowCoordinator.AutoFollowResult>> handler = results -> {
             invoked[0] = true;
-            assertThat(e, nullValue());
+
+            assertThat(results.size(), equalTo(1));
+            assertThat(results.get(0).clusterStateFetchException, nullValue());
+            List<Map.Entry<Index, Exception>> entries = new ArrayList<>(results.get(0).autoFollowExecutionResults.entrySet());
+            assertThat(entries.size(), equalTo(1));
+            assertThat(entries.get(0).getKey().getName(), equalTo("logs-20190101"));
+            assertThat(entries.get(0).getValue(), nullValue());
         };
         AutoFollower autoFollower = new AutoFollower(handler, currentState) {
             @Override
@@ -116,9 +127,12 @@ public class AutoFollowCoordinatorTests extends ESTestCase {
 
         Exception failure = new RuntimeException("failure");
         boolean[] invoked = new boolean[]{false};
-        Consumer<Exception> handler = e -> {
+        Consumer<List<AutoFollowCoordinator.AutoFollowResult>> handler = results -> {
             invoked[0] = true;
-            assertThat(e, sameInstance(failure));
+
+            assertThat(results.size(), equalTo(1));
+            assertThat(results.get(0).clusterStateFetchException, sameInstance(failure));
+            assertThat(results.get(0).autoFollowExecutionResults.entrySet().size(), equalTo(0));
         };
         AutoFollower autoFollower = new AutoFollower(handler, followerState) {
             @Override
@@ -170,9 +184,15 @@ public class AutoFollowCoordinatorTests extends ESTestCase {
 
         Exception failure = new RuntimeException("failure");
         boolean[] invoked = new boolean[]{false};
-        Consumer<Exception> handler = e -> {
+        Consumer<List<AutoFollowCoordinator.AutoFollowResult>> handler = results -> {
             invoked[0] = true;
-            assertThat(e, sameInstance(failure));
+
+            assertThat(results.size(), equalTo(1));
+            assertThat(results.get(0).clusterStateFetchException, nullValue());
+            List<Map.Entry<Index, Exception>> entries = new ArrayList<>(results.get(0).autoFollowExecutionResults.entrySet());
+            assertThat(entries.size(), equalTo(1));
+            assertThat(entries.get(0).getKey().getName(), equalTo("logs-20190101"));
+            assertThat(entries.get(0).getValue(), sameInstance(failure));
         };
         AutoFollower autoFollower = new AutoFollower(handler, followerState) {
             @Override
@@ -225,9 +245,15 @@ public class AutoFollowCoordinatorTests extends ESTestCase {
 
         Exception failure = new RuntimeException("failure");
         boolean[] invoked = new boolean[]{false};
-        Consumer<Exception> handler = e -> {
+        Consumer<List<AutoFollowCoordinator.AutoFollowResult>> handler = results -> {
             invoked[0] = true;
-            assertThat(e, sameInstance(failure));
+
+            assertThat(results.size(), equalTo(1));
+            assertThat(results.get(0).clusterStateFetchException, nullValue());
+            List<Map.Entry<Index, Exception>> entries = new ArrayList<>(results.get(0).autoFollowExecutionResults.entrySet());
+            assertThat(entries.size(), equalTo(1));
+            assertThat(entries.get(0).getKey().getName(), equalTo("logs-20190101"));
+            assertThat(entries.get(0).getValue(), sameInstance(failure));
         };
         AutoFollower autoFollower = new AutoFollower(handler, followerState) {
             @Override
@@ -317,4 +343,63 @@ public class AutoFollowCoordinatorTests extends ESTestCase {
         assertThat(AutoFollower.getFollowerIndexName(autoFollowPattern, "metrics-0"), equalTo("eu-metrics-0"));
     }
 
+    public void testStats() {
+        AutoFollowCoordinator autoFollowCoordinator = new AutoFollowCoordinator(
+            Settings.EMPTY,
+            null,
+            null,
+            mock(ClusterService.class),
+            new CcrLicenseChecker(() -> true)
+        );
+
