[KAFKA-3410] Unclean leader election and "Halting because log truncation is not allowed"

• Type: Bug
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Component/s: replication
• Labels: reliability
• Assignee: Unassigned
• Reporter: James Cheng
• Created: 16/Mar/16 06:06
• Updated: 24/Feb/23 20:02
• Resolved: 24/Feb/23 20:02

Description

I ran into a scenario where one of my brokers would continually shutdown, with the error message:
[2016-02-25 00:29:39,236] FATAL [ReplicaFetcherThread-0-1], Halting because log truncation is not allowed for topic test, Current leader 1's latest offset 0 is less than replica 2's latest offset 151 (kafka.server.ReplicaFetcherThread)

I managed to reproduce it with the following scenario:
1. Start broker1, with unclean.leader.election.enable=false
2. Start broker2, with unclean.leader.election.enable=false

3. Create topic, single partition, with replication-factor 2.
4. Write data to the topic.

5. At this point, both brokers are in the ISR. Broker1 is the partition leader.

6. Ctrl-Z on broker2. (Simulates a GC pause or a slow network) Broker2 gets dropped out of ISR. Broker1 is still the leader. I can still write data to the partition.

7. Shutdown Broker1. Hard or controlled, doesn't matter.

8. rm -rf the log directory of broker1. (This simulates a disk replacement or full hardware replacement)

9. Resume broker2. It attempts to connect to broker1, but doesn't succeed because broker1 is down. At this point, the partition is offline. Can't write to it.

10. Resume broker1. Broker1 resumes leadership of the topic. Broker2 attempts to join ISR, and immediately halts with the error message:
[2016-02-25 00:29:39,236] FATAL [ReplicaFetcherThread-0-1], Halting because log truncation is not allowed for topic test, Current leader 1's latest offset 0 is less than replica 2's latest offset 151 (kafka.server.ReplicaFetcherThread)

I am able to recover by setting unclean.leader.election.enable=true on my brokers.

I'm trying to understand a couple things:

• In step 10, why is broker1 allowed to resume leadership even though it has no data?
• In step 10, why is it necessary to stop the entire broker due to one partition that is in this state? Wouldn't it be possible for the broker to continue to serve traffic for all the other topics, and just mark this one as unavailable?
• Would it make sense to allow an operator to manually specify which broker they want to become the new master? This would give me more control over how much data loss I am willing to handle. In this case, I would want broker2 to become the new master. Or, is that possible and I just don't know how to do it?

Issue Links

• relates to: KAFKA-3955 Kafka log recovery doesn't truncate logs on non-monotonic offsets, leading to failed broker boot
• relates to: KAFKA-3861 Shrunk ISR before leader crash makes the partition unavailable
• relates to: KAFKA-4414 Unexpected "Halting because log truncation is not allowed"
• relates to: KAFKA-3918 Broker faills to start after ungraceful shutdown due to non-monotonically incrementing offsets in logs
• relates to: KAFKA-3924 Data loss due to halting when LEO is larger than leader's LEO