diff --git a/hstream-kafka/HStream/Kafka/Group/Group.hs b/hstream-kafka/HStream/Kafka/Group/Group.hs
index 502eccce4..27a3fe75c 100644
--- a/hstream-kafka/HStream/Kafka/Group/Group.hs
+++ b/hstream-kafka/HStream/Kafka/Group/Group.hs
@@ -260,20 +260,18 @@ newGroupFromValue value metadataManager metaHandle = do
 joinGroup :: Group -> K.RequestContext -> K.JoinGroupRequest -> IO K.JoinGroupResponse
 joinGroup group@Group{..} reqCtx req = do
   -- delayed response(join barrier)
-  Log.debug $ "received joinGroup"
+  Log.info $ "received joinGroup request:" <> Log.buildString' req
   delayedResponse <- C.newEmptyMVar
   C.withMVar lock $ \_ -> do
     -- TODO: GROUP MAX SIZE
 
     checkSupportedProtocols group req
-    Log.debug $ "checked protocols"
 
     -- check state
     IO.readIORef group.state >>= \case
       Dead -> throw (ErrorCodeException K.UNKNOWN_MEMBER_ID)
       _ -> pure ()
 
-    Log.debug $ "checked state"
     if T.null req.memberId
       then doNewMemberJoinGoup group reqCtx req delayedResponse
       else doCurrentMemeberJoinGroup group req delayedResponse
@@ -281,11 +279,11 @@ joinGroup group@Group{..} reqCtx req = do
   -- waiting other consumers
   resp <- C.takeMVar delayedResponse
   Log.info $ "joinGroup: received delayed response:" <> Log.buildString' resp
+    <> "group:" <> Log.build groupId
   return resp
 
 checkSupportedProtocols :: Group -> K.JoinGroupRequest -> IO ()
 checkSupportedProtocols Group{..} req = do
-  Log.debug $ "checking protocols"
   IO.readIORef state >>= \case
     Empty -> do
       M.when (T.null req.protocolType) $ do
@@ -305,6 +303,7 @@ cancelDelayedSyncResponses Group{..} reason = do
   lst <- H.toList delayedSyncResponses
   M.forM_ lst $ \(memberId, delayed) -> do
     Log.info $ "cancel delayed sync response for " <> Log.buildString' memberId
+      <> ", group:" <> Log.build groupId
       <> ", reason:" <> Log.build reason
     _ <- C.tryPutMVar delayed $ K.SyncGroupResponse {throttleTimeMs=0, errorCode=K.REBALANCE_IN_PROGRESS, assignment=BS.empty}
     H.delete delayedSyncResponses memberId
@@ -320,7 +319,8 @@ cancelDelayedSync Group{..} = do
 doNewMemberJoinGoup :: Group -> K.RequestContext -> K.JoinGroupRequest -> C.MVar K.JoinGroupResponse -> IO ()
 doNewMemberJoinGoup group reqCtx req delayedResponse = do
   newMemberId <- generateMemberId reqCtx
-  Log.debug $ "generated member id:" <> Log.buildString' newMemberId
+  Log.info $ "generated member id:" <> Log.buildString' newMemberId
+    <> " for group:" <> Log.build group.groupId
   doDynamicNewMemberJoinGroup group reqCtx req newMemberId delayedResponse
 
 generateMemberId :: K.RequestContext -> IO T.Text
@@ -350,15 +350,17 @@ addMemberAndRebalance group reqCtx req newMemberId delayedResponse = do
   member <- newMemberFromReq reqCtx req newMemberId (refineProtocols req.protocols)
   addMember group member (Just delayedResponse)
   -- TODO: check state
-  prepareRebalance group
+  prepareRebalance group $ "add member:" <> member.memberId
 
 updateMemberAndRebalance :: Group -> Member -> K.JoinGroupRequest -> C.MVar K.JoinGroupResponse -> IO ()
 updateMemberAndRebalance group member req delayedResponse = do
   updateMember group member req delayedResponse
-  prepareRebalance group
+  prepareRebalance group $ "update member:" <> member.memberId
 
