From b328dbc65b1cbf200cc291e452cfdd71c889b78b Mon Sep 17 00:00:00 2001
From: Harsh Garg <gkharsh@amazon.com>
Date: Wed, 23 Oct 2024 10:19:48 +0530
Subject: [PATCH] Avoid making further stats calls if paginationStrategy
 outputs empty entities

Signed-off-by: Harsh Garg <gkharsh@amazon.com>
---
 .../shards/TransportCatShardsAction.java      | 19 +++++++++++
 .../rest/action/cat/RestIndicesAction.java    | 32 +++++++++++++++----
 2 files changed, 44 insertions(+), 7 deletions(-)

diff --git a/server/src/main/java/org/opensearch/action/admin/cluster/shards/TransportCatShardsAction.java b/server/src/main/java/org/opensearch/action/admin/cluster/shards/TransportCatShardsAction.java
index 3dc8c38152a16..1ac4d6068725b 100644
--- a/server/src/main/java/org/opensearch/action/admin/cluster/shards/TransportCatShardsAction.java
+++ b/server/src/main/java/org/opensearch/action/admin/cluster/shards/TransportCatShardsAction.java
@@ -12,6 +12,7 @@
 import org.opensearch.action.admin.cluster.state.ClusterStateResponse;
 import org.opensearch.action.admin.indices.stats.IndicesStatsRequest;
 import org.opensearch.action.admin.indices.stats.IndicesStatsResponse;
+import org.opensearch.action.admin.indices.stats.ShardStats;
 import org.opensearch.action.pagination.PageParams;
 import org.opensearch.action.pagination.ShardPaginationStrategy;
 import org.opensearch.action.support.ActionFilters;
@@ -27,6 +28,7 @@
 import org.opensearch.tasks.Task;
 import org.opensearch.transport.TransportService;
 
+import java.util.Collections;
 import java.util.Objects;
 
 import static org.opensearch.common.breaker.ResponseLimitSettings.LimitEntity.SHARDS;
@@ -40,6 +42,13 @@ public class TransportCatShardsAction extends HandledTransportAction<CatShardsRe
 
     private final NodeClient client;
     private final ResponseLimitSettings responseLimitSettings;
+    private static final IndicesStatsResponse EMPTY_INDICES_STATS_RESPONSE = new IndicesStatsResponse(
+        new ShardStats[0],
+        0,
+        0,
+        0,
+        Collections.emptyList()
+    );
 
     @Inject
     public TransportCatShardsAction(
@@ -108,6 +117,12 @@ public void onResponse(ClusterStateResponse clusterStateResponse) {
                                 : paginationStrategy.getRequestedEntities()
                         );
                         catShardsResponse.setPageToken(Objects.isNull(paginationStrategy) ? null : paginationStrategy.getResponseToken());
+                        // For paginated queries, if strategy outputs no shards to be returned, avoid fetching IndicesStats.
+                        if (shouldSkipIndicesStatsRequest(paginationStrategy)) {
+                            catShardsResponse.setIndicesStatsResponse(EMPTY_INDICES_STATS_RESPONSE);
+                            cancellableListener.onResponse(catShardsResponse);
+                            return;
+                        }
                         IndicesStatsRequest indicesStatsRequest = new IndicesStatsRequest();
                         indicesStatsRequest.setShouldCancelOnTimeout(true);
                         indicesStatsRequest.all();
@@ -159,4 +174,8 @@ private void validateRequestLimit(
             }
         }
     }
+
+    private boolean shouldSkipIndicesStatsRequest(ShardPaginationStrategy paginationStrategy) {
+        return Objects.nonNull(paginationStrategy) && paginationStrategy.getRequestedEntities().isEmpty();
+    }
 }
diff --git a/server/src/main/java/org/opensearch/rest/action/cat/RestIndicesAction.java b/server/src/main/java/org/opensearch/rest/action/cat/RestIndicesAction.java
index b566ba9bbb8e4..805ea9130aff1 100644
--- a/server/src/main/java/org/opensearch/rest/action/cat/RestIndicesAction.java
+++ b/server/src/main/java/org/opensearch/rest/action/cat/RestIndicesAction.java
@@ -43,6 +43,7 @@
 import org.opensearch.action.admin.indices.stats.IndexStats;
 import org.opensearch.action.admin.indices.stats.IndicesStatsRequest;
 import org.opensearch.action.admin.indices.stats.IndicesStatsResponse;
+import org.opensearch.action.admin.indices.stats.ShardStats;
 import org.opensearch.action.pagination.IndexPaginationStrategy;
 import org.opensearch.action.pagination.PageToken;
 import org.opensearch.action.support.GroupedActionListener;
@@ -104,6 +105,13 @@ public class RestIndicesAction extends AbstractListAction {
         "Parameter [master_timeout] is deprecated and will be removed in 3.0. To support inclusive language, please use [cluster_manager_timeout] instead.";
     private static final String DUPLICATE_PARAMETER_ERROR_MESSAGE =
         "Please only use one of the request parameters [master_timeout, cluster_manager_timeout].";
+    private static final IndicesStatsResponse EMPTY_INDICES_STATS_RESPONSE = new IndicesStatsResponse(
+        new ShardStats[0],
+        0,
+        0,
+        0,
+        Collections.emptyList()
+    );
 
     private final ResponseLimitSettings responseLimitSettings;
 
@@ -212,13 +220,19 @@ public void onResponse(ClusterStateResponse clusterStateResponse) {
                                     groupedListener.onResponse(getSettingsResponse);
                                     groupedListener.onResponse(clusterStateResponse);
 
-                                    sendIndicesStatsRequest(
-                                        indicesToBeQueried,
-                                        subRequestIndicesOptions,
-                                        includeUnloadedSegments,
-                                        client,
-                                        ActionListener.wrap(groupedListener::onResponse, groupedListener::onFailure)
-                                    );
+                                    // For paginated queries, if strategy outputs no indices to be returned,
+                                    // avoid fetching indices stats.
+                                    if (shouldSkipIndicesStatsRequest(paginationStrategy)) {
+                                        groupedListener.onResponse(EMPTY_INDICES_STATS_RESPONSE);
+                                    } else {
+                                        sendIndicesStatsRequest(
+                                            indicesToBeQueried,
+                                            subRequestIndicesOptions,
+                                            includeUnloadedSegments,
+                                            client,
+                                            ActionListener.wrap(groupedListener::onResponse, groupedListener::onFailure)
+                                        );
+                                    }
 
                                     sendClusterHealthRequest(
                                         indicesToBeQueried,
@@ -1093,4 +1107,8 @@ public Tuple<String, Settings> next() {
         };
     }
 
+    private boolean shouldSkipIndicesStatsRequest(IndexPaginationStrategy paginationStrategy) {
+        return Objects.nonNull(paginationStrategy) && paginationStrategy.getRequestedEntities().isEmpty();
+    }
+
 }