flink-dev mailing list archives

Site index · List index
Message view « Date » · « Thread »
Top « Date » · « Thread »
From Paris Carbone <par...@kth.se>
Subject Re: [DISCUSS] FLIP-14: Loops API and Termination
Date Thu, 15 Dec 2016 15:16:46 GMT
Hey Gabor,

These are some meaningful concerns! 

Joining data outside one's scope is somewhat similar to a “goto” statement when you write
imperative code. It certainly allows you to have more complex data flow but then...programs
become arbitrary (nested structures have no meaning).
On top of that, in a decentralised execution there are practical reasons for scopes such as
decentralized progress tracking and termination establishment (Timely Dataflow went to strict
scopes for the same reasons in the past).

Given the current proposal:
For multiple outputs you can always put out everything as one stream and split and repartition
it outside the loop scope in a structured way so this is not a big issue.
Regarding binary input operators across scopes you can pre-join those streams before starting
a new scope/loop. 

Another take we look into is to decouple scopes and loops which can allow you to define a
custom scope with all streams you want to use registered in it.
We are preparing a prototype for that approach too so we can have it as an alternative, hopefully
in the same FLIP soon.

cheers
Paris


> On 15 Dec 2016, at 15:52, Gábor Gévay <ggab90@gmail.com> wrote:
> 
> Hi Paris and Fouad,
> 
> I finally had some time to delve into this. Thanks for the nice proposal!
> 
> +1 for also having a CoLoopFunction. That might be useful even if the
> input and feedback have the same type, as it might happen that I want
> to treat the elements coming on the feedback in a different way from
> the input.
> 
> I have some questions about StreamScope and the restrictions based on them:
> 
>> All binary operations can and should be restricted on data streams of the same StreamScope.
> 
> I'm wondering whether this might prevent some valid use cases, as it
> essentially restricts information getting into the loop to only the
> "official" input stream (i.e. the stream that is given to the loop
> method of the LoopFunction). For example, it might happen that I'm in
> some intermediate node in an iteration, but I also want to use some
> info from a stream in scope 0 that has nothing to do with the stream
> that is the "official" input of my iteration. So what I essentially
> mean is what if my iteration has multiple inputs? (Note: I actually do
> have such topologies in the paper that I'm currently writing.)
> 
> A similar concern is what if I would like to get out some information
> from the loop besides the "official" output stream that is being
> returned from LoopFunction.loop. That is, what if I want to have more
> than one output stream from an iteration?
> 
> Best,
> Gábor
> 
> 
> 
> 
> 2016-12-12 12:13 GMT+01:00 Paris Carbone <parisc@kth.se>:
>> Hi again folks,
>> 
>> It has been almost a month so I would like to remind you about the loop redesign
which many of you have checked already.
>> Anyone else excited about iterative streams and wants to add a comment or simply
+1 this? It would be more than welcome :)
>> 
>> You can find the description in the wiki [1] and the implementation as well.
>> We have put a lot of time on this and I think there is potential for even more future
improvements when it comes to iterative computation.
>> 
>> cheers
>> Paris
>> 
>> [1] https://cwiki.apache.org/confluence/display/FLINK/FLIP-15+Scoped+Loops+and+Job+Termination
>> 
>> On 17 Nov 2016, at 14:00, Paris Carbone <parisc@kth.se<mailto:parisc@kth.se>>
wrote:
>> 
>> That was fast!  Seems to be working.
>> Thank you Fabian!
>> 
>> On 17 Nov 2016, at 13:58, Fabian Hueske <fhueske@gmail.com<mailto:fhueske@gmail.com>>
wrote:
>> 
>> Hi Paris,
>> 
>> just gave you the permissions (I hope).
>> Let me know if something does not work.
>> 
>> Cheers, Fabian
>> 
>> 2016-11-17 13:48 GMT+01:00 Paris Carbone <parisc@kth.se<mailto:parisc@kth.se>>:
>> 
>> We do not have to schedule this for an early Flink release, just saying.
>> I would just like to get the changes out and you people can review it and
>> integrate it anytime at your own pace.
>> 
>> Who is the admin of the wiki? It would be nice to get write access.
>> 
>> On 17 Nov 2016, at 13:45, Paris Carbone <parisc@kth.se<mailto:parisc@kth.se>>
wrote:
>> 
>> Sounds like a plan!
>> 
>> Can someone grant me access to write in the wiki please?
>> My username is “senorcarbone”.
