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From "Wright, Eron" <ewri...@live.com>
Subject Re: Some thoughts about the lower-level Flink APIs
Date Tue, 16 Aug 2016 14:34:13 GMT
I think you raise a valid concern but I would hesitate to accept the suggestion that the low-level
API be promoted to app developers.

Higher-level abstractions tend to be more constrained and more optimized, whereas lower-level
abstractions tend to be more powerful, be more laborious to use and provide the system with
less knowledge.   It is a classic tradeoff.    

I think it important to consider, what are the important/distinguishing characteristics of
the Flink framework.    Exactly-once guarantees, event-time support, support for job upgrade
without data loss, fault tolerance, etc.    I’m speculating that the high-level abstraction
provided to app developers is probably needed to retain those charactistics.  

I think Vasia makes a good point that SQL might be a good alternative way to ease into Flink.

Finally, I believe the low-level API is primarily intended for extension purposes (connectors,
operations, etc) not app development.    It could use better documentation to ensure that
third-party extensions support those key characteristics.


> On Aug 16, 2016, at 3:12 AM, Vasiliki Kalavri <vasilikikalavri@gmail.com> wrote:
> Hi Jamie,
> thanks for sharing your thoughts on this! You're raising some interesting
> points.
> Whether users find the lower-level primitives more intuitive depends on
> their background I believe. From what I've seen, if users are coming from
> the S4/Storm world and are used to the "compositional" way of streaming,
> then indeed it's easier for them to think and operate on that level. These
> are usually people who have seen/built streaming things before trying out
> Flink.
> But if we're talking about analysts and people coming from the "batch" way
> of thinking or people used to working with SQL/python, then the
> higher-level declarative API is probably easier to understand.
> I do think that we should make the lower-level API more visible and
> document it properly, but I'm not sure if we should teach Flink on this
> level first. I think that presenting it as a set of "advanced" features
> makes more sense actually.
> Cheers,
> -Vasia.
> On 16 August 2016 at 04:24, Jamie Grier <jamie@data-artisans.com> wrote:
>> You lost me at lattice, Aljoscha ;)
>> I do think something like the more powerful N-way FlatMap w/ Timers
>> Aljoscha is describing here would probably solve most of the problem.
>> Often Flink's higher level primitives work well for people and that's
>> great.  It's just that I also spend a fair amount of time discussing with
>> people how to map what they know they want to do onto operations that
>> aren't a perfect fit and it sometimes liberates them when they realize they
>> can just implement it the way they want by dropping down a level.  They
>> usually don't go there themselves, though.
>> I mention teaching this "first" and then the higher layers I guess because
>> that's just a matter of teaching philosophy.  I think it's good to to see
>> the basic operations that are available first and then understand that the
>> other abstractions are built on top of that.  That way you're not afraid to
>> drop-down to basics when you know what you want to get done.
>> -Jamie
>> On Mon, Aug 15, 2016 at 2:11 AM, Aljoscha Krettek <aljoscha@apache.org>
>> wrote:
>>> Hi All,
>>> I also thought about this recently. A good think would be to add a good
>>> user facing operator that behaves more or less like an enhanced FlatMap
>>> with multiple inputs, multiple outputs, state access and keyed timers.
>> I'm
>>> a bit hesitant, though, since users rarely think about the implications
>>> that come with state updating and out-of-order events. If you don't
>>> implement a stateful operator correctly you have pretty much arbitrary
>>> results.
>>> The problem with out-of-order event arrival and state update is that the
>>> state basically has to monotonically transition "upwards" through a
>> lattice
>>> for the computation to make sense. I know this sounds rather theoretical
>> so
>>> I'll try to explain with an example. Say you have an operator that waits
>>> for timestamped elements A, B, C to arrive in timestamp order and then
>> does
>>> some processing. The naive approach would be to have a small state
>> machine
>>> that tracks what element you have seen so far. The state machine has
>> three
>>> states: NOTHING, SEEN_A, SEEN_B and SEEN_C. The state machine is supposed
>>> to traverse these states linearly as the elements arrive. This doesn't
>>> work, however, when elements arrive in an order that does not match their
>>> timestamp order. What the user should do is to have a "Set" state that
>>> keeps track of the elements that it has seen. Once it has seen {A, B, C}
>>> the operator must check the timestamps and then do the processing, if
>>> required. The set of possible combinations of A, B, and C forms a lattice
>>> when combined with the "subset" operation. And traversal through these
>> sets
>>> is monotonically "upwards" so it works regardless of the order that the
>>> elements arrive in. (I recently pointed this out on the Beam mailing list
>>> and Kenneth Knowles rightly pointed out that what I was describing was in
>>> fact a lattice.)
