lianetm commented on code in PR #14413:
URL: https://github.com/apache/kafka/pull/14413#discussion_r1333324044


##########
clients/src/main/java/org/apache/kafka/clients/consumer/internals/MembershipManagerImpl.java:
##########
@@ -19,56 +19,91 @@
 
 import org.apache.kafka.common.message.ConsumerGroupHeartbeatResponseData;
 import org.apache.kafka.common.protocol.Errors;
+import org.apache.kafka.common.requests.ConsumerGroupHeartbeatRequest;
+import org.apache.kafka.common.utils.LogContext;
+import org.slf4j.Logger;
 
 import java.util.Optional;
 
 /**
- * Membership manager that maintains group membership for a single member 
following the new
+ * Membership manager that maintains group membership for a single member, 
following the new
  * consumer group protocol.
  * <p/>
- * This keeps membership state and assignment updated in-memory, based on the 
heartbeat responses
- * the member receives. It is also responsible for computing assignment for 
the group based on
- * the metadata, if the member has been selected by the broker to do so.
+ * This is responsible for:
+ * <li>Keeping member info (ex. member id, member epoch, assignment, etc.)</li>
+ * <li>Keeping member state as defined in {@link MemberState}.</li>
+ * <p/>
+ * Member info and state are updated based on the heartbeat responses the 
member receives.
  */
 public class MembershipManagerImpl implements MembershipManager {
 
+    /**
+     * ID of the consumer group the member will be part of., provided when 
creating the current
+     * membership manager.
+     */
     private final String groupId;
+
+    /**
+     * Group instance ID to be used by the member, provided when creating the 
current membership manager.
+     */
     private final Optional<String> groupInstanceId;
+
+    /**
+     * Member ID assigned by the server to the member, received in a heartbeat 
response when
+     * joining the group specified in {@link #groupId}
+     */
     private String memberId;
+
+    /**
+     * Current epoch of the member. It will be set to 0 by the member, and 
provided to the server
+     * on the heartbeat request, to join the group. It will be then maintained 
by the server,
+     * incremented as the member reconciles and acknowledges the assignments 
it receives.
+     */
     private int memberEpoch;
+
+    /**
+     * Current state of this member a part of the consumer group, as defined 
in {@link MemberState}
+     */
     private MemberState state;
+
+    /**
+     * Assignor type selection for the member. If non-null, the member will 
send its selection to
+     * the server on the {@link ConsumerGroupHeartbeatRequest}. If null, the 
server will select a
+     * default assignor for the member, which the member does not need to 
track.
+     */
     private AssignorSelection assignorSelection;
 
     /**
      * Assignment that the member received from the server and successfully 
processed.
      */
     private ConsumerGroupHeartbeatResponseData.Assignment currentAssignment;
+
     /**
      * Assignment that the member received from the server but hasn't 
completely processed
      * yet.
      */
     private Optional<ConsumerGroupHeartbeatResponseData.Assignment> 
targetAssignment;
+
     /**
-     * Latest assignment that the member received from the server while a 
{@link #targetAssignment}
-     * was in process.
+     * slf4j logger.
      */
-    private Optional<ConsumerGroupHeartbeatResponseData.Assignment> 
nextTargetAssignment;
+    private final Logger log;
 
-    public MembershipManagerImpl(String groupId) {
-        this(groupId, null, null);
+    public MembershipManagerImpl(String groupId, LogContext logContext) {
+        this(groupId, null, null, logContext);
     }
 
-    public MembershipManagerImpl(String groupId, String groupInstanceId, 
AssignorSelection assignorSelection) {
+    public MembershipManagerImpl(String groupId, String groupInstanceId,
+                                 AssignorSelection assignorSelection, 
LogContext logContext) {
+        if (groupId == null) {
+            throw new IllegalArgumentException("Group ID cannot be null.");

Review Comment:
   It is truly to make this component solid. This is a component to manage 
membership to a group, it cannot do its job without a group so making sure it's 
explicit and fails fast. (As it is currently used, this would be a programming 
issue as you said, and would fail when creating this for the heartbeat manager)



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