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https://issues.apache.org/jira/browse/KAFKA-1555?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Gwen Shapira updated KAFKA-1555:
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Attachment: KAFKA-1555-DOCS.3.patch
Thank you for the detailed review [~jjkoshy]. I think the design documentation
is far clearer now.
One comment I did not incorporate into the docs:
{quote}
* The sentence that follows ("Note, however...") contains details that seem
redundant to what has already been said in parantheses. So we can remove one or
the other.
{quote}
I added this note since this is a specific topic of confusion - why isn't
"acks=-1" enough to guarantee consistency. I've explained this in detail to
multiple customers, users on mailing list, product managers, support, etc. I
think it deserves a sentence in our documentation.
Regarding the design itself - perhaps its worth its own JIRA with discussion.
IMO, when a user specifies acks=[0,1], they are basically declaring that they
don't care much if the message is lost due to a replica failure. Therefore
rejecting the message due to lack of replicas will be surprising and
counter-intuitive. However, most of my users are in the strong consistency camp
and simply don't use acks=[0,1]. Perhaps you have example scenarios where these
combinations make sense?
> 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: Gwen Shapira
> Fix For: 0.8.2
>
> Attachments: KAFKA-1555-DOCS.0.patch, KAFKA-1555-DOCS.1.patch,
> KAFKA-1555-DOCS.2.patch, KAFKA-1555-DOCS.3.patch, KAFKA-1555.0.patch,
> KAFKA-1555.1.patch, KAFKA-1555.2.patch, KAFKA-1555.3.patch,
> KAFKA-1555.4.patch, KAFKA-1555.5.patch, KAFKA-1555.5.patch,
> KAFKA-1555.6.patch, KAFKA-1555.8.patch, KAFKA-1555.9.patch
>
>
> 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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