abbccdda commented on a change in pull request #9270:
URL: https://github.com/apache/kafka/pull/9270#discussion_r492896997



##########
File path: 
core/src/test/scala/unit/kafka/coordinator/group/GroupCoordinatorTest.scala
##########
@@ -3882,6 +3942,21 @@ class GroupCoordinatorTest {
     Await.result(responseFuture, Duration(rebalanceTimeout + 100, 
TimeUnit.MILLISECONDS))
   }
 
+  private def staticJoinGroupWithPersistence(groupId: String,
+                                 memberId: String,

Review comment:
       nit: alignment

##########
File path: core/src/main/scala/kafka/coordinator/group/GroupCoordinator.scala
##########
@@ -1040,21 +1040,36 @@ class GroupCoordinator(val brokerId: Int,
 
     group.currentState match {
       case Stable =>
-        info(s"Static member joins during Stable stage will not trigger 
rebalance.")
-        group.maybeInvokeJoinCallback(member, JoinGroupResult(
-          members = List.empty,
-          memberId = newMemberId,
-          generationId = group.generationId,
-          protocolType = group.protocolType,
-          protocolName = group.protocolName,
-          // We want to avoid current leader performing trivial assignment 
while the group
-          // is in stable stage, because the new assignment in leader's next 
sync call
-          // won't be broadcast by a stable group. This could be guaranteed by
-          // always returning the old leader id so that the current leader 
won't assume itself
-          // as a leader based on the returned message, since the new 
member.id won't match
-          // returned leader id, therefore no assignment will be performed.
-          leaderId = currentLeader,
-          error = Errors.NONE))
+        // check if group's selectedProtocol of next generation will change, 
if not, simply store group to persist the
+        // updated static member, if yes, rebalance should be triggered to let 
the group's assignment and selectProtocol consistent
+        val selectedProtocolOfNextGeneration = group.selectProtocol
+        if (group.protocolName.contains(selectedProtocolOfNextGeneration)) {
+          info(s"Static member which joins during Stable stage and doesn't 
affect selectProtocol will not trigger rebalance.")
+          val groupAssignment: Map[String, Array[Byte]] = 
group.allMemberMetadata.map(member => member.memberId -> 
member.assignment).toMap
+          groupManager.storeGroup(group, groupAssignment, error => {
+            group.inLock {
+              if (error != Errors.NONE) {
+                warn(s"Failed to persist metadata for group ${group.groupId}: 
${error.message}")
+              }
+            }
+          })
+          group.maybeInvokeJoinCallback(member, JoinGroupResult(
+            members = List.empty,
+            memberId = newMemberId,
+            generationId = group.generationId,
+            protocolType = group.protocolType,
+            protocolName = group.protocolName,
+            // We want to avoid current leader performing trivial assignment 
while the group
+            // is in stable stage, because the new assignment in leader's next 
sync call
+            // won't be broadcast by a stable group. This could be guaranteed 
by
+            // always returning the old leader id so that the current leader 
won't assume itself
+            // as a leader based on the returned message, since the new 
member.id won't match
+            // returned leader id, therefore no assignment will be performed.
+            leaderId = currentLeader,
+            error = Errors.NONE))
+        } else {
+          maybePrepareRebalance(group, s"Group's selectedProtocol will change 
because static member ${member.memberId} with instance id $groupInstanceId 
joined with change of protocol")

Review comment:
       Could you elaborate why this case is possible? We do have checks for 
`!group.supportsProtocols(protocolType, 
MemberMetadata.plainProtocolSet(protocols)` in the caller, so if the group 
protocol is incompatible, won't we just reject the rejoin?

##########
File path: core/src/main/scala/kafka/coordinator/group/GroupCoordinator.scala
##########
@@ -1040,21 +1040,36 @@ class GroupCoordinator(val brokerId: Int,
 
     group.currentState match {
       case Stable =>
-        info(s"Static member joins during Stable stage will not trigger 
rebalance.")
-        group.maybeInvokeJoinCallback(member, JoinGroupResult(
-          members = List.empty,
-          memberId = newMemberId,
-          generationId = group.generationId,
-          protocolType = group.protocolType,
-          protocolName = group.protocolName,
-          // We want to avoid current leader performing trivial assignment 
while the group
-          // is in stable stage, because the new assignment in leader's next 
sync call
-          // won't be broadcast by a stable group. This could be guaranteed by
-          // always returning the old leader id so that the current leader 
won't assume itself
-          // as a leader based on the returned message, since the new 
member.id won't match
-          // returned leader id, therefore no assignment will be performed.
-          leaderId = currentLeader,
-          error = Errors.NONE))
+        // check if group's selectedProtocol of next generation will change, 
if not, simply store group to persist the
+        // updated static member, if yes, rebalance should be triggered to let 
the group's assignment and selectProtocol consistent
+        val selectedProtocolOfNextGeneration = group.selectProtocol
+        if (group.protocolName.contains(selectedProtocolOfNextGeneration)) {
+          info(s"Static member which joins during Stable stage and doesn't 
affect selectProtocol will not trigger rebalance.")
+          val groupAssignment: Map[String, Array[Byte]] = 
group.allMemberMetadata.map(member => member.memberId -> 
member.assignment).toMap
+          groupManager.storeGroup(group, groupAssignment, error => {
+            group.inLock {
+              if (error != Errors.NONE) {

Review comment:
       Should we reply join failure if the persistence of the group metadata 
failed?




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