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From Maximilian Michels <...@apache.org>
Subject Re: execute() and collect()/print()/count()
Date Tue, 23 Jun 2015 09:43:40 GMT
@Stephan I understand your concerns that the user might wonder that nothing
happens when executing. However, in this case a warning will provide a hint
to the user that he didn't define any sinks. In the case where he
immediately calls execute() after an eager execution, the program is
actually executed and he still gets an Exception although everything
already ran. I think that's much worse because the user sees that something
executed and thinks it failed.

@Alexander I like your proposal. It adds a bit more logic to the
ExecutionEnvironment but I think that's acceptable for the sake of the user
experience.





On Mon, Jun 22, 2015 at 9:37 PM, Alexander Alexandrov <
alexander.s.alexandrov@gmail.com> wrote:

> What about adding some state state to the DataBag internals that tracks the
> following conditions
>
> 1. whether the last job execution was triggered by an "enforcer" API method
> like print() / collect();
> 2. whether a DataSource / lazy operator was created after that;
>
> If 1 is true and 2 is false, a WARN can be displayed. Otherwise, we can
> still throw an error.
>
>
> 2015-06-22 18:17 GMT+02:00 Stephan Ewen <sewen@apache.org>:
>
> > We have two situations to trade off here, and fixing one will make the
> > other worse:
> >
> > 1) env.execute() after collect() - see Max's mail
> >
> > 2) env.execute() on empty sinks program. Not throwing an exception makes
> > people wonder why nothing happens (if they write the program to just test
> > whether it runs or if they want to measure time).
> >
> > Both choices make one behave nice and the other not. So far, the idea was
> > that throwing an exception on empty sinks is that the error message will
> > help people figure out what is wrong fast. Debugging why nothing happens
> > can be slow.
> >
> >
> > It is hard to say if we would not introduce another source of confusion
> by
> > fixing one...
> >
> >
> >
> >
> >
> > On Mon, Jun 22, 2015 at 10:26 AM, Maximilian Michels <mxm@apache.org>
> > wrote:
> >
> > > +1 for cleaning up the documentation
> > > +1 for adding a link to the documentation (should be a permalink)
> > > +1 for printing a warning instead of an exception
> > >
> > > On Sun, Jun 21, 2015 at 12:25 AM, Robert Metzger <rmetzger@apache.org>
> > > wrote:
> > >
> > > > We could also add a link to the documentation into the exception that
> > > > explains the behavior.
> > > >
> > > > On Fri, Jun 19, 2015 at 5:52 AM, Chiwan Park <chiwanpark@icloud.com>
> > > > wrote:
> > > >
> > > > > +1 for ignoring execute() call with warning.
> > > > >
> > > > > But I'm concerned for how the user catches the error in program
> > without
> > > > > any data sinks.
> > > > >
> > > > > By the way, eager execution is not well documented in data sinks
> > > section
> > > > > but is in program
> > > > > skeleton section. [1] This makes the user’s confusion. We should
> > clean
> > > up
> > > > > documents.
> > > > > There are many codes calling execute() method after print() method.
> > > > [2][3]
> > > > >
> > > > > We should add a description for count() method to documents too.
> > > > >
> > > > > [1]
> > > > >
> > > >
> > >
> >
> http://ci.apache.org/projects/flink/flink-docs-master/apis/programming_guide.html#data-sinks
> > > > > [2]
> > > > >
> > > >
> > >
> >
> http://ci.apache.org/projects/flink/flink-docs-master/apis/programming_guide.html#parallel-execution
> > > > > [3]
> > > > >
> > > >
> > >
> >
> http://ci.apache.org/projects/flink/flink-docs-master/apis/programming_guide.html#iteration-operators
> > > > >
> > > > > Regards,
> > > > > Chiwan Park
> > > > >
> > > > > > On Jun 19, 2015, at 9:15 PM, Maximilian Michels <mxm@apache.org>
> > > > wrote:
> > > > > >
> > > > > > Dear Flink community,
> > > > > >
> > > > > > I have stopped to count how many people on the user list and
> during
> > > > Flink
> > > > > > trainings have asked why their Flink program throws an Exception
> > when
> > > > > they
> > > > > > just one to print a DataSet. The reason for this is that print()
> > now
> > > > > > executes eagerly, thus, executes the Flink program. Subsequent
> > calls
> > > to
> > > > > > execute() need to define new DataSinks and throw an exception
> > > > otherwise.
> > > > > >
> > > > > > We have recently introduced a flag in the ExecutionEnvironment
> that
> > > > > checks
> > > > > > whether the user executed before (explicitly via execute() or
> > > > implicitly
> > > > > > through collect()/print()/count()). That enabled us to print
a
> > nicer
> > > > > > exception message. However, users either do not read the
> exception
> > > > > message
> > > > > > or do not understand it. They do ask this question a lot.
> > > > > >
> > > > > > That's why I propose to ignore calls to execute() entirely if
no
> > > sinks
> > > > > are
> > > > > > defined. That will get rid of one of the core annoyances for
> Flink
> > > > > users. I
> > > > > > know, that this is painfully for us programmers because we
> > understand
> > > > how
> > > > > > Flink works internally but let's step back once and see that
it
> > > > wouldn't
> > > > > be
> > > > > > so bad if execute didn't do anything in case of no new sinks.
> > > > > >
> > > > > > What would be the downside of this change? Users might call
> > execute()
> > > > and
> > > > > > wonder that nothing happens. We would then simply print a warning
> > > that
> > > > > > their program didn't define any sinks. That is a big difference
> to
> > > the
> > > > > > behavior before because users are scared of exceptions. If they
> > just
> > > > get
> > > > > a
> > > > > > warning they will double-check their program and investigate
why
> > > > nothing
> > > > > > happens. Most of the cases they do actually have defined sinks
> but
> > > > simply
> > > > > > left a call to execute() when they were printing a DataSet.
> > > > > >
> > > > > > What are you opinions on this issue? I have opened a JIRA for
> this
> > as
> > > > > well:
> > > > > > https://issues.apache.org/jira/browse/FLINK-2249
> > > > > >
> > > > > > Best,
> > > > > > Max
> > > > >
> > > > >
> > > > >
> > > > >
> > > > >
> > > >
> > >
> >
>

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