lucasbru commented on code in PR #18397:
URL: https://github.com/apache/kafka/pull/18397#discussion_r1908706477


##########
group-coordinator/src/main/java/org/apache/kafka/coordinator/group/streams/topics/CopartitionedTopicsEnforcer.java:
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@@ -0,0 +1,189 @@
+/*
+ * 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.coordinator.group.streams.topics;
+
+import org.apache.kafka.common.errors.StreamsInvalidTopologyException;
+import org.apache.kafka.common.utils.LogContext;
+
+import org.slf4j.Logger;
+
+import java.util.Collection;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.Map;
+import java.util.Map.Entry;
+import java.util.OptionalInt;
+import java.util.Set;
+import java.util.TreeMap;
+import java.util.function.Function;
+import java.util.stream.Collectors;
+
+/**
+ * This class is responsible for enforcing the number of partitions in 
copartitioned topics. For each copartition group, it checks whether
+ * the number of partitions for all repartition topics is the same, and 
enforces copartitioning for repartition topics whose number of
+ * partitions is not enforced by the topology.
+ */
+public class CopartitionedTopicsEnforcer {
+
+    private final Logger log;
+    private final Function<String, OptionalInt> topicPartitionCountProvider;
+
+    /**
+     * The constructor for the class.
+     *
+     * @param logContext                  The context for emitting log 
messages.
+     * @param topicPartitionCountProvider Returns the number of partitions for 
a given topic, representing the current state of the broker
+     *                                    as well as any partition number 
decisions that have already been made. In particular, we expect
+     *                                    the number of partitions for all 
repartition topics defined, even if they do not exist in the
+     *                                    broker yet.
+     */
+    public CopartitionedTopicsEnforcer(final LogContext logContext,
+                                       final Function<String, OptionalInt> 
topicPartitionCountProvider) {
+        this.log = logContext.logger(getClass());
+        this.topicPartitionCountProvider = topicPartitionCountProvider;
+    }
+
+    /**
+     * Enforces the number of partitions for copartitioned topics.
+     *
+     * @param copartitionedTopics          The set of copartitioned topics 
(external source topics and repartition topics).
+     * @param fixedRepartitionTopics       The set of repartition topics whose 
partition count is fixed by the topology.

Review Comment:
   I don't want too restrictive - the number of partitions is fixed in the 
topology, and on the broker we don't necessarily know why it was fixed. But I 
think right now you are right, this only happens when the user explicitly 
specifies it in `repartition`, so I updated it this way:
   
   ```
   The set of repartition topics whose partition count is fixed by the topology 
sent by the client (in particular, when the user uses `repartition` in the DSL).
   ```
   



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