-prepareRebalance :: Group -> IO ()
-prepareRebalance group@Group{..} = do
+prepareRebalance :: Group -> T.Text -> IO ()
+prepareRebalance group@Group{..} reason = do
+  Log.info $ "prepare rebalance, group:" <> Log.build groupId
+    <> "reason:" <> Log.build reason
   -- check state CompletingRebalance and cancel delayedSyncResponses
   Utils.whenIORefEq state CompletingRebalance $ do
     cancelDelayedSyncResponses group "rebalance"
@@ -374,6 +376,7 @@ prepareRebalance group@Group{..} = do
     Nothing -> do
       delayed <- makeDelayedRebalance group 5000
       Log.info $ "created delayed rebalance thread:" <> Log.buildString' delayed
+        <> ", group:" <> Log.build groupId
       IO.atomicWriteIORef delayedRebalance (Just delayed)
       IO.atomicWriteIORef state PreparingRebalance
     _ -> pure ()
@@ -388,7 +391,7 @@ makeDelayedRebalance group rebalanceDelayMs = do
 rebalance :: Group -> IO ()
 rebalance group@Group{..} = do
   C.withMVar lock $ \() -> do
-    Log.info "rebalancing is starting"
+    Log.info $ "rebalancing is starting, group:" <> Log.build groupId
     -- remove all members who haven't joined(and maybe elect new leader)
     removeNotYetRejoinedMembers group
 
@@ -397,7 +400,7 @@ rebalance group@Group{..} = do
       <> ", generationId:" <> Log.build nextGenerationId
     IO.readIORef leader >>= \case
       Nothing -> do
-        Log.info "cancel rebalance without any join request"
+        Log.info $ "cancel rebalance without any join request, " <> Log.build groupId
 
         IO.atomicWriteIORef group.protocolName Nothing
         transitionTo group Empty
@@ -415,22 +418,24 @@ transitionTo :: Group -> GroupState -> IO ()
 transitionTo group state = do
   oldState <- IO.atomicModifyIORef' group.state (state, )
   Log.info $ "group state changed, " <> Log.buildString' oldState <> " -> " <> Log.buildString' state
+    <> ", group:" <> Log.build group.groupId
 
 doRelance :: Group -> T.Text -> IO ()
 doRelance group@Group{..} leaderMemberId = do
   selectedProtocolName <- computeProtocolName group
-  Log.info $ "selected protocolName:" <> Log.buildString' selectedProtocolName
 
   -- state changes
   transitionTo group CompletingRebalance
 
   generationId <- IO.readIORef groupGenerationId
   leaderMembersInResponse <- H.toList members >>= M.mapM (\(_, m) -> getJoinResponseMember selectedProtocolName m)
-  Log.debug $ "members in join responses" <> Log.buildString' leaderMembersInResponse
+  Log.info $ "members in join responses:" <> Log.buildString' leaderMembersInResponse
+    <> ", group:" <> Log.build groupId
 
   delayedJoinResponseList <- H.toList delayedJoinResponses
 
-  Log.info $ "set all delayed responses, response list size:" <> Log.buildString' (length delayedJoinResponseList)
+  Log.info $ "set all delayed responses, response list:" <> Log.buildString' (length delayedJoinResponseList)
+    <> ", group:" <> Log.build groupId
   -- response all delayedJoinResponses
   M.forM_ delayedJoinResponseList $ \(memberId, delayed) -> do
     let memebersInResponse = if leaderMemberId == memberId then leaderMembersInResponse else []
@@ -443,24 +448,33 @@ doRelance group@Group{..} leaderMemberId = do
                 , members = K.KaArray (Just $ V.fromList memebersInResponse)
                 , throttleTimeMs = 0
                 }
-    Log.debug $ "set delayed response:" <> Log.buildString' resp
-      <> " for " <> Log.buildString' memberId
     _ <- C.tryPutMVar delayed resp
     H.delete delayedJoinResponses memberId
 
   rebalanceTimeoutMs <- computeRebalnceTimeoutMs group
   delayedSyncTid <- makeDelayedSync group generationId rebalanceTimeoutMs
   IO.atomicWriteIORef delayedSync (Just delayedSyncTid)
+  Log.info $ "create delayed sync for group:" <> Log.build groupId
+    <> ", threadId:" <> Log.buildString' delayedSyncTid
 
   IO.atomicWriteIORef delayedRebalance Nothing
-  Log.info "rebalancing is finished"
+  Log.info $ "rebalancing is finished, group:" <> Log.build groupId
 
 removeNotYetRejoinedMembers :: Group -> IO ()
 removeNotYetRejoinedMembers group@Group{..} = do
   (flip H.mapM_) members $ \(mid, member) -> do
     H.lookup delayedJoinResponses mid >>= \case
       Nothing -> do
-        Log.info $ "remove member: " <> Log.build mid <> " from " <> Log.build groupId
+        Log.info $ "remove not yet joined member: " <> Log.build mid <> " from " <> Log.build groupId
+        removeMember group member
+      Just _ -> pure ()
+
+removeNotYetSyncedMembers :: Group -> IO ()
+removeNotYetSyncedMembers group@Group{..} = do
+  (flip H.mapM_) members $ \(mid, member) -> do
+    H.lookup delayedSyncResponses mid >>= \case
+      Nothing -> do
+        Log.info $ "remove not yet synced member: " <> Log.build mid <> " from " <> Log.build groupId
         removeMember group member
       Just _ -> pure ()
 
