Adding a setting for data stream reindexing concurrency

x-pack/plugin/migrate/src/main/java/org/elasticsearch/xpack/migrate/MigratePlugin.java

@@ -16,6 +16,7 @@
 import org.elasticsearch.common.io.stream.NamedWriteableRegistry;
 import org.elasticsearch.common.settings.ClusterSettings;
 import org.elasticsearch.common.settings.IndexScopedSettings;
+import org.elasticsearch.common.settings.Setting;
 import org.elasticsearch.common.settings.Settings;
 import org.elasticsearch.common.settings.SettingsFilter;
 import org.elasticsearch.common.settings.SettingsModule;
@@ -58,6 +59,7 @@
 import java.util.function.Supplier;
 
 import static org.elasticsearch.xpack.migrate.action.ReindexDataStreamAction.REINDEX_DATA_STREAM_FEATURE_FLAG;
+import static org.elasticsearch.xpack.migrate.task.ReindexDataStreamPersistentTaskExecutor.MAX_CONCURRENT_INDICES_REINDEXED_PER_DATA_STREAM_SETTING;
 
 public class MigratePlugin extends Plugin implements ActionPlugin, PersistentTaskPlugin {
 
@@ -153,4 +155,11 @@ public List<PersistentTasksExecutor<?>> getPersistentTasksExecutor(
             return List.of();
         }
     }
+
+    @Override
+    public List<Setting<?>> getSettings() {
+        List<Setting<?>> pluginSettings = new ArrayList<>();
+        pluginSettings.add(MAX_CONCURRENT_INDICES_REINDEXED_PER_DATA_STREAM_SETTING);
+        return pluginSettings;
+    }
 }

x-pack/plugin/migrate/src/main/java/org/elasticsearch/xpack/migrate/task/ReindexDataStreamPersistentTaskExecutor.java

@@ -7,6 +7,8 @@
 
 package org.elasticsearch.xpack.migrate.task;
 
+import org.apache.logging.log4j.LogManager;
+import org.apache.logging.log4j.Logger;
 import org.elasticsearch.ElasticsearchException;
 import org.elasticsearch.action.ActionListener;
 import org.elasticsearch.action.admin.indices.rollover.RolloverAction;
@@ -19,6 +21,7 @@
 import org.elasticsearch.cluster.metadata.DataStream;
 import org.elasticsearch.cluster.metadata.DataStreamAction;
 import org.elasticsearch.cluster.service.ClusterService;
+import org.elasticsearch.common.settings.Setting;
 import org.elasticsearch.core.Nullable;
 import org.elasticsearch.core.TimeValue;
 import org.elasticsearch.index.Index;
@@ -39,6 +42,19 @@
 import static org.elasticsearch.xpack.core.deprecation.DeprecatedIndexPredicate.getReindexRequiredPredicate;
 
 public class ReindexDataStreamPersistentTaskExecutor extends PersistentTasksExecutor<ReindexDataStreamTaskParams> {
+    /*
+     * This setting controls how many indices we reindex concurrently for a single data stream. This is not an overall limit -- if five
+     * data streams are being reindexed, then each of them can have this many indices being reindexed at once. This setting is dynamic,
+     * but changing it does not have an impact if the task is already running (unless the task is restarted or moves to another node).
+     */
+    public static final Setting<Integer> MAX_CONCURRENT_INDICES_REINDEXED_PER_DATA_STREAM_SETTING = Setting.intSetting(
+        "migrate.max_concurrent_indices_reindexed_per_data_stream",
+        1,
+        1,
+        Setting.Property.Dynamic,
+        Setting.Property.NodeScope
+    );
+    private static final Logger logger = LogManager.getLogger(ReindexDataStreamPersistentTaskExecutor.class);
     private static final TimeValue TASK_KEEP_ALIVE_TIME = TimeValue.timeValueDays(1);
     private final Client client;
     private final ClusterService clusterService;
@@ -165,8 +181,9 @@ private void reindexIndices(
         CountDownActionListener listener = new CountDownActionListener(indicesToBeReindexed.size() + 1, ActionListener.wrap(response1 -> {
             completeSuccessfulPersistentTask(reindexDataStreamTask, updatedState);
         }, exception -> { completeFailedPersistentTask(reindexDataStreamTask, updatedState, exception); }));
+        final int maxConcurrentIndices = clusterService.getClusterSettings().get(MAX_CONCURRENT_INDICES_REINDEXED_PER_DATA_STREAM_SETTING);
         List<Index> indicesRemaining = Collections.synchronizedList(new ArrayList<>(indicesToBeReindexed));
-        final int maxConcurrentIndices = 1;
+        logger.debug("Reindexing {} indices, with up to {} handled concurrently", indicesRemaining.size(), maxConcurrentIndices);
         for (int i = 0; i < maxConcurrentIndices; i++) {
             maybeProcessNextIndex(indicesRemaining, reindexDataStreamTask, reindexClient, sourceDataStream, listener, parentTaskId);
         }