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From Guozhang Wang <wangg...@gmail.com>
Subject Re: Producer fails when old brokers are replaced by new
Date Tue, 04 Mar 2014 00:00:31 GMT
Hi Chris,

In 0.9 we will have just one "broker list", i.e. the list of brokers read
from the config file will be updated during bootstraping and all the future
metadata refresh operations. This feature should lift this limit you are
describing, for example, if your broker list in config is {1,2,3}, and
later on you extend the cluster to {1,2,3,4,5,6}, then now you can shut
down 1,2,3 all at once.

Guozhang


On Mon, Mar 3, 2014 at 1:35 PM, Christofer Hedbrandh <christofer@knewton.com
> wrote:

> 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
> >
>



-- 
-- Guozhang

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