philipnee commented on code in PR #13265:
URL: https://github.com/apache/kafka/pull/13265#discussion_r1114928370


##########
clients/src/main/java/org/apache/kafka/clients/consumer/StubbedAsyncKafkaConsumer.java:
##########
@@ -0,0 +1,548 @@
+/*
+ * 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.clients.consumer;
+
+import org.apache.kafka.clients.consumer.internals.Fetcher;
+import org.apache.kafka.clients.consumer.internals.SubscriptionState;
+import 
org.apache.kafka.clients.consumer.internals.NoOpConsumerRebalanceListener;
+import 
org.apache.kafka.clients.consumer.internals.events.ApplicationEventProcessor;
+import 
org.apache.kafka.clients.consumer.internals.events.AssignPartitionsEvent;
+import org.apache.kafka.clients.consumer.internals.events.CommitAsyncEvent;
+import org.apache.kafka.clients.consumer.internals.events.CommitSyncEvent;
+import org.apache.kafka.clients.consumer.internals.events.EventHandler;
+import 
org.apache.kafka.clients.consumer.internals.events.FetchCommittedOffsetsEvent;
+import org.apache.kafka.clients.consumer.internals.events.FetchOffsetsEvent;
+import org.apache.kafka.clients.consumer.internals.events.FetchRecordsEvent;
+import 
org.apache.kafka.clients.consumer.internals.events.RequestRebalanceEvent;
+import 
org.apache.kafka.clients.consumer.internals.events.SubscribePatternEvent;
+import org.apache.kafka.clients.consumer.internals.events.SubscribeTopicsEvent;
+import org.apache.kafka.clients.consumer.internals.events.UnsubscribeEvent;
+import org.apache.kafka.common.Metric;
+import org.apache.kafka.common.MetricName;
+import org.apache.kafka.common.PartitionInfo;
+import org.apache.kafka.common.TopicPartition;
+import org.apache.kafka.common.errors.InvalidGroupIdException;
+import org.apache.kafka.common.errors.RecordDeserializationException;
+import org.apache.kafka.common.header.Headers;
+import org.apache.kafka.common.header.internals.RecordHeaders;
+import org.apache.kafka.common.record.Record;
+import org.apache.kafka.common.record.TimestampType;
+import org.apache.kafka.common.requests.ListOffsetsRequest;
+import org.apache.kafka.common.serialization.Deserializer;
+import org.apache.kafka.common.utils.Time;
+import org.apache.kafka.common.utils.Timer;
+import org.apache.kafka.common.utils.Utils;
+
+import java.nio.ByteBuffer;
+import java.time.Duration;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.Optional;
+import java.util.OptionalLong;
+import java.util.Set;
+import java.util.regex.Pattern;
+
+/**
+ * This {@link StubbedAsyncKafkaConsumer stub} implementation of {@link 
Consumer} serves as a <i>wireframe</i> or
+ * <i>sketch</i>, showing the interaction between the foreground and 
background threads. Each of the main API methods
+ * will need to answer the following questions:
+ *
+ * <ol>
+ *     <li>Does this method block?</li>
+ *     <li>Does this method interact with the background thread?</li>
+ *     <li>If yes, what data is passed as input to the background thread?</li>
+ *     <li>If yes, what data is returned as output from the background 
thread?</li>
+ * </ol>
+ *
+ * @param <K> Key
+ * @param <V> Value
+ * @see ApplicationEventProcessor for the logic of the background event handler
+ */
+public class StubbedAsyncKafkaConsumer<K, V> implements Consumer<K, V> {
+
+    /**
+     * These instance variables are intentionally left unassigned, to avoid 
clutter...
+     */
+    private Time time;
+
+    private EventHandler eventHandler;
+
+    private SubscriptionState subscriptions;
+
+    private Deserializer<K> keyDeserializer;
+
+    private Deserializer<V> valueDeserializer;
+
+    private long defaultApiTimeoutMs;
+
+    private List<ConsumerPartitionAssignor> assignors;
+
+    private Optional<String> groupId;
+
+    /**
+     * Answers to the above questions:
+     *
+     * <ol>
+     *     <li>No</li>
+     *     <li>No</li>
+     *     <li><i>n/a</i></li>
+     *     <li><i>n/a</i></li>
+     * </ol>
+     */
+    @Override
+    public Set<TopicPartition> assignment() {

Review Comment:
   I feel we need an explicit synchronization barrier to ensure state changes 
happen in a deterministic sequence.  I mentioned this idea in our previous 
meeting, but I just want to bring it up again for discussion: (feel free to 
strike down this idea)
   
   Is it reasonable to ask users to invoke poll() after any sort of assignment 
to trigger the rebalance? I think we could use poll() as an explicit barrier to 
synchronize both threads.
   
   So back to your example,
   1. User updates assignment via `assign()`
   2. Read calls (`assignments()`) here will still returns the previous state
   3.  client invoke poll() - update client and background state and triggers 
rebalance. Rebalance callback triggered.
   4. `assignments()` returns the updated version of the assignment. Invalid 
fetches will be dropped here.
   
   I think it kind of breaks the current contract, as `assign()` -> 
`assignment()` should already reflect to the current assignment.  But I do 
think regardless of how we handle this, there will be some kind of behavioral 
changes as the threading model is now asynchronous, which kind of implies data 
update is expected to be asynchronous.



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