>> 
>> Paris
>> 
>> On 16 Nov 2016, at 14:30, Gyula Fóra <gyula.fora@gmail.com<mailto:gyula.fora@gmail.com>>
wrote:
>> 
>> I am not completely sure whether we should deprecate the old API for
>> 1.2 or
>> remove it completely. Personally I am in favor of removing it, I don't
>> think it is a huge burden to move to the new one if it makes for a much
>> nicer user experience.
>> 
>> I think you can go ahead add the FLIP to the wiki and open the PR so we
>> can
>> start the review if you have it ready anyways.
>> 
>> Gyula
>> 
>> Paris Carbone <parisc@kth.se<mailto:parisc@kth.se>> ezt írta (időpont:
2016. nov. 16., Sze,
>> 11:55):
>> 
>> Thanks for reviewing, Gyula.
>> 
>> One thing that is still up to discussion is whether we should remove
>> completely the old iterations API or simply mark it as deprecated till
>> v2.0.
>> Also, not sure what is the best process now. We have the changes ready.
>> Should I copy the FLIP to the wiki and trigger the PRs or wait for a
>> few
>> more days in case someone has objections?
>> 
>> @Stephan, what is your take on our interpretation of the approach you
>> suggested? Should we proceed or is there anything that you do not find
>> nice?
>> 
>> Paris
>> 
>> On 15 Nov 2016, at 10:01, Gyula Fóra <gyfora@apache.org<mailto:gyfora@apache.org>>
wrote:
>> 
>> Hi Paris,
>> 
>> I like the proposed changes to the iteration API, this cleans up
>> things
>> in
>> the Java API without any strict restriction I think (it was never a
>> problem
>> in the Scala API).
>> 
>> The termination algorithm based on the proposed scoped loops seems to
>> be
>> fairly simple and looks good :)
>> 
>> Cheers,
>> Gyula
>> 
>> Paris Carbone <parisc@kth.se<mailto:parisc@kth.se>> ezt írta (időpont:
2016. nov. 14., H,
>> 8:50):
>> 
>> That would be great Shi! Let's take that offline.
>> 
>> Anyone else interested in the iteration changes? It would be nice to
>> incorporate these to v1.2 if possible so I count on your review asap.
>> 
>> cheers,
>> Paris
>> 
>> On Nov 14, 2016, at 2:59 AM, xiaogang.sxg <
>> xiaogang.sxg@alibaba-inc.com<mailto:xiaogang.sxg@alibaba-inc.com>
>> <mailto:xiaogang.sxg@alibaba-inc.com>> wrote:
>> 
>> Hi Paris
>> 
>> Unfortunately, the project is not public yet.
>> But i can provide you a primitive implementation of the update
>> protocol
>> in
>> the paper. It’s implemented in Storm. Since the protocol assumes the
>> communication channels between different tasks are dual, i think it’s
>> not
>> easy to adapt it to Flink.
>> 
>> Regards
>> Xiaogang
>> 
>> 
>> 在 2016年11月12日,上午3:03,Paris Carbone <parisc@kth.se<mailto:parisc@
>> kth.se
>> 
>> 写道:
>> 
>> Hi Shi,
>> 
>> Naiad/Timely Dataflow and other projects use global coordination
>> which
>> is
>> very convenient for asynchronous progress tracking in general but it
>> has
>> some downsides in a production systems that count on in-flight
>> transactional control mechanisms and rollback recovery guarantees.
>> This
>> is
>> why we generally prefer decentralized approaches (despite their our
>> downsides).
>> 
>> Regarding synchronous/structured iterations, this is a bit off topic
>> and
>> they are a bit of a different story as you already know.
>> We maintain a graph streaming (gelly-streams) library on Flink that
>> you
>> might find interesting [1]. Vasia, another Flink committer is also
>> working
>> on that among others.
>> You can keep an eye on it since we are planning to use this project
>> as a
>> showcase for a new way of doing structured and fixpoint iterations on
>> streams in the future.
>> 
>> P.S. many thanks for sharing your publication, it was an interesting
>> read.
>> Do you happen to have your source code public? We could most
>> certainly
>> use
>> it in an benchmark soon.
>> 
>> [1] https://github.com/vasia/gelly-streaming
>> 
>> 
>> On 11 Nov 2016, at 19:18, SHI Xiaogang <shixiaogangg@gmail.com<
>> mailto:
>> shixiaogangg@gmail.com><mailto:shixiaogangg@gmail.com>> wrote:
>> 
>> Hi, Fouad
>> 
>> Thank you for the explanation. Now the centralized method seems
>> correct
>> to
>> me.
