Jun Rao created KAFKA-574:
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Summary: KafkaController unnecessarily reads leaderAndIsr info
from ZK
Key: KAFKA-574
URL: https://issues.apache.org/jira/browse/KAFKA-574
Project: Kafka
Issue Type: Bug
Components: core
Affects Versions: 0.8
Reporter: Jun Rao
Priority: Blocker
KafkaController calls updateLeaderAndIsrCache() in onBrokerFailure(). This is
unnecessary since in onBrokerFailure(), we will make leader and isr change
anyway so there is no need to first read that information from ZK. Latency is
critical in onBrokerFailure() since it determines how quickly a leader can be
made online.
Similarly, updateLeaderAndIsrCache() is called in onBrokerStartup()
unnecessarily. In this case, the controller does not change the leader or the
isr. It just needs to send the current leader and the isr info to the newly
started broker. We already cache leader in the controller. Isr in theory could
change any time by the leader. So, reading from ZK doesn't guarantee that we
can get the latest isr anyway. Instead, we just need to get the isr last
selected by the controller (which can be cached together with the leader in the
controller). If the leader epoc in a broker is at or larger than the epoc in
the leaderAndIsr request, the broker can just ignore it. Otherwise, the leader
and the isr selected by the controller should be used.
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