+        autoFollowCoordinator.updateStats(Collections.singletonList(
+            new AutoFollowCoordinator.AutoFollowResult("_alias1"))
+        );
+        AutoFollowStats autoFollowStats = autoFollowCoordinator.getStats();
+        assertThat(autoFollowStats.getNumberOfFailedFollowIndices(), equalTo(0L));
+        assertThat(autoFollowStats.getNumberOfFailedRemoteClusterStateRequests(), equalTo(0L));
+        assertThat(autoFollowStats.getNumberOfSuccessfulFollowIndices(), equalTo(0L));
+        assertThat(autoFollowStats.getRecentAutoFollowErrors().size(), equalTo(0));
+
+        autoFollowCoordinator.updateStats(Collections.singletonList(
+            new AutoFollowCoordinator.AutoFollowResult("_alias1", new RuntimeException("error")))
+        );
+        autoFollowStats = autoFollowCoordinator.getStats();
+        assertThat(autoFollowStats.getNumberOfFailedFollowIndices(), equalTo(0L));
+        assertThat(autoFollowStats.getNumberOfFailedRemoteClusterStateRequests(), equalTo(1L));
+        assertThat(autoFollowStats.getNumberOfSuccessfulFollowIndices(), equalTo(0L));
+        assertThat(autoFollowStats.getRecentAutoFollowErrors().size(), equalTo(1));
+        assertThat(autoFollowStats.getRecentAutoFollowErrors().get("_alias1").getCause().getMessage(), equalTo("error"));
+
+        autoFollowCoordinator.updateStats(Arrays.asList(
+            new AutoFollowCoordinator.AutoFollowResult("_alias1",
+                Collections.singletonList(Tuple.tuple(new Index("index1", "_na_"), new RuntimeException("error")))),
+            new AutoFollowCoordinator.AutoFollowResult("_alias2",
+                Collections.singletonList(Tuple.tuple(new Index("index2", "_na_"), new RuntimeException("error"))))
+        ));
+        autoFollowStats = autoFollowCoordinator.getStats();
+        assertThat(autoFollowStats.getNumberOfFailedFollowIndices(), equalTo(2L));
+        assertThat(autoFollowStats.getNumberOfFailedRemoteClusterStateRequests(), equalTo(1L));
+        assertThat(autoFollowStats.getNumberOfSuccessfulFollowIndices(), equalTo(0L));
+        assertThat(autoFollowStats.getRecentAutoFollowErrors().size(), equalTo(3));
+        assertThat(autoFollowStats.getRecentAutoFollowErrors().get("_alias1").getCause().getMessage(), equalTo("error"));
+        assertThat(autoFollowStats.getRecentAutoFollowErrors().get("_alias1:index1").getCause().getMessage(), equalTo("error"));
+        assertThat(autoFollowStats.getRecentAutoFollowErrors().get("_alias2:index2").getCause().getMessage(), equalTo("error"));
+
+        autoFollowCoordinator.updateStats(Arrays.asList(
+            new AutoFollowCoordinator.AutoFollowResult("_alias1",
+                Collections.singletonList(Tuple.tuple(new Index("index1", "_na_"), null))),
+            new AutoFollowCoordinator.AutoFollowResult("_alias2",
+                Collections.singletonList(Tuple.tuple(new Index("index2", "_na_"), null)))
+        ));
+        autoFollowStats = autoFollowCoordinator.getStats();
+        assertThat(autoFollowStats.getNumberOfFailedFollowIndices(), equalTo(2L));
+        assertThat(autoFollowStats.getNumberOfFailedRemoteClusterStateRequests(), equalTo(1L));
+        assertThat(autoFollowStats.getNumberOfSuccessfulFollowIndices(), equalTo(2L));
+        assertThat(autoFollowStats.getRecentAutoFollowErrors().size(), equalTo(3));
+        assertThat(autoFollowStats.getRecentAutoFollowErrors().get("_alias1").getCause().getMessage(), equalTo("error"));
+        assertThat(autoFollowStats.getRecentAutoFollowErrors().get("_alias1:index1").getCause().getMessage(), equalTo("error"));
+        assertThat(autoFollowStats.getRecentAutoFollowErrors().get("_alias2:index2").getCause().getMessage(), equalTo("error"));
+    }
+
 }

x-pack/plugin/ccr/src/test/java/org/elasticsearch/xpack/ccr/action/AutoFollowStatsTests.java

@@ -0,0 +1,77 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the Elastic License;
+ * you may not use this file except in compliance with the Elastic License.