>> The passing of StatusUpdate events will lead to synchronous
>> iterations
>> and
>> we are using the information in each iterations to terminate the
>> computation.
>> 
>> Actually, i prefer the centralized method because in many
>> applications,
>> the
>> convergence may depend on some global statistics.
>> For example, a PageRank program may terminate the computation when
>> 99%
>> vertices are converged.
>> I think those learning programs which cannot reach the fixed-point
>> (oscillating around the fixed-point) can benefit a lot from such
>> features.
>> The decentralized method makes it hard to support such convergence
>> conditions.
>> 
>> 
>> Another concern is that Flink cannot produce periodical results in
>> the
>> iteration over infinite data streams.
>> Take a concrete example. Given an edge stream constructing a graph,
>> the
>> user may need the PageRank weight of each vertex in the graphs
>> formed at
>> certain instants.
>> Currently Flink does not provide any input or iteration information
>> to
>> users, making users hard to implement such real-time iterative
>> applications.
>> Such features are supported in both Naiad and Tornado. I think Flink
>> should
>> support it as well.
>> 
>> What do you think?
>> 
>> Regards
>> Xiaogang
>> 
>> 
>> 2016-11-11 19:27 GMT+08:00 Fouad ALi <fouad.alsayadi@gmail.com<
>> mailto:
>> fouad.alsayadi@gmail.com><mailto:fouad.alsayadi@gmail.com>>:
>> 
>> Hi Shi,
>> 
>> It seems that you are referring to the centralized algorithm which
>> is no
>> longer the proposed version.
>> In the decentralized version (check last doc) there is no master
>> node or
>> global coordination involved.
>> 
>> Let us keep this discussion to the decentralized one if possible.
>> 
>> To answer your points on the previous approach, there is a catch in
>> your
>> trace at t7. Here is what is happening :
>> - Head,as well as RS, will receive  a 'BroadcastStatusUpdate' from
>> runtime (see 2.1 in the steps).
>> - RS and Heads will broadcast StatusUpdate  event and will not notify
>> its
>> status.
>> - When StatusUpdate event gets back to the head it will notify its
>> WORKING  status.
>> 
>> Hope that answers your concern.
>> 
>> Best,
>> Fouad
>> 
>> On Nov 11, 2016, at 6:21 AM, SHI Xiaogang <shixiaogangg@gmail.com
>> <mailto:
>> shixiaogangg@gmail.com><mailto:shixiaogangg@gmail.com>>
>> wrote:
>> 
>> Hi Paris
>> 
>> I have several concerns about the correctness of the termination
>> protocol.
>> I think the termination protocol put an end to the computation even
>> when
>> the computation has not converged.
>> 
>> Suppose there exists a loop context constructed by a OP operator, a
>> Head
>> operator and a Tail operator (illustrated in Figure 2 in the first
>> draft).
>> The stream only contains one record. OP will pass the record to its
>> downstream operators 10 times. In other words, the loop should
>> iterate
>> 10
>> times.
>> 
>> If I understood the protocol correctly, the following event sequence
>> may
>> happen in the computation:
>> t1:  RS emits Record to OP. Since RS has reached the "end-of-stream",
>> the
>> system enters into Speculative Phase.
>> t2:  OP receives Record and emits it to TAIL.
>> t3:  HEAD receives the UpdateStatus event, and notifies with an IDLE
>> state.
>> t4. OP receives the UpdateStatus event from HEAD, and notifies with
>> an
>> WORKING state.
>> t5. TAIL receives Record and emits it to HEAD.
>> t6. TAIL receives the UpdateStatus event from OP, and notifies with
>> an
>> WORKING state.
>> t7. The system starts a new attempt. HEAD receives the UpdateStatus
>> event
>> and notifies with an IDLE state.  (Record is still in transition.)
>> t8. OP receives the UpdateStatus event from HEAD and notifies with an
>> IDLE
>> state.
>> t9. TAIL receives the UpdateStatus event from OP and notifies with an
>> IDLE
>> state.
>> t10. HEAD receives Record from TAIL and emits it to OP.
>> t11. System puts an end to the computation.
>> 
>> Though the computation is expected to iterate 10 times, it ends
>> earlier.
>> The cause is that the communication channels of MASTER=>HEAD and
>> TAIL=>HEAD
>> are not synchronized.
