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From Guozhang Wang <wangg...@gmail.com>
Subject Re: Producer fails when old brokers are replaced by new
Date Wed, 26 Feb 2014 17:04:25 GMT
Hello Chris,

The broker.metadata.list, once read in at start up time, will not be
changed. In other words, during the life time of a producer it has two
lists of brokers:

1. The current brokers in the cluster that is returned in the metadata
request response, which is dynamic

2. The broker list that is used for bootstraping, this is read from
broker.metadata.list and is fixed. This list could for example be a VIP and
a hardware load balancer behind it will distribute the metadata requests to
the brokers.

So in your case, the metadata list only has broker B, and once it is taken
out and the producer failed to send message to it and hence tries to
refresh its metadata, it has no broker to go.

Therefore, when you are trying to do a rolling bounce of the cluster to,
for example, do a in-place upgrade, you need to make sure at least one
broker in the list is alive during the rolling bounce.

Hope this helps.


On Wed, Feb 26, 2014 at 8:19 AM, Christofer Hedbrandh <
christofer@knewton.com> wrote:

> Hi all,
> I ran into a problem with the Kafka producer when attempting to replace all
> the nodes in a 0.8.0 Beta1 Release Kafka cluster, with 0.8.0 Release nodes.
> I started a producer/consumer test program to measure the clusters
> performance during the process, I added new brokers, I ran
> kafka-reassign-partitions.sh, and I removed the old brokers. When I removed
> the old brokers my producer failed.
> The simplest scenario that I could come up with where I still see this
> behavior is this. Using version 0.8.0 Release, we have a 1 partition topic
> with 2 replicas on 2 brokers, broker A and broker B. Broker A is taken
> down. A producer is started with only broker B in the metadata.broker.list.
> Broker A is brought back up. We let
> topic.metadata.refresh.interval.msamount of time pass. Broker B is
> taken down, and we get
> kafka.common.FailedToSendMessageException after all the (many) retries have
> failed.
> During my experimentation I have made sure that the producer fetches meta
> data before the old broker is taken down. And I have made sure that enough
> retries with enough backoff time were used for the producer to not give up
> prematurely.
> The documentation for the producer config metadata.broker.list suggests to
> me that this list of brokers is only used at startup. "This is for
> bootstrapping and the producer will only use it for getting metadata
> (topics, partitions and replicas)". And when I read about
> topic.metadata.refresh.interval.ms and retry.backoff.ms I learn that meta
> data is indeed fetched at later times. Based on this documentation, I make
> the assumption that the producer would learn about any new brokers when new
> meta data is fetched.
> I also want to point out that the cluster seems to work just fine during
> this process, it only seems to be a problem with the producer. Between all
> these steps I run kafka-list-topic.sh, I try the console producer and
> consumer, and everything is as expected.
> Also I found another interesting thing when experimenting with running
> kafka-preferred-replica-election.sh before taking down the old broker. This
> script only causes any changes when the leader and the preferred replica
> are different. In the scenario when they are in fact different, and the new
> broker takes the role of leader from the old broker, the producer does NOT
> fail. This makes me think that perhaps the producer only keeps meta data
> about topic leaders and not all replicas, as the documentation suggests to
> me.
> It is clear that I am making a lot of assumptions here, and I am relatively
> new to Kafka so I could very well me missing something important. The way I
> see it, there are a few possibilities.
> 1. Broker discovery is a supposed producer feature, and it has a bug.
> 2. Broker discovery is not a producer feature, in which case I think many
> people might benefit from a clearer documentation.
> 3. I am doing something dumb e.g. forgetting about some important
> configuration.
> Please let me know what you make of this.
> Thanks,
> Christofer Hedbrandh

-- Guozhang

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