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From Stephan Ewen <se...@apache.org>
Subject Re: [DISCUSS] FLIP-13: Consistent Processing with Loops
Date Thu, 27 Oct 2016 08:55:17 GMT
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
  - 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!


On Thu, Oct 27, 2016 at 10:19 AM, Stephan Ewen <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> 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@k
>> 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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