>> 
>> I think the protocol follows the idea of the Chandy-Lamport
>> algorithm to
>> determine a global state.
>> But the information of whether a node has processed any record to
>> since
>> the
>> last request is not STABLE.
>> Hence i doubt the correctness of the protocol.
>> 
>> To determine the termination correctly, we need some information
>> that is
>> stable.
>> In timelyflow, Naiad collects the progress made in each iteration and
>> terminates the loop when a little progress is made in an iteration
>> (identified by the timestamp vector).
>> The information is stable because the result of an iteration cannot
>> be
>> changed by the execution of later iterations.
>> 
>> A similar method is also adopted in Tornado.
>> You may see my paper for more details about the termination of loops:
>> http://net.pku.edu.cn/~cuibin/Papers/2016SIGMOD.pdf <
>> http://net.pku.edu.cn/~cuibin/Papers/2016SIGMOD.pdf>
>> 
>> Regards
>> Xiaogang
>> 
>> 2016-11-11 3:19 GMT+08:00 Paris Carbone <parisc@kth.se<mailto:
>> parisc@kth.se><mailto:parisc@kth.se> <mailto:
>> parisc@kth.se<mailto:parisc@kth.se><mailto:parisc@kth.se>>>:
>> 
>> Hi again Flink folks,
>> 
>> Here is our new proposal that addresses Job Termination - the loop
>> fault
>> tolerance proposal will follow shortly.
>> As Stephan hinted, we need operators to be aware of their scope
>> level.
>> 
>> Thus, it is time we make loops great again! :)
>> 
>> Part of this FLIP basically introduces a new functional,
>> compositional
>> API
>> for defining asynchronous loops for DataStreams.
>> This is coupled with a decentralized algorithm for job termination
>> with
>> loops - along the lines of what Stephan described.
>> We are already working on the actual prototypes as you can observe in
>> the
>> links of the doc.
>> 
>> Please let us know if you like (or don't like) it and why, in this
>> mail
>> discussion.
>> 
>> https://docs.google.com/document/d/1nzTlae0AFimPCTIV1LB3Z2y-
>> PfTHtq3173EhsAkpBoQ
>> 
>> cheers
>> Paris and Fouad
>> 
>> On 31 Oct 2016, at 12:53, Paris Carbone <parisc@kth.se<mailto:
>> parisc@kth.se> <mailto:
>> parisc@kth.se<mailto:parisc@kth.se><mailto:parisc@kth.se>><mailto:
>> parisc@
>> kth.se<http://kth.se/><http://kth.se<http://kth.se/>> <
>> http://kth.se/
>> 
>> wrote:
>> 
>> Hey Stephan,
>> 
>> Thanks for looking into it!
>> 
>> +1 for breaking this up, will do that.
>> 
>> I can see your point and maybe it makes sense to introduce part of
>> scoping
>> to incorporate support for nested loops (otherwise it can’t work).
>> Let us think about this a bit. We will share another draft for a more
>> detail description of the approach you are suggesting asap.
>> 
>> 
>> On 27 Oct 2016, at 10:55, Stephan Ewen <sewen@apache.org<mailto:
>> sewen@apache.org><mailto:sewen@apache.org> <mailto:
>> sewen@apache.org<mailto:sewen@apache.org><mailto:sewen@apache.org
>> <mailto:sewen
>> @apache.org<http://apache.org/>>> wrote:
>> 
>> How about we break this up into two FLIPs? There are after all two
>> orthogonal problems (termination, fault tolerance) with quite
>> different
>> discussion states.
>> 
>> Concerning fault tolerance, I like the ideas.
>> For the termination proposal, I would like to iterate a bit more.
>> 
>> *Termination algorithm:*
>> 
>> My main concern here is the introduction of a termination coordinator
>> and
>> any involvement of RPC messages when deciding termination.
>> That would be such a fundamental break with the current runtime
>> architecture, and it would make the currently very elegant and simple
>> model
>> much more complicated and harder to maintain. Given that Flink's
>> runtime is
>> complex enough, I would really like to avoid that.
>> 
>> The current runtime paradigm coordinates between operators strictly
>> via
>> in-band events. RPC calls happen between operators and the master for
>> triggering and acknowledging execution and checkpoints.
>> 
>> I was wondering whether we can keep following that paradigm and still
>> get
>> most of what you are proposing here. In some sense, all we need to
>> do is
>> replace RPC calls with in-band events, and "decentralize" the
>> coordinator
>> such that every operator can make its own termination decision by
>> itself.
