consumer group that does not join group

weed/mq/kafka/protocol/offset_management.go

@@ -115,11 +115,10 @@ func (h *Handler) handleOffsetCommit(correlationID uint32, apiVersion uint16, re
 		return h.buildOffsetCommitErrorResponse(correlationID, ErrorCodeInvalidGroupID, apiVersion), nil
 	}
 
-	// Get consumer group
-	group := h.groupCoordinator.GetGroup(req.GroupID)
-	if group == nil {
-		return h.buildOffsetCommitErrorResponse(correlationID, ErrorCodeInvalidGroupID, apiVersion), nil
-	}
+	// Get or create consumer group
+	// Some Kafka clients (like kafka-go Reader) commit offsets without formally joining
+	// the group via JoinGroup/SyncGroup. We need to support these "simple consumer" use cases.
+	group := h.groupCoordinator.GetOrCreateGroup(req.GroupID)
 
 	group.Mu.Lock()
 	defer group.Mu.Unlock()
@@ -127,8 +126,14 @@ func (h *Handler) handleOffsetCommit(correlationID uint32, apiVersion uint16, re
 	// Update group's last activity
 	group.LastActivity = time.Now()
 
-	// Require matching generation to store commits; return IllegalGeneration otherwise
-	generationMatches := (req.GenerationID == group.Generation)
+	// Check generation compatibility
+	// Allow commits for empty groups (no active members) to support simple consumers
+	// that commit offsets without formal group membership
+	groupIsEmpty := len(group.Members) == 0
+	generationMatches := groupIsEmpty || (req.GenerationID == group.Generation)
+
+	glog.V(1).Infof("[OFFSET_COMMIT] Group check: id=%s reqGen=%d groupGen=%d members=%d empty=%v matches=%v",
+		req.GroupID, req.GenerationID, group.Generation, len(group.Members), groupIsEmpty, generationMatches)
 
 	// Process offset commits
 	resp := OffsetCommitResponse{
@@ -166,13 +171,18 @@ func (h *Handler) handleOffsetCommit(correlationID uint32, apiVersion uint16, re
 					glog.V(2).Infof("[OFFSET_COMMIT] SMQ storage not available: %v", err)
 				}
 
-				glog.V(0).Infof("[OFFSET_COMMIT] Committed: group=%s topic=%s partition=%d offset=%d",
-					req.GroupID, t.Name, p.Index, p.Offset)
+				if groupIsEmpty {
+					glog.V(0).Infof("[OFFSET_COMMIT] ✓ Committed (empty group): group=%s topic=%s partition=%d offset=%d",
+						req.GroupID, t.Name, p.Index, p.Offset)
+				} else {
+					glog.V(0).Infof("[OFFSET_COMMIT] ✓ Committed: group=%s topic=%s partition=%d offset=%d gen=%d",
+						req.GroupID, t.Name, p.Index, p.Offset, group.Generation)
+				}
 			} else {
 				// Do not store commit if generation mismatch
 				errCode = 22 // IllegalGeneration
-				glog.V(0).Infof("[OFFSET_COMMIT] Generation mismatch: group=%s expected=%d got=%d",
-					req.GroupID, group.Generation, req.GenerationID)
+				glog.V(0).Infof("[OFFSET_COMMIT] ❌ Rejected - generation mismatch: group=%s expected=%d got=%d members=%d",
+					req.GroupID, group.Generation, req.GenerationID, len(group.Members))
 			}
 
 			topicResp.Partitions = append(topicResp.Partitions, OffsetCommitPartitionResponse{