Jiang Wu created KAFKA-1555:
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Summary: provide strong consistency with reasonable availability
Key: KAFKA-1555
URL: https://issues.apache.org/jira/browse/KAFKA-1555
Project: Kafka
Issue Type: Improvement
Components: controller
Affects Versions: 0.8.1.1
Reporter: Jiang Wu
Assignee: Neha Narkhede
In a mission critical application, we expect a kafka cluster with 3 brokers can
satisfy two requirements:
1. When 1 broker is down, no message loss or service blocking happens.
2. In worse cases such as two brokers are down, service can be blocked, but no
message loss happens.
We found that current kafka versoin (0.8.1.1) cannot achieve the requirements
due to its two behaviors:
1. when choosing a new leader from 2 followers in ISR, the one with less
messages may be chosen as the leader.
2. even when replica.lag.max.messages=0, a follower can stay in ISR when it has
less messages than the leader.
The following is an analytical proof.
We consider a cluster with 3 brokers and a topic with 3 replicas, and assume
that at the beginning, all 3 replicas, leader A, followers B and C, are in
sync, i.e., they have the same messages and are all in ISR.
According to the value of request.required.acks (acks for short), there are the
following cases.
1. acks=0, 1, 3. Obviously these settings do not satisfy the requirement.
2. acks=2. Producer sends a message m. It's acknowledged by A and B. At this
time, although C hasn't received m, C is still in ISR. If A is killed, C can be
elected as the new leader, and consumers will miss m.
3. acks=-1. Suppose replica.lag.max.messages=M. There are two sub-cases:
3.1 M>0. Suppose C be killed. C will be out of ISR after
replica.lag.time.max.ms. Then the producer publishes M messages to A and B. C
restarts. C will join in ISR since it is M messages behind A and B. Before C
replicates all messages, A is killed, and C becomes leader, then message loss
happens.
3.2 M=0. In this case, when the producer publishes at a high speed, B and C
will fail out of ISR; only A keeps receiving messages. Then A is killed. Either
message loss or service blocking will happen, depending on whether unclean
leader election is disabled (a new feature will be in 0.8.2, see jira
KAFKA-1028).
In summary, any existing configuration cannot satisfy the requirements.
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