+ */
+package org.elasticsearch.xpack.ccr.action;
+
+import org.elasticsearch.ElasticsearchException;
+import org.elasticsearch.common.io.stream.Writeable;
+import org.elasticsearch.common.xcontent.XContentParser;
+import org.elasticsearch.test.AbstractSerializingTestCase;
+import org.elasticsearch.xpack.core.ccr.AutoFollowStats;
+
+import java.io.IOException;
+import java.util.Map;
+import java.util.NavigableMap;
+import java.util.TreeMap;
+
+import static org.hamcrest.Matchers.anyOf;
+import static org.hamcrest.Matchers.containsString;
+import static org.hamcrest.Matchers.equalTo;
+import static org.hamcrest.Matchers.instanceOf;
+
+public class AutoFollowStatsTests extends AbstractSerializingTestCase<AutoFollowStats> {
+
+    @Override
+    protected AutoFollowStats doParseInstance(XContentParser parser) throws IOException {
+        return AutoFollowStats.fromXContent(parser);
+    }
+
+    @Override
+    protected AutoFollowStats createTestInstance() {
+        return new AutoFollowStats(
+            randomNonNegativeLong(),
+            randomNonNegativeLong(),
+            randomNonNegativeLong(),
+            randomReadExceptions()
+        );
+    }
+
+    private static NavigableMap<String, ElasticsearchException> randomReadExceptions() {
+        final int count = randomIntBetween(0, 16);
+        final NavigableMap<String, ElasticsearchException> readExceptions = new TreeMap<>();
+        for (int i = 0; i < count; i++) {
+            readExceptions.put("" + i, new ElasticsearchException(new IllegalStateException("index [" + i + "]")));
+        }
+        return readExceptions;
+    }
+
+    @Override
+    protected Writeable.Reader<AutoFollowStats> instanceReader() {
+        return AutoFollowStats::new;
+    }
+
+    @Override
+    protected void assertEqualInstances(AutoFollowStats expectedInstance, AutoFollowStats newInstance) {
+        assertNotSame(expectedInstance, newInstance);
+
+        assertThat(newInstance.getRecentAutoFollowErrors().size(), equalTo(expectedInstance.getRecentAutoFollowErrors().size()));
+        assertThat(newInstance.getRecentAutoFollowErrors().keySet(), equalTo(expectedInstance.getRecentAutoFollowErrors().keySet()));
+        for (final Map.Entry<String, ElasticsearchException> entry : newInstance.getRecentAutoFollowErrors().entrySet()) {
+            // x-content loses the exception
+            final ElasticsearchException expected = expectedInstance.getRecentAutoFollowErrors().get(entry.getKey());
+            assertThat(entry.getValue().getMessage(), containsString(expected.getMessage()));
+            assertNotNull(entry.getValue().getCause());
+            assertThat(
+                entry.getValue().getCause(),
+                anyOf(instanceOf(ElasticsearchException.class), instanceOf(IllegalStateException.class)));
+            assertThat(entry.getValue().getCause().getMessage(), containsString(expected.getCause().getMessage()));
+        }
+    }
+
+    @Override
+    protected boolean assertToXContentEquivalence() {
+        return false;
+    }
+}

x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/ccr/AutoFollowStats.java

@@ -0,0 +1,194 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the Elastic License;
+ * you may not use this file except in compliance with the Elastic License.
