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Jun Rao commented on KAFKA-1555: -------------------------------- In case 3.1, when C restarts, the protocol is that C can only join ISR if it has received all messages up to the current high watermark. For example, let's assume that M is 10. Let's say A, B, C all have messages at offset 100 and all those messages are committed (therefore high watermark is at 100). Then C dies. After that, we commit 5 more messages with both A and B (high watermark is at 105). Now, C is restarted. C is actually not allowed to rejoin ISR until its log end offset has passed 105. This means that C must first fetch the 5 newly committed messages before being added to ISR. > 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 three 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. > 3. ISR can contains only 1 broker, therefore acknowledged messages may be > stored in only 1 broker. > 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. B and C restart and are removed from ISR. Producer sends a > message m to A, and receives an acknowledgement. Disk failure happens in A > before B and C replicate m. Message m is lost. > In summary, any existing configuration cannot satisfy the requirements. -- This message was sent by Atlassian JIRA (v6.2#6252)