GEORGE LI created KAFKA-8638:
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Summary: Preferred Leader Blacklist (deprioritized list)
Key: KAFKA-8638
URL: https://issues.apache.org/jira/browse/KAFKA-8638
Project: Kafka
Issue Type: Improvement
Components: config, controller, core
Affects Versions: 2.2.1, 2.3.0, 1.1.1
Reporter: GEORGE LI
Assignee: GEORGE LI
Currently, the kafka preferred leader election will pick the broker_id in the
topic/partition replica assignments in a priority order when the broker is in
ISR. The preferred leader is the broker id in the first position of replica.
There are use-cases that, even the first broker in the replica assignment is in
ISR, there is a need for it to be moved to the end of ordering (lowest
priority) when deciding leadership during preferred leader election.
Let’s use topic/partition replica (1,2,3) as an example. 1 is the preferred
leader. When preferred leadership is run, it will pick 1 as the leader if it's
ISR, if 1 is not online and in ISR, then pick 2, if 2 is not in ISR, then pick
3 as the leader. There are use cases that, even 1 is in ISR, we would like it
to be moved to the end of ordering (lowest priority) when deciding leadership
during preferred leader election. Below is a list of use cases:
# If broker_id 1 is a swapped failed host and brought up with last segments or
latest offset without historical data (There is another effort on this), it's
better for it to not serve leadership till it's caught-up.
# The cross-data center cluster has AWS instances which have less computing
power than the on-prem bare metal machines. We could put the AWS broker_ids in
Preferred Leader Blacklist, so on-prem brokers can be elected leaders, without
changing the reassignments ordering of the replicas.
# If the broker_id 1 is constantly losing leadership after some time:
"Flapping". we would want to exclude 1 to be a leader unless all other brokers
of this topic/partition are offline. The “Flapping” effect was seen in the
past when 2 or more brokers were bad, when they lost leadership
constantly/quickly, the sets of partition replicas they belong to will see
leadership constantly changing. The ultimate solution is to swap these bad
hosts. But for quick mitigation, we can also put the bad hosts in the
Preferred Leader Blacklist to move the priority of its being elected as leaders
to the lowest.
# If the controller is busy serving an extra load of metadata requests and
other tasks. we would like to put the controller's leaders to other brokers to
lower its CPU load. currently bouncing to lose leadership would not work for
Controller, because after the bounce, the controller fails over to another
broker.
# Avoid bouncing broker in order to lose its leadership: it would be good if we
have a way to specify which broker should be excluded from serving
traffic/leadership (without changing the replica assignment ordering by
reassignments, even though that's quick), and run preferred leader election. A
bouncing broker will cause temporary URP, and sometimes other issues. Also a
bouncing of broker (e.g. broker_id 1) can temporarily lose all its leadership,
but if another broker (e.g. broker_id 2) fails or gets bounced, some of its
leaderships will likely failover to broker_id 1 on a replica with 3 brokers.
If broker_id 1 is in the blacklist, then in such a scenario even broker_id 2
offline, the 3rd broker can take leadership.
The current work-around of the above is to change the topic/partition's replica
reassignments to move the broker_id 1 from the first position to the last
position and run preferred leader election. e.g. (1, 2, 3) => (2, 3, 1). This
changes the replica reassignments, and we need to keep track of the original
one and restore if things change (e.g. controller fails over to another broker,
the swapped empty broker caught up). That’s a rather tedious task.
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