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Posted to user@flink.apache.org by "Raja.Aravapalli" <Ra...@target.com> on 2017/08/11 19:41:48 UTC
Re: [EXTERNAL] Re: difference between checkpoints & savepoints
Thanks for the discussion. That answered many questions I have.
Also, in the same line, can someone detail the difference between State Backend & External checkpoint?
Also, programmatic API, thru which methods we can configure those.
Regards,
Raja.
From: Stefan Richter <s....@data-artisans.com>
Date: Thursday, August 10, 2017 at 11:38 AM
To: Henri Heiskanen <he...@gmail.com>
Cc: "user@flink.apache.org" <us...@flink.apache.org>
Subject: [EXTERNAL] Re: difference between checkpoints & savepoints
I most of the things you are asking for are already there: you can configure checkpoint interval + externalized cp, the backend, and the location for savepoints and externalized checkpoints. You can restart from savepoints and externalized checkpoints from the CLI. One point I am not entirely sure about are automatic CP or SP when a job is shut down. IIRC, this is either already available, or in the making.
Resolving the last external checkpoint is as easy as listing the configured directory, especially if you only retain the last one. Otherwise the timestamp gives the required information. It is true that there could also be an CLI option to automatically does the work to pick the latest.
And there is a command line parameter switch to supply savepoints and externalized checkpoints for restarts. I think that makes more sense than a general configuration of automatic restart behaviour because the user might also intend to start a new, clean run for the job.
Am 10.08.2017 um 15:45 schrieb Henri Heiskanen <he...@gmail.com>>:
Hi,
But I still need to resolve the latest checkpoint and pass that as an argument. My question still is that why all this can not be handled by Flink core? Why not just have parameters enable savepoints, location of savepoints and state backend and system would then automatically do checkpoints / savepoints on exit and also start from the first available checkpoint?
Br,
Henkka
On Thu, Aug 10, 2017 at 3:15 PM, Stefan Richter <s....@data-artisans.com>> wrote:
Hi,
but I think this is exactly the case for externalized checkpoints. Periodic savepoints are problematic because, their lifecycle is meant to be under the control of the user and Flink can not make any assumptions when they can be dropped. So in the conservative scenario, savepoints would quickly pile up. With externalized checkpoints, you can control the number of retained checkpoints. if you set this number to one, that should be exactly what you want.
As for rescalability, this limitation is more of a future than a current problem. Right now, you should be able to rescale from all externalized checkpoints. But this might not hold in the future, because you can optimize checkpoints in some cases if this is feature dropped.
Right now, externalized checkpoints should offer all that you want.
Best,
Stefan
Am 10.08.2017 um 11:46 schrieb Henri Heiskanen <he...@gmail.com>>:
Hi,
It would be super helpful if Flink would provide out of the box functionality for writing automatic savepoints and then starting from the latest savepoint. If external checkpoints would support rescaling then 1st requirement is met, but one would still need to e.g. find the latest checkpoint from some folder and pass that as argument. We are currently writing our own functionality for this. Why not just tell Flink that this job uses persistent states and default functionality is then to start from the latest snapshot.
Br,
Henri H
On Thu, Aug 10, 2017 at 11:20 AM, Stefan Richter <s....@data-artisans.com>> wrote:
Hi,
I would explain the main conceptual difference as follows:
- Checkpoints are periodically triggered by the system for fault tolerance. They are used to automatically recover from failures. Because of their automatic and periodical nature, they should be lightweight to produce and will restore the same job without any changes to the jobgraph, parallelism, etc. Checkpoints are usually dropped after the job was terminated by the user.
- Savepoints are triggered by the user to store the state of the job for a manual resume and backup. Savepoints are usually not periodical but typically taken before some user actions to the job or the system. For example, this could be an update of your Flink version, changing your job graph, changing parallelism, forking a second job like for a red/blue deployment, and so on. Of course, savepoints must survive job termination. Conceptually, savepoints can be a bit more expensive to produce, because they should have a format that makes all those „changes to the job“ features possible.
Besides this conceptual difference, the current implementations are basically using the same code and produce the same „format". However, there is currently one exception from this, but I would expect more differences in the future. This exception are incremental checkpoints with the RocksDB state backend. They are using some RocksDB internal format instead of Flink’s „savepoint format“. This makes them the first instance of a more lightweight checkpointing mechanism, compared to savepoints, at the cost of dropping support for certain features such as changing the parallelism.
Furthermore, there also exists „externalized checkpoints“, which are somewhere in between checkpoints and savepoints. They are triggered by Flink, but can survive job termination and can then be used by the user to restart the job, similar to savepoints. They use the checkpointing code path, so there are for example externalized incremental checkpoints. However, exactly like a normal checkpoints, they might also lack certain features like rescalability.
Best,
Stefan
Am 10.08.2017 um 05:32 schrieb Raja.Aravapalli <Ra...@target.com>>:
Hi,
Can someone please help me understand the difference between Flink's Checkpoints & Savepoints.
