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From SHI Xiaogang <shixiaoga...@gmail.com>
Subject Re: [DISCUSS] FLIP-14: Loops API and Termination
Date Fri, 11 Nov 2016 05:21:56 GMT
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

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
t9. TAIL receives the UpdateStatus event from OP and notifies with an IDLE
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
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:


2016-11-11 3:19 GMT+08:00 Paris Carbone <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>> 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>> 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>> 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>> 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@k
> 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

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