+ */
+package org.elasticsearch.xpack.core.ccr;
+
+import org.elasticsearch.ElasticsearchException;
+import org.elasticsearch.common.ParseField;
+import org.elasticsearch.common.io.stream.StreamInput;
+import org.elasticsearch.common.io.stream.StreamOutput;
+import org.elasticsearch.common.io.stream.Writeable;
+import org.elasticsearch.common.xcontent.ConstructingObjectParser;
+import org.elasticsearch.common.xcontent.ToXContentObject;
+import org.elasticsearch.common.xcontent.XContentBuilder;
+import org.elasticsearch.common.xcontent.XContentParser;
+
+import java.io.IOException;
+import java.util.AbstractMap;
+import java.util.List;
+import java.util.Map;
+import java.util.NavigableMap;
+import java.util.Objects;
+import java.util.TreeMap;
+import java.util.stream.Collectors;
+
+public class AutoFollowStats implements Writeable, ToXContentObject {
+
+    private static final ParseField NUMBER_OF_SUCCESSFUL_INDICES_AUTO_FOLLOWED = new ParseField("number_of_successful_follow_indices");
+    private static final ParseField NUMBER_OF_FAILED_INDICES_AUTO_FOLLOWED = new ParseField("number_of_failed_follow_indices");
+    private static final ParseField NUMBER_OF_FAILED_REMOTE_CLUSTER_STATE_REQUESTS =
+        new ParseField("number_of_failed_remote_cluster_state_requests");
+    private static final ParseField RECENT_AUTO_FOLLOW_ERRORS = new ParseField("recent_auto_follow_errors");
+    private static final ParseField LEADER_INDEX = new ParseField("leader_index");
+    private static final ParseField AUTO_FOLLOW_EXCEPTION = new ParseField("auto_follow_exception");
+
+    @SuppressWarnings("unchecked")
+    private static final ConstructingObjectParser<AutoFollowStats, Void> STATS_PARSER = new ConstructingObjectParser<>("auto_follow_stats",
+        args -> new AutoFollowStats(
+            (Long) args[0],
+            (Long) args[1],
+            (Long) args[2],
+            new TreeMap<>(
+                ((List<Map.Entry<String, ElasticsearchException>>) args[3])
+                    .stream()
+                    .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue)))
+        ));
+
+    private static final ConstructingObjectParser<Map.Entry<String, ElasticsearchException>, Void> AUTO_FOLLOW_EXCEPTIONS_PARSER =
+        new ConstructingObjectParser<>(
+            "auto_follow_stats_errors",
+            args -> new AbstractMap.SimpleEntry<>((String) args[0], (ElasticsearchException) args[1]));
+
+    static {
+        AUTO_FOLLOW_EXCEPTIONS_PARSER.declareString(ConstructingObjectParser.constructorArg(), LEADER_INDEX);
+        AUTO_FOLLOW_EXCEPTIONS_PARSER.declareObject(
+            ConstructingObjectParser.constructorArg(),
+            (p, c) -> ElasticsearchException.fromXContent(p),
+            AUTO_FOLLOW_EXCEPTION);
+
+        STATS_PARSER.declareLong(ConstructingObjectParser.constructorArg(), NUMBER_OF_FAILED_INDICES_AUTO_FOLLOWED);
+        STATS_PARSER.declareLong(ConstructingObjectParser.constructorArg(), NUMBER_OF_FAILED_REMOTE_CLUSTER_STATE_REQUESTS);
+        STATS_PARSER.declareLong(ConstructingObjectParser.constructorArg(), NUMBER_OF_SUCCESSFUL_INDICES_AUTO_FOLLOWED);
+        STATS_PARSER.declareObjectArray(ConstructingObjectParser.constructorArg(), AUTO_FOLLOW_EXCEPTIONS_PARSER,
+            RECENT_AUTO_FOLLOW_ERRORS);
+    }
+
+    public static AutoFollowStats fromXContent(final XContentParser parser) {
+        return STATS_PARSER.apply(parser, null);
+    }
+
+    private final long numberOfFailedFollowIndices;
+    private final long numberOfFailedRemoteClusterStateRequests;
+    private final long numberOfSuccessfulFollowIndices;
+    private final NavigableMap<String, ElasticsearchException> recentAutoFollowErrors;
+
+    public AutoFollowStats(
+            long numberOfFailedFollowIndices,
+            long numberOfFailedRemoteClusterStateRequests,
+            long numberOfSuccessfulFollowIndices,
+            NavigableMap<String, ElasticsearchException> recentAutoFollowErrors
+    ) {
+        this.numberOfFailedFollowIndices = numberOfFailedFollowIndices;
+        this.numberOfFailedRemoteClusterStateRequests = numberOfFailedRemoteClusterStateRequests;
+        this.numberOfSuccessfulFollowIndices = numberOfSuccessfulFollowIndices;
+        this.recentAutoFollowErrors = recentAutoFollowErrors;
+    }
+
+    public AutoFollowStats(StreamInput in) throws IOException {
+        numberOfFailedFollowIndices = in.readVLong();
+        numberOfFailedRemoteClusterStateRequests = in.readVLong();
+        numberOfSuccessfulFollowIndices = in.readVLong();
+        recentAutoFollowErrors= new TreeMap<>(in.readMap(StreamInput::readString, StreamInput::readException));