While I read the documentation, couldn't understand the difference! :s
Thanks a lot.
Regards,
Raja.
Re: [EXTERNAL] difference between checkpoints & savepoints
Posted by Hao Sun <ha...@zendesk.com>.
Hi team, I have one follow up question on this.
There is a discussion on resuming jobs from *a saved external checkpoint*,
I feel there are two aspects of that topic.
*1. I do not have changes to the job, just want to resume the job from a
failure.*
I can see this automatically happen with ZK enabled. I do not have to
manually do anything.
======
2017-12-01 05:02:26,603 DEBUG
org.apache.flink.runtime.jobmanager.ZooKeeperSubmittedJobGraphStore -
Recovering job graph f824eabe58d180d79416d9637ac6aa32 from
fraud_prevention_service/flink/jobgraphs/f824eabe58d180d79416d9637ac6aa32.
======
*2. I want to submit a new job and resume the previous process for whatever
reason. e.g. JobGraph changed, need to change parallelism, etc.*
https://ci.apache.org/projects/flink/flink-docs-release-1.3/setup/savepoints.html#faq
I am wondering for Flink 1.3.2, 1.4 and 1.5, does external checkpoint
identical to savepoint? Does it mean everything in the FAQ section, also
applies to the externalized checkpoint? *How about allowNonRestoredState,
do we have things like this for externalized chkpnt?*
I am running Flink 1.3.2 on K8S, so I am wondering what is the best
practice to do the deployment for new code releases. And Flip6 is awesome,
can't wait to use it.
Thanks as always.
On Wed, Aug 16, 2017 at 5:23 PM Raja.Aravapalli <Ra...@target.com>
wrote:
>
>
> Thanks very much for the detailed explanation Stefan.
>
>
>
>
>
> Regards,
>
> Raja.
>
>
>
> *From: *Stefan Richter <s....@data-artisans.com>
> *Date: *Monday, August 14, 2017 at 7:47 AM
> *To: *Raja Aravapalli <Ra...@target.com>
> *Cc: *"user@flink.apache.org" <us...@flink.apache.org>
> *Subject: *Re: [EXTERNAL] difference between checkpoints & savepoints
>
>
>
> Just noticed that I forgot to include also a reference to the
> documentation about externalized checkpoints:
> https://ci.apache.org/projects/flink/flink-docs-master/ops/state/checkpoints.html
> <https://ci.apache.org/projects/flink/flink-docs-master/ops/state/checkpoints.html>
>
>
>
> Am 14.08.2017 um 14:17 schrieb Stefan Richter <s.richter@data-artisans.com
> >:
>
>
>
>
>
> Hi,
>
>
>
>
>
> Also, in the same line, can someone detail the difference between State
> Backend & External checkpoint?
>
>
>
>
>
> Those are two very different things. If we talk about state backends in
> Flink, we mean the entity that is responsible for storing and managing the
> state inside an operator. This could for example be something like the
> FsStateBackend that is based on hash maps and keeps state on the heap, or
> the RocksDBStateBackend which is using RocksDB as a store internally and
> operates on native memory and disk.
>
>
>
> An externalized checkpoint, like a normal checkpoint, is the collection of
> all state in a job persisted to stable storage for recovery. A little more
> concrete, this typically means writing out the contents of the state
> backends to a save place so that we can restore them from there.
>
>
>
> Also, programmatic API, thru which methods we can configure those.
>
>
>
> This explains how to set the backend programatically:
>
>
>
>
> https://ci.apache.org/projects/flink/flink-docs-master/ops/state/state_backends.html
> <https://ci.apache.org/projects/flink/flink-docs-master/ops/state/state_backends.html>
>
>
>
> To activate externalized checkpoints, you activate normal checkpoints,
> plus the following line:
>
>
>
> env.getCheckpointConfig().enableExternalizedCheckpoints(CheckpointConfig.ExternalizedCheckpointCleanup.*RETAIN_ON_CANCELLATION*);
>
>
>
> where env is your StreamExecutionEnvironment.
>
>
>
> If you need an example, please take a look at the
> org.apache.flink.test.checkpointing.ExternalizedCheckpointITCase. This
> class configures everything you asked about programatically.
>
>
>
> Best,
>
> Stefan
>
>
>
>
>
Re: [EXTERNAL] difference between checkpoints & savepoints
Posted by "Raja.Aravapalli" <Ra...@target.com>.
Thanks very much for the detailed explanation Stefan.
Regards,
Raja.