>>> I know this is a bit off-topic but I think it's very easy for users to
>>> write wrong operations when they are dealing with state. We should still
>>> have a good API for it, though. Just wanted to make people aware of this.
>>> Cheers,
>>> Aljoscha
>>> On Mon, 15 Aug 2016 at 08:18 Matthias J. Sax <mjsax@apache.org> wrote:
>>>> It really depends on the skill level of the developer. Using low-level
>>>> API requires to think about many details (eg. state handling etc.) that
>>>> could be done wrong.
>>>> As Flink gets a broader community, more people will use it who might
>> not
>>>> have the required skill level to deal with low-level API. For more
>>>> trained uses, it is of course a powerful tool!
>>>> I guess it boils down to the question, what type of developer Flink
>>>> targets, if low-level API should be offensive advertised or not. Also
>>>> keep in mind, that many people criticized Storm's low-level API as hard
>>>> to program etc.
>>>> -Matthias
>>>> On 08/15/2016 07:46 AM, Gyula Fóra wrote:
>>>>> Hi Jamie,
>>>>> I agree that it is often much easier to work on the lower level APIs
>> if
>>>> you
>>>>> know what you are doing.
>>>>> I think it would be nice to have very clean abstractions on that
>> level
>>> so
>>>>> we could teach this to the users first but currently I thinm its not
>>> easy
>>>>> enough to be good starting point.
>>>>> The user needs to understand a lot about the system if the dont want
>> to
>>>>> hurt other parts of the pipeline. For insance working with the
>>>>> streamrecords, propagating watermarks, working with state internals
>>>>> This all might be overwhelming at the first glance. But maybe we can
>>> slim
>>>>> some abstractions down to the point where this becomes kind of the
>>>>> extension of the RichFunctions.
>>>>> Cheers,
>>>>> Gyula
>>>>> On Sat, Aug 13, 2016, 17:48 Jamie Grier <jamie@data-artisans.com>
>>> wrote:
>>>>>> Hey all,
>>>>>> I've noticed a few times now when trying to help users implement
>>>> particular
>>>>>> things in the Flink API that it can be complicated to map what they
>>> know
>>>>>> they are trying to do onto higher-level Flink concepts such as
>>>> windowing or
>>>>>> Connect/CoFlatMap/ValueState, etc.
>>>>>> At some point it just becomes easier to think about writing a Flink
>>>>>> operator yourself that is integrated into the pipeline with a
>>>> transform()
>>>>>> call.
>>>>>> It can just be easier to think at a more basic level.  For example
>>> can
>>>>>> write an operator that can consume one or two input streams (should
>>>>>> probably be N), update state which is managed for me fault
>> tolerantly,
>>>> and
>>>>>> output elements or setup timers/triggers that give me callbacks from
>>>> which
>>>>>> I can also update state or emit elements.
>>>>>> When you think at this level you realize you can program just about
>>>>>> anything you want.  You can create whatever fault-tolerant data
>>>> structures
>>>>>> you want, and easily execute robust stateful computation over data
>>>> streams
>>>>>> at scale.  This is the real technology and power of Flink IMO.
>>>>>> Also, at this level I don't have to think about the complexities
>>>>>> windowing semantics, learn as much API, etc.  I can easily have some
>>>> inputs
>>>>>> that are broadcast, others that are keyed, manage my own state in
>>>> whatever
>>>>>> data structure makes sense, etc.  If I know exactly what I actually
>>>> want to
>>>>>> do I can just do it with the full power of my chosen language, data
>>>>>> structures, etc.  I'm not "restricted" to trying to map everything
>>> onto
>>>>>> higher-level Flink constructs which is sometimes actually more
>>>> complicated.
>>>>>> Programming at this level is actually fairly easy to do but people
>>> seem
>>>> a
>>>>>> bit afraid of this level of the API.  They think of it as low-level
>> or
>>>>>> custom hacking..
>>>>>> Anyway, I guess my thought is this..  Should we explain Flink to
>>> people
>>>> at
>>>>>> this level *first*?  Show that you have nearly unlimited power and
>>>>>> flexibility to build what you want *and only then* from there
>> explain
>>>> the
>>>>>> higher level APIs they can use *if* those match their use cases
>> well.
>>>>>> Would this better demonstrate to people the power of Flink and maybe
>>>>>> *liberate* them a bit from feeling they have to map their problem
>>> onto a
>>>>>> more complex set of higher level primitives?  I see people trying
>>>>>> shoe-horn what they are really trying to do, which is simple to
>>> explain
>>>> in
>>>>>> english, onto windows, triggers, CoFlatMaps, etc, and this get's
>>>>>> complicated sometimes.  It's like an impedance mismatch.  You could
>>> just
>>>>>> solve the problem very easily programmed in straight Java/Scala.
>>>>>> Anyway, it's very easy to drop down a level in the API and program
>>>> whatever
>>>>>> you want but users don't seem to *perceive* it that way.
>>>>>> Just some thoughts...  Any feedback?  Have any of you had similar
>>>>>> experiences when working with newer Flink users or as a newer Flink
>>> user
>>>>>> yourself?  Can/should we do anything to make the *lower* level API
>>> more
>>>>>> accessible/visible to users?
>>>>>> -Jamie
>> --
>> Jamie Grier
>> data Artisans, Director of Applications Engineering
>> @jamiegrier <https://twitter.com/jamiegrier>
>> jamie@data-artisans.com

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