@@ -470,18 +484,20 @@ makeDelayedSync group@Group{..} generationId timeoutMs = do
     C.threadDelay (fromIntegral timeoutMs * 1000)
     C.withMVar lock $ \() -> do
       Utils.unlessIORefEq groupGenerationId generationId $ \currentGid -> do
-        Log.fatal $ "unexpected delayed sync with wrong generationId:" <> Log.build generationId
+        Log.warning $ "unexpected delayed sync with wrong generationId:" <> Log.build generationId
           <> ", current group generation id:" <> Log.build currentGid
           <> ", groupId:" <> Log.build groupId
       IO.readIORef state >>= \case
         CompletingRebalance -> do
-          Log.info $ "delayed sync timeout, try to prepare Rebalance, groupId:" <> Log.build groupId
+          Log.info $ "delayed sync timeout, try to prepare Rebalance, group:" <> Log.build groupId
+          removeNotYetSyncedMembers group
+
           -- remove itself (to avoid killing itself in prepareRebalance)
           IO.atomicWriteIORef delayedSync Nothing
-          prepareRebalance group
+          prepareRebalance group $ "delayed sync timeout"
         s -> do
-          Log.fatal $ "unexpected delayed sync with wrong state:" <> Log.buildString' s
-            <> ", groupId:" <> Log.build groupId
+          Log.warning $ "unexpected delayed sync with wrong state:" <> Log.buildString' s
+            <> ", group:" <> Log.build groupId
 
 -- select max rebalanceTimeoutMs from all members
 computeRebalnceTimeoutMs :: Group -> IO Int32
@@ -503,7 +519,6 @@ computeProtocolName group@Group{..} = do
   IO.readIORef protocolName >>= \case
     Nothing -> do
       pn <- chooseProtocolName group
-      Log.debug $ "choosed protocolName" <> Log.buildString' pn
       IO.atomicWriteIORef protocolName (Just pn)
       pure pn
     Just pn -> pure pn
@@ -512,8 +527,11 @@ computeProtocolName group@Group{..} = do
 chooseProtocolName :: Group -> IO T.Text
 chooseProtocolName Group {..} = do
   ps <- IO.readIORef supportedProtocols
-  Log.debug $ "protocols:" <> Log.buildString' ps
-  return . head $ Set.toList ps
+  let pn = head $ Set.toList ps
+  Log.info $ "choose protocol:" <> Log.build pn
+    <> ", current supported protocols:" <> Log.buildString' ps
+    <> ", group:" <> Log.build groupId
+  return pn
 
 updateSupportedProtocols :: Group -> [(T.Text, BS.ByteString)] -> IO ()
 updateSupportedProtocols Group{..} protocols = do
@@ -535,14 +553,13 @@ addMember group@Group{..} member delayedResponse = do
 
   Utils.whenIORefEq leader Nothing $ do
     IO.atomicWriteIORef leader (Just member.memberId)
-    Log.info $ "group updated leader, groupId:" <> Log.build groupId
+    Log.info $ "updated leader, group:" <> Log.build groupId
       <> "leader:" <> Log.build member.memberId
 
   H.insert members member.memberId member
   updateSupportedProtocols group memberProtocols
 
   M.forM_ delayedResponse $ \delayed -> do
-    Log.debug $ "add delayed response into response list for member:" <> Log.buildString' member.memberId
     H.insert delayedJoinResponses member.memberId delayed
 
 updateMember :: Group -> Member -> K.JoinGroupRequest -> C.MVar K.JoinGroupResponse -> IO ()
@@ -554,7 +571,8 @@ updateMember group@Group{..} member req delayedResponse = do
   IO.atomicWriteIORef member.sessionTimeoutMs req.sessionTimeoutMs
 
