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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 18:33:32 GMT
Hi David,

When you (re)-start the producer/consumer, the broker list is used for
bootstrap, so it should guarantee that some of the brokers listed is alive
when the client starts. When you migrate from 1,2,3 to 4,5,6 (in fact, in
this case it is better to just keep the broker id but just replace the
host/port), and then bounce the clients, then it is better to also change
the bootstrap broker list since clients are memoryless. Does this make
sense?

I agree that using ZK the clients can get ride of the bootstrap broker
list, but that would add the dependency of ZooKeeper on the clients code
base. So we decided to remove the ZK dependency from the client instead,
and if people do want to use ZK for bootstraping, they can always add a
simple script which reads the broker list from ZK and give that to the
broker config.

Guozhang


On Mon, Mar 3, 2014 at 4:16 PM, David Birdsong <david.birdsong@gmail.com>wrote:

> On Mon, Mar 3, 2014 at 4:00 PM, Guozhang Wang <wangguoz@gmail.com> wrote:
>
> > 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.
> >
>
> But if you producer or consumer ever restarts and only knows about {1,2,3},
> the problem still exists no?
>
> This is why I bootstrap off of zk and expect to have to maintain an
> accurate list of zk nodes to all processes.
>
>
> >
> > 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
> >
>



-- 
-- Guozhang

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