From: Stefan Richter <s....@data-artisans.com>
Date: Monday, August 14, 2017 at 7:47 AM
To: Raja Aravapalli <Ra...@target.com>
Cc: "user@flink.apache.org" <us...@flink.apache.org>
Subject: Re: [EXTERNAL] difference between checkpoints & savepoints
Just noticed that I forgot to include also a reference to the documentation about externalized checkpoints: https://ci.apache.org/projects/flink/flink-docs-master/ops/state/checkpoints.html
Am 14.08.2017 um 14:17 schrieb Stefan Richter <s....@data-artisans.com>>:
Hi,
Also, in the same line, can someone detail the difference between State Backend & External checkpoint?
Those are two very different things. If we talk about state backends in Flink, we mean the entity that is responsible for storing and managing the state inside an operator. This could for example be something like the FsStateBackend that is based on hash maps and keeps state on the heap, or the RocksDBStateBackend which is using RocksDB as a store internally and operates on native memory and disk.
An externalized checkpoint, like a normal checkpoint, is the collection of all state in a job persisted to stable storage for recovery. A little more concrete, this typically means writing out the contents of the state backends to a save place so that we can restore them from there.
Also, programmatic API, thru which methods we can configure those.
This explains how to set the backend programatically:
https://ci.apache.org/projects/flink/flink-docs-master/ops/state/state_backends.html
To activate externalized checkpoints, you activate normal checkpoints, plus the following line:
env.getCheckpointConfig().enableExternalizedCheckpoints(CheckpointConfig.ExternalizedCheckpointCleanup.RETAIN_ON_CANCELLATION);
where env is your StreamExecutionEnvironment.
If you need an example, please take a look at the org.apache.flink.test.checkpointing.ExternalizedCheckpointITCase. This class configures everything you asked about programatically.
Best,
Stefan
Re: [EXTERNAL] difference between checkpoints & savepoints
Posted by Stefan Richter <s....@data-artisans.com>.
Just noticed that I forgot to include also a reference to the documentation about externalized checkpoints: https://ci.apache.org/projects/flink/flink-docs-master/ops/state/checkpoints.html <https://ci.apache.org/projects/flink/flink-docs-master/ops/state/checkpoints.html>
> Am 14.08.2017 um 14:17 schrieb Stefan Richter <s....@data-artisans.com>:
>
>
> Hi,
>
>>
>> Also, in the same line, can someone detail the difference between State Backend & External checkpoint?
>>
>
> Those are two very different things. If we talk about state backends in Flink, we mean the entity that is responsible for storing and managing the state inside an operator. This could for example be something like the FsStateBackend that is based on hash maps and keeps state on the heap, or the RocksDBStateBackend which is using RocksDB as a store internally and operates on native memory and disk.
>
> An externalized checkpoint, like a normal checkpoint, is the collection of all state in a job persisted to stable storage for recovery. A little more concrete, this typically means writing out the contents of the state backends to a save place so that we can restore them from there.
>
>> Also, programmatic API, thru which methods we can configure those.
>
> This explains how to set the backend programatically:
>
> https://ci.apache.org/projects/flink/flink-docs-master/ops/state/state_backends.html <https://ci.apache.org/projects/flink/flink-docs-master/ops/state/state_backends.html>
>
> To activate externalized checkpoints, you activate normal checkpoints, plus the following line:
>
> env.getCheckpointConfig().enableExternalizedCheckpoints(CheckpointConfig.ExternalizedCheckpointCleanup.RETAIN_ON_CANCELLATION);
>
> where env is your StreamExecutionEnvironment.
>
> If you need an example, please take a look at the org.apache.flink.test.checkpointing.ExternalizedCheckpointITCase. This class configures everything you asked about programatically.
>
> Best,
> Stefan
>
Re: [EXTERNAL] difference between checkpoints & savepoints
Posted by Stefan Richter <s....@data-artisans.com>.
Hi,
>
> Also, in the same line, can someone detail the difference between State Backend & External checkpoint?
>
Those are two very different things. If we talk about state backends in Flink, we mean the entity that is responsible for storing and managing the state inside an operator. This could for example be something like the FsStateBackend that is based on hash maps and keeps state on the heap, or the RocksDBStateBackend which is using RocksDB as a store internally and operates on native memory and disk.
An externalized checkpoint, like a normal checkpoint, is the collection of all state in a job persisted to stable storage for recovery. A little more concrete, this typically means writing out the contents of the state backends to a save place so that we can restore them from there.
> Also, programmatic API, thru which methods we can configure those.
This explains how to set the backend programatically:
https://ci.apache.org/projects/flink/flink-docs-master/ops/state/state_backends.html <https://ci.apache.org/projects/flink/flink-docs-master/ops/state/state_backends.html>
To activate externalized checkpoints, you activate normal checkpoints, plus the following line:
env.getCheckpointConfig().enableExternalizedCheckpoints(CheckpointConfig.ExternalizedCheckpointCleanup.RETAIN_ON_CANCELLATION);
where env is your StreamExecutionEnvironment.
If you need an example, please take a look at the org.apache.flink.test.checkpointing.ExternalizedCheckpointITCase. This class configures everything you asked about programatically.
Best,
Stefan