frankvicky commented on code in PR #22511:
URL: https://github.com/apache/kafka/pull/22511#discussion_r3381016208


##########
server/src/main/java/org/apache/kafka/server/streams/InMemoryTopologyDescriptionPlugin.java:
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@@ -0,0 +1,73 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.kafka.server.streams;
+
+import 
org.apache.kafka.coordinator.group.api.streams.StreamsGroupTopologyDescription;
+import 
org.apache.kafka.coordinator.group.api.streams.StreamsGroupTopologyDescriptionPlugin;
+
+import java.util.Map;
+import java.util.concurrent.CompletableFuture;
+import java.util.concurrent.ConcurrentHashMap;
+
+/**
+ * Reference {@link StreamsGroupTopologyDescriptionPlugin} that stores 
topology descriptions
+ * in an in-memory map. Intended for testing and as a starting point for real 
implementations;
+ * not suitable for production because its state is lost on broker restart and 
is not shared
+ * across brokers.
+ *
+ * <p>Stores at most one description per group, identified by its topology 
epoch. A
+ * {@code setTopology} call overwrites any previously stored description for 
that group;
+ * {@code getTopology} returns the stored description only when the requested 
topology epoch
+ * matches the stored epoch, and {@code null} otherwise.
+ */
+public class InMemoryTopologyDescriptionPlugin implements 
StreamsGroupTopologyDescriptionPlugin {
+
+    private record Entry(int topologyEpoch, StreamsGroupTopologyDescription 
description) { }
+
+    private final ConcurrentHashMap<String, Entry> store = new 
ConcurrentHashMap<>();
+
+    @Override
+    public void configure(Map<String, ?> configs) {
+        // No-op.
+    }
+
+    @Override
+    public CompletableFuture<Void> setTopology(String groupId, int 
topologyEpoch, StreamsGroupTopologyDescription description) {
+        store.put(groupId, new Entry(topologyEpoch, description));
+        return CompletableFuture.completedFuture(null);
+    }
+
+    @Override
+    public CompletableFuture<Void> deleteTopology(String groupId) {
+        store.remove(groupId);
+        return CompletableFuture.completedFuture(null);
+    }

Review Comment:
   https://github.com/apache/kafka/pull/22511#discussion_r3381009919



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