>> 
>> This is only a rough sketch, you probably need to flesh it out more.
>> 
>> - I assume that the OP in the diagram knows that it is in a loop and
>> that
>> it is the one connected to the head and tail
>> 
>> - When OP receives and EndOfStream Event from the regular source
>> (RS),
>> it
>> emits an "AttemptTermination" event downstream to the operators
>> involved in
>> the loop. It attaches an attempt sequence number and memorizes that
>> - Tail and Head forward these events
>> - When OP receives the event back with the same attempt sequence
>> number,
>> and no records came in the meantime, it shuts down and emits
>> EndOfStream
>> downstream
>> - When other records came back between emitting the
>> AttemptTermination
>> event and receiving it back, then it emits a new AttemptTermination
>> event
>> with the next sequence number.
>> - This should terminate as soon as the loop is empty.
>> 
>> Might this model even generalize to nested loops, where the
>> "AttemptTermination" event is scoped by the loop's nesting level?
>> 
>> Let me know what you think!
>> 
>> 
>> Best,
>> Stephan
>> 
>> 
>> On Thu, Oct 27, 2016 at 10:19 AM, Stephan Ewen <sewen@apache.org
>> <mailto:
>> sewen@apache.org><mailto:sewen@apache.org>
>> <mailto:sewen@apache.org><mailto:
>> sewen@apache.org<mailto:sewen@apache.org><mailto:sewen@apache.org>
>> <mailto:sewen@apache.org>>> wrote:
>> 
>> Hi!
>> 
>> I am still scanning it and compiling some comments. Give me a bit ;-)
>> 
>> Stephan
>> 
>> 
>> On Wed, Oct 26, 2016 at 6:13 PM, Paris Carbone <parisc@kth.se
>> <mailto:
>> parisc@kth.se><mailto:parisc@kth.se> <mailto:
>> parisc@kth.se<mailto:parisc@kth.se><mailto:parisc@kth.se>><mailto:
>> parisc@kth.se<mailto:parisc@kth.se><mailto:parisc@kth.se> <mailto:
>> parisc@kth.se>>> wrote:
>> 
>> Hey all,
>> 
>> Now that many of you have already scanned the document (judging from
>> the
>> views) maybe it is time to give back some feedback!
>> Did you like it? Would you suggest an improvement?
>> 
>> I would suggest not to leave this in the void. It has to do with
>> important properties that the system promises to provide.
>> Me and Fouad will do our best to answer your questions and discuss
>> this
>> further.
>> 
>> cheers
>> Paris
>> 
>> On 21 Oct 2016, at 08:54, Paris Carbone <parisc@kth.se<mailto:
>> parisc@kth.se><mailto:parisc@kth.se> <mailto:
>> parisc@kth.se<mailto:parisc@kth.se><mailto:parisc@kth.se>><mailto:
>> parisc@
>> kth.se<http://kth.se/><http://kth.se<http://kth.se/>> <
>> http://kth.se/
>> <mailto:parisc@k
>> th.se<http://th.se/><http://th.se<http://th.se/>> <http://th.se/><
>> http://th.se<http://th.se/> <http://th.se/>>>> wrote:
>> 
>> Hello everyone,
>> 
>> Loops in Apache Flink have a good potential to become a much more
>> powerful thing in future version of Apache Flink.
>> There is generally high demand to make them usable and first of all
>> production-ready for upcoming releases.
>> 
>> As a first commitment we would like to propose FLIP-13 for consistent
>> processing with Loops.
>> We are also working on scoped loops for Q1 2017 which we can share if
>> there is enough interest.
>> 
>> For now, that is an improvement proposal that solves two pending
>> major
>> issues:
>> 
>> 1) The (not so trivial) problem of correct termination of jobs with
>> iterations
>> 2) The applicability of the checkpointing algorithm to iterative
>> dataflow
>> graphs.
>> 
>> We would really appreciate it if you go through the linked draft
>> (motivation and proposed changes) for FLIP-13 and point out comments,
>> preferably publicly in this devlist discussion before we go ahead and
>> update the wiki.
>> 
>> https://docs.google.com/document/d/1M6ERj-TzlykMLHzPSwW5L9b0
>> BhDbtoYucmByBjRBISs/edit?usp=sharing
>> 
>> cheers
>> 
>> Paris and Fouad
>> 
>> 
>> 
>> 
>> 
>> 
>> 
>> 
>> 

Mime
View raw message