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From Stephan Ewen <se...@apache.org>
Subject Re: High Availability on Yarn
Date Mon, 08 May 2017 16:00:32 GMT
@Ankit:

ZooKeeper is required in YARN setups still. Even if there is only one
JobManager in the normal case, Yarn can accidentally create a second one
when there is a network partition.
To prevent that this leads to inconsistencies, we use ZooKeeper.

Flink uses ZooKeeper very little, so you can just let Flink attach to any
existing ZooKeeper, or user one ZooKeeper cluster for very many Flink
clusters/jobs.

Stephan


On Mon, May 8, 2017 at 2:11 PM, Aljoscha Krettek <aljoscha@apache.org>
wrote:

> Hi,
> Yes, it’s recommended to use one ZooKeeper cluster for all Flink clusters.
>
> Best,
> Aljoscha
>
> On 5. May 2017, at 16:56, Jain, Ankit <ankit.jain@here.com> wrote:
>
> Thanks for the update Aljoscha.
>
> @Till Rohrmann <trohrmann@apache.org>,
> Can you please chim in?
>
> Also, we currently have a long running EMR cluster where we create one
> flink cluster per job – can we just choose to install Zookeeper when
> creating the EMR cluster and use one Zookeeper instance for ALL of flink
> jobs?
> Or
> Recommendation is to have a dedicated Zookeeper instance per flink job?
>
> Thanks
> Ankit
>
> *From: *Aljoscha Krettek <aljoscha@apache.org>
> *Date: *Thursday, May 4, 2017 at 1:19 AM
> *To: *"Jain, Ankit" <ankit.jain@here.com>
> *Cc: *"user@flink.apache.org" <user@flink.apache.org>, Till Rohrmann <
> trohrmann@apache.org>
> *Subject: *Re: High Availability on Yarn
>
> Hi,
> Yes, for YARN there is only one running JobManager. As far as I Know, In
> this case ZooKeeper is only used to keep track of checkpoint metadata and
> the execution graph of the running job. Such that a restoring JobManager
> can pick up the data again.
>
> I’m not 100 % sure on this, though, so maybe Till can shed some light on
> this.
>
> Best,
> Aljoscha
>
> On 3. May 2017, at 16:58, Jain, Ankit <ankit.jain@here.com> wrote:
>
> Thanks for your reply Aljoscha.
>
> After building better understanding of Yarn and spending copious amount of
> time on Flink codebase, I think I now get how Flink & Yarn interact – I
> plan to document this soon in case it could help somebody starting afresh
> with Flink-Yarn.
>
> Regarding Zookeper, in YARN mode there is only one JobManager running, do
> we still need leader election?
>
> If the ApplicationMaster goes down (where JM runs) it is restarted by Yarn
> RM and while restarting, Flink AM will bring back previous running
> containers.  So, where does Zookeeper sit in this setup?
>
> Thanks
> Ankit
>
> *From: *Aljoscha Krettek <aljoscha@apache.org>
> *Date: *Wednesday, May 3, 2017 at 2:05 AM
> *To: *"Jain, Ankit" <ankit.jain@here.com>
> *Cc: *"user@flink.apache.org" <user@flink.apache.org>, Till Rohrmann <
> trohrmann@apache.org>
> *Subject: *Re: High Availability on Yarn
>
> Hi,
> As a first comment, the work mentioned in the FLIP-6 doc you linked is
> still work-in-progress. You cannot use these abstractions yet without going
> into the code and setting up a cluster “by hand”.
>
> The documentation for one-step deployment of a Job to YARN is available
> here: https://ci.apache.org/projects/flink/flink-docs-
> release-1.2/setup/yarn_setup.html#run-a-single-flink-job-on-yarn
> <https://emea01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fci.apache.org%2Fprojects%2Fflink%2Fflink-docs-release-1.2%2Fsetup%2Fyarn_setup.html%23run-a-single-flink-job-on-yarn&data=01%7C01%7C%7Cfb13823970ba4476ebbf08d49203846c%7C6d4034cd72254f72b85391feaea64919%7C1&sdata=fzBiQtv7MR2%2Fehg6GepwPa1uWxpqEgPJakto2B8k0Zk%3D&reserved=0>
>
> Regarding your third question, ZooKeeper is mostly used for discovery and
> leader election. That is, JobManagers use it to decide who is the main JM
> and who are standby JMs. TaskManagers use it to discover the leading
> JobManager that they should connect to.
>
> I’m also cc’ing Till, who should know this stuff better and can maybe
> explain it in a bit more detail.
>
> Best,
> Aljoscha
>
> On 1. May 2017, at 18:59, Jain, Ankit <ankit.jain@here.com> wrote:
>
> Hi fellow users,
> We are trying to straighten out high availability story for flink.
>
> Our setup includes a long running EMR cluster, job submission is a
> two-step process – 1) Flink cluster is first created using flink yarn
> client on the EMR cluster already running 2) Flink job is submitted.
>
> I also saw references that with 1.2, these two steps have been combined
> into 1 – is that change in FlinkYarnSessionCli.java? Can somebody point to
> documentation please?
>
> W/o worrying about Yarn RM (not Flink Yarn RM that seems to be newly
> introduced) failure for now, I want to understand first how task manager &
> job manager failures are handled.
>
> My questions-
> 1)       https://cwiki.apache.org/confluence/pages/viewpage.
> action?pageId=65147077
> <https://emea01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fcwiki.apache.org%2Fconfluence%2Fpages%2Fviewpage.action%3FpageId%3D65147077&data=01%7C01%7C%7Cfb13823970ba4476ebbf08d49203846c%7C6d4034cd72254f72b85391feaea64919%7C1&sdata=29Se7mWQZ09ukF3rkQNmSRPXY4RkA8RCNO4ec4Glj8I%3D&reserved=0>
>  suggests a new RM has been added and now there is one JobManager for
> each job. Since Yarn RM will now talk to Flink RM( instead of JobManager
> previously), will Yarn automatically restart failing Flink RM?
> 2)       Is there any documentation on behavior of new Flink RM that will
> come up? How will previously running JobManagers & TaskManagers find out
> about new RM?
> 3)       https://ci.apache.org/projects/flink/flink-docs-
> release-1.3/setup/jobmanager_high_availability.html#configuration
> <https://emea01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fci.apache.org%2Fprojects%2Fflink%2Fflink-docs-release-1.3%2Fsetup%2Fjobmanager_high_availability.html%23configuration&data=01%7C01%7C%7Cfb13823970ba4476ebbf08d49203846c%7C6d4034cd72254f72b85391feaea64919%7C1&sdata=nYTYWaaWA4T1D7EwvL%2B7mwhrVcqn6xTzCv8SS6x%2FqLM%3D&reserved=0>
>  requires configuring Zookeeper even for Yarn – Is this needed for
> handling Task Manager failures or JM or both? Will Yarn not take care of JM
> failures?
>
> It may sound like I am little confused between role of Yarn and Flink
> components– who has the most burden of HA? Documentation in current state
> is lacking clarity – I know it is still evolving.
>
> Please let me know if somebody can help clear the confusion.
>
> Thanks
> Ankit
>
>
>
>
>
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>
>

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