+    }
+
+    @Override
+    public void writeTo(StreamOutput out) throws IOException {
+        out.writeVLong(numberOfFailedFollowIndices);
+        out.writeVLong(numberOfFailedRemoteClusterStateRequests);
+        out.writeVLong(numberOfSuccessfulFollowIndices);
+        out.writeMap(recentAutoFollowErrors, StreamOutput::writeString, StreamOutput::writeException);
+    }
+
+    public long getNumberOfFailedFollowIndices() {
+        return numberOfFailedFollowIndices;
+    }
+
+    public long getNumberOfFailedRemoteClusterStateRequests() {
+        return numberOfFailedRemoteClusterStateRequests;
+    }
+
+    public long getNumberOfSuccessfulFollowIndices() {
+        return numberOfSuccessfulFollowIndices;
+    }
+
+    public NavigableMap<String, ElasticsearchException> getRecentAutoFollowErrors() {
+        return recentAutoFollowErrors;
+    }
+
+    @Override
+    public XContentBuilder toXContent(XContentBuilder builder, Params params) throws IOException {
+        builder.startObject();
+        {
+            builder.field(NUMBER_OF_FAILED_INDICES_AUTO_FOLLOWED.getPreferredName(), numberOfFailedFollowIndices);
+            builder.field(NUMBER_OF_FAILED_REMOTE_CLUSTER_STATE_REQUESTS.getPreferredName(), numberOfFailedRemoteClusterStateRequests);
+            builder.field(NUMBER_OF_SUCCESSFUL_INDICES_AUTO_FOLLOWED.getPreferredName(), numberOfSuccessfulFollowIndices);
+            builder.startArray(RECENT_AUTO_FOLLOW_ERRORS.getPreferredName());
+            {
+                for (final Map.Entry<String, ElasticsearchException> entry : recentAutoFollowErrors.entrySet()) {
+                    builder.startObject();
+                    {
+                        builder.field(LEADER_INDEX.getPreferredName(), entry.getKey());
+                        builder.field(AUTO_FOLLOW_EXCEPTION.getPreferredName());
+                        builder.startObject();
+                        {
+                            ElasticsearchException.generateThrowableXContent(builder, params, entry.getValue());
+                        }
+                        builder.endObject();
+                    }
+                    builder.endObject();
+                }
+            }
+            builder.endArray();
+        }
+        builder.endObject();
+        return builder;
+    }
+
+    @Override
+    public boolean equals(Object o) {
+        if (this == o) return true;
+        if (o == null || getClass() != o.getClass()) return false;
+        AutoFollowStats that = (AutoFollowStats) o;
+        return numberOfFailedFollowIndices == that.numberOfFailedFollowIndices &&
+            numberOfFailedRemoteClusterStateRequests == that.numberOfFailedRemoteClusterStateRequests &&
+            numberOfSuccessfulFollowIndices == that.numberOfSuccessfulFollowIndices &&
+            /*
+             * ElasticsearchException does not implement equals so we will assume the fetch exceptions are equal if they are equal
+             * up to the key set and their messages.  Note that we are relying on the fact that the auto follow exceptions are ordered by
+             * keys.
+             */
+            recentAutoFollowErrors.keySet().equals(that.recentAutoFollowErrors.keySet()) &&
+            getFetchExceptionMessages(this).equals(getFetchExceptionMessages(that));
+    }
+
+    @Override
+    public int hashCode() {
+        return Objects.hash(
+            numberOfFailedFollowIndices,
+            numberOfFailedRemoteClusterStateRequests,
+            numberOfSuccessfulFollowIndices,
+            /*
+             * ElasticsearchException does not implement hash code so we will compute the hash code based on the key set and the
+             * messages. Note that we are relying on the fact that the auto follow exceptions are ordered by keys.
+             */
+            recentAutoFollowErrors.keySet(),
+            getFetchExceptionMessages(this)
+        );
+    }
+
+    private static List<String> getFetchExceptionMessages(final AutoFollowStats status) {
+        return status.getRecentAutoFollowErrors().values().stream().map(ElasticsearchException::getMessage).collect(Collectors.toList());
+    }
+
+    @Override
+    public String toString() {
+        return "AutoFollowStats{" +
+            "numberOfFailedFollowIndices=" + numberOfFailedFollowIndices +
+            ", numberOfFailedRemoteClusterStateRequests=" + numberOfFailedRemoteClusterStateRequests +
+            ", numberOfSuccessfulFollowIndices=" + numberOfSuccessfulFollowIndices +
+            ", recentAutoFollowErrors=" + recentAutoFollowErrors +
+            '}';
+    }
+}