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From Stephan Ewen <se...@apache.org>
Subject Re: intermediate result reuse
Date Sun, 13 Sep 2015 00:13:43 GMT
Hi!

In most places where you use collect(), you should be able to use a
broadcast variable to the same extend. This keeps the plan as one DAG,
executed in one unit, so no re-computation will happen.

Intermediate result caching is actually a work that has been in progress
for a while, but has stalled for a bit due to prioritization of some other
issues. It will be resumed in the near future, definitely. Too many parts
are already in place to not complete this feature...

Greetings,
Stephan


On Sat, Sep 12, 2015 at 6:44 PM, Michele Bertoni <
michele1.bertoni@mail.polimi.it> wrote:

> ok, I think I got the point: I don’t have two execute but a collect in
> some branch
> I will look for a way to remove it
>
>
> What I am doing is to keep all the elements of A that as value equal to
> something in B, where B (at this point) is very small
> Is it better to collect or a cogroup?
>
>
> btw is something you expect to solve i further versions?
>
>
> thanks
> michele
>
>
>
>
> Il giorno 12/set/2015, alle ore 16:27, Stephan Ewen <sewen@apache.org> ha
> scritto:
>
> Fabian has explained it well. All functions are executed lazily as one
> DAG, when "env.execute()" is called.
>
> Beware that there are three exceptions:
>   - count()
>   - collect()
>   - print()
>
> These functions trigger an immediate program execution (they are "eager"
> functions). They will execute all that is needed for produce their result.
> Summing up:
>
> ---------------------------
>
> One execution in this case (result "a" is reused by "b" and "c")
>
> a = env.createInput() -> map() -> reduce() -> filter()
>
> b = a.flatmap()
> c = a.groupBy() -> reduce()
>
> b.writeAsText()
> c.writeAsCsv()
>
> env.execute();
>
> ---------------------------
>
> Two executions in this case ("a" is computed twice, once for "b" and once
> for "c")
>
> a = env.createInput() -> map() -> reduce() -> filter()
>
> b = a -> flatmap() -> count()
> c = a -> groupBy() -> reduce().collect()
>
> ---------------------------
>
> Greetings,
> Stephan
>
>
> On Sat, Sep 12, 2015 at 11:31 AM, Fabian Hueske <fhueske@gmail.com> wrote:
>
>> Hi Michele,
>>
>> Flink programs can have multiple sinks.
>> In your program, the intermediate result a will be streamed to both
>> filters (b and c) at the same time and both sinks will be written at the
>> same time.
>> So in this case, there is no need to materialize the intermediate result
>> a.
>>
>> If you call execute() after you defined b, the program will compute a and
>> stream the result only to b.
>> If you call execute() again after you defined c, the program will compute
>> a again and stream the result to c.
>>
>> Summary:
>> Flink programs can usually stream intermediate results without
>> materializing them. There are a few cases where it needs to materialize
>> intermediate results in order to avoid deadlocks, but these are fully
>> transparently handled.
>> It is not possible (yet!) to share results across program executions,
>> i.e., whenever you call execute().
>>
>> I suppose, you call execute() between defining b and c. If you execute
>> that call, a will be computed once and both b and c are computed at the
>> same time.
>>
>> Best, Fabian
>>
>> 2015-09-12 11:02 GMT+02:00 Michele Bertoni <
>> michele1.bertoni@mail.polimi.it>:
>>
>>> Hi everybody, I have a question about internal optimization
>>> is flink able to reuse intermediate result that are used twice in the
>>> graph?
>>>
>>> i.e.
>>> a = readsource -> filter -> reduce -> something else even more
>>> complicated
>>>
>>> b = a filter(something)
>>> store b
>>>
>>> c = a filter(something else)
>>> store c
>>>
>>> what happens to a? is it computed twice?
>>>
>>> in my read function I have a some logging commands and I see the printed
>>> twice, but it sounds strange to me
>>>
>>>
>>>
>>> thanks
>>> cheers
>>> michele
>>
>>
>>
>
>

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