   -- TODO: check delayedJoinResponses
-  Log.debug $ "add delayed response into response list for member:" <> Log.buildString' member.memberId
+  Log.info $ "updated member, add delayed response into response list for member:"
+    <> Log.buildString' member.memberId <> ", group:" <> Log.build groupId
   H.insert delayedJoinResponses member.memberId delayedResponse
 
 removeMember :: Group -> Member -> IO ()
@@ -572,9 +590,14 @@ removeMember Group{..} member = do
       [] -> do
         -- select from members
         H.toList members >>= \ms -> do
-          IO.atomicWriteIORef leader (listToMaybe (fmap fst ms))
+          let newLeader = listToMaybe (fmap fst ms)
+          IO.atomicWriteIORef leader newLeader
+          Log.info $ "selcet new leader from members, group:" <> Log.build groupId
+            <> ", new leader:" <> Log.buildString' newLeader <> ", old leader:" <> Log.build member.memberId
       (mid, _):_ -> do
         IO.atomicWriteIORef leader (Just mid)
+        Log.info $ "select new leader from delayedJoinResponses, group:" <> Log.build groupId
+               <> ", new leader:" <> Log.build mid <> ", old leader:" <> Log.build member.memberId
 
 
 plainProtocols :: [(T.Text, BS.ByteString)] -> Set.Set T.Text
@@ -592,6 +615,7 @@ refineProtocols protocols = case K.unKaArray protocols of
 
 syncGroup :: Group -> K.SyncGroupRequest -> IO K.SyncGroupResponse
 syncGroup group req@K.SyncGroupRequest{..} = do
+  Log.info $ "received sync group request:" <> Log.buildString' req
   delayed <- C.newEmptyMVar
   C.withMVar (group.lock) $ \() -> do
     -- check member id
@@ -605,7 +629,10 @@ syncGroup group req@K.SyncGroupRequest{..} = do
         M.void $ C.tryPutMVar delayed (K.SyncGroupResponse {throttleTimeMs=0, errorCode=0, assignment=assignment})
       PreparingRebalance -> throw (ErrorCodeException K.REBALANCE_IN_PROGRESS)
       _ -> throw (ErrorCodeException K.UNKNOWN_MEMBER_ID)
-  C.readMVar delayed
+  resp <- C.readMVar delayed
+  Log.info $ "received delayed sync group response:" <> Log.buildString' resp
+    <> ", group:" <> Log.build group.groupId
+  return resp
 
 doSyncGroup :: Group -> K.SyncGroupRequest -> C.MVar K.SyncGroupResponse -> IO ()
 doSyncGroup group@Group{..} req@K.SyncGroupRequest{memberId=memberId} delayedResponse = do
@@ -620,8 +647,9 @@ doSyncGroup group@Group{..} req@K.SyncGroupRequest{memberId=memberId} delayedRes
   (Just leaderMemberId) <- IO.readIORef leader
   Log.info $ "sync group leaderMemberId: " <> Log.buildString' leaderMemberId
     <> " memberId:" <> Log.buildString' memberId
+    <> " group:" <> Log.build groupId
+    <> " isLeader:" <> Log.buildString' (memberId == leaderMemberId)
   when (memberId == leaderMemberId) $ do
-    Log.info $ "received leader SyncGroup request, " <> Log.buildString' memberId
     let assignmentMap = getAssignmentMap req
 
     storeGroup group assignmentMap
@@ -645,7 +673,7 @@ setAndPropagateAssignment :: Group -> Map.Map T.Text BS.ByteString -> IO ()
 setAndPropagateAssignment group@Group{..} assignments = do
   -- set assignments
   Log.info $ "setting assignments:" <> Log.buildString' assignments
-    <> "group Id:" <> Log.build groupId
+    <> "group:" <> Log.build groupId
   (flip H.mapM_) members $ \(memberId, member) -> do
     -- set assignment
     let assignment = fromMaybe "" $ Map.lookup memberId assignments
@@ -658,14 +686,14 @@ setAndPropagateAssignment group@Group{..} assignments = do
         M.void $ C.tryPutMVar delayed (K.SyncGroupResponse {throttleTimeMs=0, errorCode=0, assignment=assignment})
         H.delete delayedSyncResponses memberId
 
-  -- delayedJoinResponses should have been empty,
+  -- delayedSyncResponses should have been empty,
   -- so it actually does nothing,
-  -- but it is useful if delayedJoinResponses is NOT empty(bug):
+  -- but it is useful if delayedSyncResponses is NOT empty(bug):
   --  * log error information
   --  * avoid inconsistent state
   --  * avoid clients are getting stuck
-  cancelDelayedSyncResponses group "ERROR: delayedJoinResponses should be empty after propagated assignments"
-  Log.info $ "setAndPropagateAssignment completed"
+  cancelDelayedSyncResponses group "ERROR: delayedSyncResponses should be empty after propagated assignments"
+  Log.info $ "setAndPropagateAssignment completed, group:" <> Log.build groupId
 
