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From Alexander Alexandrov <alexander.s.alexand...@gmail.com>
Subject Re: execute() and collect()/print()/count()
Date Mon, 22 Jun 2015 19:37:38 GMT
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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