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From Christofer Hedbrandh <christo...@knewton.com>
Subject Re: Producer fails when old brokers are replaced by new
Date Mon, 03 Mar 2014 21:35:20 GMT
Thanks again Guozhang.

There are still some details here that are unclear to me, but if what I am
describing is not a bug, do you think it is reasonable to file this as a
feature request? We agree that it is not ideal to have to keep "at least
one broker in the list is alive", when replacing a cluster i.e. migrating
from one set of brokers to another?

Christofer



On Wed, Feb 26, 2014 at 9:16 PM, Guozhang Wang <wangguoz@gmail.com> wrote:

> kafka-preferred-replica-election.sh is only used to move leaders between
> brokers, as long as the broker in the broker.metadata.list, i.e. the second
> broker list I mentioned in previous email is still alive then the producer
> can learn the leader change from it.
>
> In terms of broker discovery, I think it depends on how you "define" the
> future. For example, originally there are 3 brokers 1,2,3, and you start
> the producer with metadata list = {1,2,3}, and later on another three
> brokers 4,5,6 are added, the producer can still find these newly added
> brokers. It is just that if all the brokers in the metadata list, i.e.
> 1,2,3 are gone, then the producer will not be able to refresh its metadata.
>
> Guozhang
>
>
> On Wed, Feb 26, 2014 at 11:04 AM, Christofer Hedbrandh <
> christofer@knewton.com> wrote:
>
> > Thanks for your response Guozhang.
> >
> > I did make sure that new meta data is fetched before taking out the old
> > broker. I set the topic.metadata.refresh.interval.ms to something very
> > low,
> > and I confirm in the producer log that new meta data is actually fetched,
> > after the new broker is brought up, and before the old broker is taken
> > down. Does this not mean that the dynamic current brokers list would hold
> > the new broker at this point?
> >
> > If you are saying that the dynamic current brokers list is never used for
> > fetching meta data, this does not explain how the producer does NOT fail
> > when kafka-preferred-replica-election.sh makes the new broker become the
> > leader.
> >
> > Lastly, if broker discovery is not a producer feature in 0.8.0 Release,
> and
> > I have to "make sure at least one broker in the list is alive during the
> > rolling bounce", is this a feature you are considering for future
> versions?
> >
> >
> >
> > On Wed, Feb 26, 2014 at 12:04 PM, Guozhang Wang <wangguoz@gmail.com>
> > wrote:
> >
> > > 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.
> > >
> > > Guozhang
> > >
> > >
> > >
> > >
> > >
> > > 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
> > >
> >
>
>
>
> --
> -- Guozhang
>

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