[jira] [Updated] (KAFKA-1555) provide strong consistency with reasonable availability

["Joe Stein (JIRA)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/KAFKA-1555?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Joe Stein updated KAFKA-1555:
-----------------------------
    Fix Version/s: 0.8.2

> 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
>             Fix For: 0.8.2
>
>
> 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.



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