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Author: pawankartik-elastic

server/src/main/java/org/elasticsearch/transport/RemoteClusterService.java

@@ -203,6 +203,16 @@ public Map<String, OriginalIndices> groupIndices(
     ) {
         final Map<String, OriginalIndices> originalIndicesMap = new HashMap<>();
         final Map<String, List<String>> groupedIndices;
+        /*
+         * returnLocalAll is used to control whether we'd like to fallback to the local cluster.
+         * While this is acceptable in a few cases, there are cases where we should not fallback to the local
+         * cluster. Consider _resolve/cluster where the specified patterns do not match any remote clusters.
+         * Falling back to the local cluster and returning its details in such cases is not ok. This is why
+         * TransportResolveClusterAction sets returnLocalAll to false wherever it uses groupIndices().
+         *
+         * If such a fallback isn't allowed and the given indices match a pattern whose semantics mean that
+         * it's ok to return an empty result (denoted via ["*", "-*"]), empty groupIndices.
+         */
         if (returnLocalAll == false && IndexNameExpressionResolver.isNoneExpression(indices)) {
             groupedIndices = Map.of();
         } else {