jeffkbkim commented on code in PR #13870:
URL: https://github.com/apache/kafka/pull/13870#discussion_r1256110776


##########
group-coordinator/src/main/java/org/apache/kafka/coordinator/group/GroupMetadataManager.java:
##########
@@ -1043,4 +1230,1358 @@ public void onNewMetadataImage(MetadataImage newImage, 
MetadataDelta delta) {
             }
         });
     }
+
+    /**
+     * Replays GroupMetadataKey/Value to update the soft state of
+     * the generic group.
+     *
+     * @param key   A GroupMetadataKey key.
+     * @param value A GroupMetadataValue record.
+     */
+    public void replay(
+        GroupMetadataKey key,
+        GroupMetadataValue value,
+        short version
+    ) {
+        String groupId = key.group();
+
+        if (value == null)  {
+            // Tombstone. Group should be removed.
+            groups.remove(groupId);
+        } else {
+            List<GenericGroupMember> loadedMembers = new ArrayList<>();
+            for (GroupMetadataValue.MemberMetadata member : value.members()) {
+                int rebalanceTimeout = version == 0 ? member.sessionTimeout() 
: member.rebalanceTimeout();
+
+                JoinGroupRequestProtocolCollection supportedProtocols = new 
JoinGroupRequestProtocolCollection();
+                supportedProtocols.add(new JoinGroupRequestProtocol()
+                    .setName(value.protocol())
+                    .setMetadata(member.subscription()));
+
+                GenericGroupMember loadedMember = new GenericGroupMember(
+                    member.memberId(),
+                    Optional.ofNullable(member.groupInstanceId()),
+                    member.clientId(),
+                    member.clientHost(),
+                    rebalanceTimeout,
+                    member.sessionTimeout(),
+                    value.protocolType(),
+                    supportedProtocols,
+                    member.assignment()
+                );
+
+                loadedMembers.add(loadedMember);
+            }
+
+            String protocolType = value.protocolType();
+
+            GenericGroup genericGroup = new GenericGroup(
+                this.logContext,
+                groupId,
+                loadedMembers.isEmpty() ? EMPTY : STABLE,
+                time,
+                value.generation(),
+                protocolType == null || protocolType.isEmpty() ? 
Optional.empty() : Optional.of(protocolType),
+                Optional.ofNullable(value.protocol()),
+                Optional.ofNullable(value.leader()),
+                value.currentStateTimestamp() == -1 ? Optional.empty() : 
Optional.of(value.currentStateTimestamp())
+            );
+
+            loadedMembers.forEach(member -> {
+                genericGroup.add(member, null);
+                log.info("Loaded member {} in group {} with generation {}.",
+                    member.memberId(), groupId, genericGroup.generationId());
+            });
+
+            genericGroup.setSubscribedTopics(
+                genericGroup.computeSubscribedTopics()
+            );
+        }
+    }
+
+    /**
+     * Handle a JoinGroupRequest.
+     *
+     * @param context The request context.
+     * @param request The actual JoinGroup request.
+     *
+     * @return The result that contains records to append if the join group 
phase completes.
+     */
+    public CoordinatorResult<Void, Record> genericGroupJoin(
+        RequestContext context,
+        JoinGroupRequestData request,
+        CompletableFuture<JoinGroupResponseData> responseFuture
+    ) {
+        CoordinatorResult<Void, Record> result = EMPTY_RESULT;
+
+        String groupId = request.groupId();
+        String memberId = request.memberId();
+        int sessionTimeoutMs = request.sessionTimeoutMs();
+
+        if (sessionTimeoutMs < genericGroupMinSessionTimeoutMs ||
+            sessionTimeoutMs > genericGroupMaxSessionTimeoutMs
+        ) {
+            responseFuture.complete(new JoinGroupResponseData()
+                .setMemberId(memberId)
+                .setErrorCode(Errors.INVALID_SESSION_TIMEOUT.code())
+            );
+        } else {
+            boolean isUnknownMember = memberId.equals(UNKNOWN_MEMBER_ID);
+            // Group is created if it does not exist and the member id is 
UNKNOWN. if member
+            // is specified but group does not exist, request is rejected with 
GROUP_ID_NOT_FOUND
+            GenericGroup group;
+            boolean isNewGroup = groups.get(groupId) == null;
+            try {
+                group = getOrMaybeCreateGenericGroup(groupId, isUnknownMember);
+            } catch (Throwable t) {
+                responseFuture.complete(new JoinGroupResponseData()
+                    .setMemberId(memberId)
+                    .setErrorCode(Errors.forException(t).code())
+                );
+                return EMPTY_RESULT;
+            }
+
+            CoordinatorResult<Void, Record> newGroupResult = EMPTY_RESULT;
+            if (isNewGroup) {
+                // If a group was newly created, we need to append records to 
the log

Review Comment:
   the issue is that the records need to be generated while the group is empty. 
after performing `genericGroupJoinNewMember()` the group will have added the 
member metadata. the existing protocol only allows records for empty groups or 
groups that have a defined protocol.
   
   this only applies to join group requests with `requireKnownMemberId = false` 
or group instance id.



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