 leaveGroup :: Group -> K.LeaveGroupRequest -> IO K.LeaveGroupResponse
 leaveGroup group@Group{..} req = do
@@ -694,8 +722,22 @@ getMember Group{..} memberId = do
 removeMemberAndUpdateGroup :: Group -> Member -> IO ()
 removeMemberAndUpdateGroup group@Group{..} member = do
   Log.info $ "member: " <> Log.build member.memberId <> " is leaving group:" <> Log.build groupId
+
+  -- New members may timeout with a pending JoinGroup while the group is still rebalancing, so we have
+  -- to invoke the callback before removing the member. We return UNKNOWN_MEMBER_ID so that the consumer
+  -- will retry the JoinGroup request if is still active.
+  cancelDelayedJoinResponse group member.memberId
+
   removeMember group member
-  prepareRebalance group
+  prepareRebalance group $ "remove member:" <> member.memberId
+
+cancelDelayedJoinResponse :: Group -> T.Text -> IO ()
+cancelDelayedJoinResponse Group{..} memberId = do
+  H.lookup delayedJoinResponses memberId >>= \case
+    Nothing -> pure ()
+    Just delayed -> do
+      _ <- C.tryPutMVar delayed (makeJoinResponseError memberId K.UNKNOWN_MEMBER_ID)
+      H.delete delayedJoinResponses memberId
 
 -- default heartbeat interval: 3s
 heartbeat :: Group -> K.HeartbeatRequest -> IO K.HeartbeatResponse
@@ -721,9 +763,10 @@ checkGroupGenerationId :: Group -> Int32 -> IO ()
 checkGroupGenerationId Group{..} generationId = do
   currentGenerationId <- IO.readIORef groupGenerationId
   M.unless (currentGenerationId == generationId) $ do
-    Log.debug $ "invalid generation id"
+    Log.info $ "invalid generation id"
       <> ", current generationId:" <> Log.buildString' currentGenerationId
-      <> ", expected generationId" <> Log.buildString' generationId
+      <> ", expected generationId:" <> Log.buildString' generationId
+      <> ", group:" <> Log.build groupId
     throw (ErrorCodeException K.ILLEGAL_GENERATION)
 
 updateLatestHeartbeat :: Member -> IO ()
@@ -739,8 +782,9 @@ setupDelayedCheckHeartbeat group@Group{..} = do
     updateLatestHeartbeat member
     memberSessionTimeoutMs <- IO.readIORef member.sessionTimeoutMs
     threadId <- C.forkIO $ delayedCheckHeart group member memberSessionTimeoutMs
-    Log.debug $ "setup delayed heartbeat check, threadId:" <> Log.buildString' threadId
+    Log.info $ "setup delayed heartbeat check, threadId:" <> Log.buildString' threadId
       <> ", member:" <> Log.buildString' member.memberId
+      <> ", group:" <> Log.build groupId
     IO.atomicWriteIORef member.heartbeatThread (Just threadId)
 
 -- cancel all delayedCheckHearts
@@ -751,6 +795,7 @@ cancelDelayedCheckHeartbeats Group{..} = do
       Nothing -> pure ()
       Just tid -> do
         Log.info $ "cancel delayedCheckHeart, member:" <> Log.buildString' mid
+          <> "group:" <> Log.build groupId
         C.killThread tid
         IO.atomicWriteIORef member.heartbeatThread Nothing
 
@@ -776,6 +821,7 @@ checkHeartbeatAndMaybeRebalance group member = do
         <> ", lastHeartbeat:" <> Log.buildString' lastUpdated
         <> ", now:" <> Log.buildString' now
         <> ", sessionTimeoutMs:" <> Log.buildString' memberSessionTimeoutMs
+        <> ", group:" <> Log.build group.groupId
       -- remove itself (to avoid killing itself in prepareRebalance)
       IO.atomicWriteIORef member.heartbeatThread Nothing
       removeMemberAndUpdateGroup group member
@@ -908,3 +954,16 @@ getMemberValue protocol assignments member = do
     , assignment=assignment
     , clientId=member.clientId
     }
+
+------------------- Group Error Response -------------------------
+makeJoinResponseError :: T.Text -> K.ErrorCode -> K.JoinGroupResponse
+makeJoinResponseError memberId errorCode =
+  K.JoinGroupResponse
+    { errorCode = errorCode
+    , generationId = -1
+    , protocolName = ""
+    , leader = ""
+    , memberId = memberId
+    , members = K.NonNullKaArray V.empty
+    , throttleTimeMs = 0
+    }