mumrah commented on code in PR #12973:
URL: https://github.com/apache/kafka/pull/12973#discussion_r1047813311
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core/src/main/scala/kafka/zk/KafkaZkClient.scala:
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@@ -166,60 +166,74 @@ class KafkaZkClient private[zk] (zooKeeperClient:
ZooKeeperClient, isSecure: Boo
* the migration.
*
* To ensure that the KRaft controller epoch exceeds the current ZK
controller epoch, this registration algorithm
- * uses a conditional update on the /controller_epoch znode. If a new ZK
controller is elected during this method,
- * the conditional update on /controller_epoch fails which causes the whole
multi-op transaction to fail.
+ * uses a conditional update on the /controller and /controller_epoch znodes.
+ *
+ * If a new controller is registered concurrently with this registration,
one of the two will fail the CAS
+ * operation on /controller_epoch. For KRaft, we have an extra guard against
the registered KRaft epoch going
+ * backwards. If a KRaft controller had previously registered, an additional
CAS operation is done on the /controller
+ * ZNode to ensure that the KRaft epoch being registered is newer.
*
* @param kraftControllerId ID of the KRaft controller node
* @param kraftControllerEpoch Epoch of the KRaft controller node
- * @return An optional of the new zkVersion of /controller_epoch. None if we
could not register the KRaft controller.
+ * @return An optional of the written epoch and new zkVersion of
/controller_epoch. None if we could not register the KRaft controller.
*/
- def tryRegisterKRaftControllerAsActiveController(kraftControllerId: Int,
kraftControllerEpoch: Int): Option[Int] = {
+ def tryRegisterKRaftControllerAsActiveController(kraftControllerId: Int,
kraftControllerEpoch: Int): Option[(Int, Int)] = {
Review Comment:
As discussed offline, the controller epoch and zkVersion are both ints
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