POC: SPI for CPS

server/src/main/java/org/elasticsearch/node/NodeConstruction.java

@@ -203,6 +203,7 @@
 import org.elasticsearch.search.SearchService;
 import org.elasticsearch.search.SearchUtils;
 import org.elasticsearch.search.aggregations.support.AggregationUsageService;
+import org.elasticsearch.search.internal.CrossClusterSearchExtension;
 import org.elasticsearch.shutdown.PluginShutdownService;
 import org.elasticsearch.snapshots.InternalSnapshotsInfoService;
 import org.elasticsearch.snapshots.RepositoryIntegrityHealthIndicatorService;
@@ -1015,6 +1016,13 @@ public Map<String, String> queryFields() {
 
         var reservedStateHandlerProviders = pluginsService.loadServiceProviders(ReservedStateHandlerProvider.class);
 
+        var crossClusterSearchExtension = pluginsService.loadSingletonServiceProvider(
+            CrossClusterSearchExtension.class,
+            CrossClusterSearchExtension.Default::new
+        );
+
+        logger.info("Cross-cluster example: [{}]", crossClusterSearchExtension.indicesExpressionRewriter().getClass());
+
         ActionModule actionModule = new ActionModule(
             settings,
             clusterModule.getIndexNameExpressionResolver(),

server/src/main/java/org/elasticsearch/search/internal/CrossClusterSearchExtension.java

@@ -0,0 +1,33 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the "Elastic License
+ * 2.0", the "GNU Affero General Public License v3.0 only", and the "Server Side
+ * Public License v 1"; you may not use this file except in compliance with, at
+ * your election, the "Elastic License 2.0", the "GNU Affero General Public
+ * License v3.0 only", or the "Server Side Public License, v 1".
+ */
+
+package org.elasticsearch.search.internal;
+
+import org.elasticsearch.action.IndicesRequest;
+
+public interface CrossClusterSearchExtension {
+
+    IndicesExpressionRewriter indicesExpressionRewriter();
+
+    interface IndicesExpressionRewriter {
+        IndicesRequest.Replaceable rewrite(IndicesRequest.Replaceable request);
+    }
+
+    class Default implements CrossClusterSearchExtension {
+        public Default() {}
+
+        @Override
+        public IndicesExpressionRewriter indicesExpressionRewriter() {
+            return request -> {
+                // Default implementation does nothing, can be overridden by extensions
+                return request;
+            };
+        }
+    }
+}