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From Chen Qin <qinnc...@gmail.com>
Subject Re: SideOutput doesn't receive anything if filter is applied after the process function
Date Tue, 16 Jan 2018 18:14:13 GMT
Thanks Chesnay,

So I think to support multi input and multiple output model like data flow paper indicates,
Flink needs to get credit based scheduling as well as side input ready and doing a new set
of data stream apis that doesn’t constrained with backwards compatibility issues. Only then
can we pass through side outputs to next operator and consumer can decide what to do with
it.

Yes, it might be too far to reach but that seems the one of directions community can consider.

Chen


> On Jan 16, 2018, at 5:18 AM, Chesnay Schepler <chesnay@apache.org> wrote:
> 
> I've opened https://issues.apache.org/jira/browse/FLINK-8437
> 
> Unfortunately i doubt we can fix this properly. The proposed solution will not work if
we ever allow arbitrary functions to use side-outputs.
> 
>> On 16.01.2018 08:59, Juho Autio wrote:
>> Could someone with knowledge of the right terms create this in JIRA, please? I guess
I could also create it if needed..
>> 
>>> On Mon, Jan 15, 2018 at 3:15 PM, Chesnay Schepler <chesnay@apache.org>
wrote:
>>> yes, i meant that process() returns the special operator. This would definitely
deserve a JIRA issue.
>>> 
>>> 
>>> On 15.01.2018 14:09, Juho Autio wrote:
>>>> Thanks for the explanation. Did you meant that process() would return a SingleOutputWithSideOutputOperator?
>>>> 
>>>> Any way, that should be enough to avoid the problem that I hit (and it also
seems like the best & only way).
>>>> 
>>>> Maybe the name should be something more                             generic
though, like ProcessedSingleOutputOperator or something, I wouldn't know..
>>>> 
>>>> Would this deserve an improvement ticket in JIRA?
>>>> 
>>>> On Mon, Jan 15, 2018 at 12:43 PM, Chesnay Schepler <chesnay@apache.org>
wrote:
>>>>> It would mean that getSideOutput() would return a SingleOutputWithSideOutputOperator
which extends SingleOutputOperator offering getSideOutput(). Other transformations would still
return a SingleOutputOperator.
>>>>> 
>>>>> With this the following code wouldn't compile.
>>>>> 
>>>>> stream
>>>>>     .process(...)
>>>>>     .filter(...)
>>>>>     .getSideOutput(...) // compile error
>>>>> 
>>>>> You would have to explicitly define the code as below, which makes the
behavior unambiguous:
>>>>> 
>>>>> processed = stream
>>>>>     .process(...)
>>>>> 
>>>>> filtered = processed
>>>>>     .filter(...)
>>>>> 
>>>>> filteredSideOutput = processed
>>>>>     .getSideOutput(...)
>>>>>     .filter(...)
>>>>> 
>>>>> 
>>>>> On 15.01.2018 09:55, Juho Autio wrote:
>>>>>> > sideoutput might deserve a seperate class which inherit form
singleoutput. It might prevent lot of confusions
>>>>>> 
>>>>>> Thanks, but how could that be done? Do you mean that if one calls
.process(), then the stream would change to another class                                
              which would only allow calls like .getMainOutput() or .getSideOutput("name")?
Of course compile time error would be even better than a runtime error, but I don't see yet
how it could be done in practice.
>>>>>> 
>>>>>>> On Sun, Jan 14, 2018 at 4:55 AM, Chen Qin <qinnchen@gmail.com>
wrote:
>>>>>>> Hi Juho,
>>>>>>> 
>>>>>>> I think sideoutput might deserve a seperate class which inherit
form singleoutput. It might prevent lot of confusions. A more generic question is whether
datastream api can be mulitple ins and mulitple outs natively. It's more like scheduling problem
when you come from single process system to multiple process system, which one should get
resource and how much sharing same hardware resources, I guess it will open gate to lots of
edge cases -> strategies-> more edge cases :)
>>>>>>> 
>>>>>>> Chen
>>>>>>> 
>>>>>>>> On Fri, Jan 12, 2018 at 6:34 AM, Juho Autio <juho.autio@rovio.com>
wrote:
>>>>>>>> Maybe I could express it in a slightly different way: if
adding the .filter() after .process() causes the side output to be somehow totally "lost",
then I believe the                                                           .getSideOutput()
could be aware that there is not such side output to be listened to from upstream, and throw
an exception. I mean, this should be possible when building the DAG, it shouldn't require
starting the stream to detect? Thanks..
>>>>>>>> 
>>>>>>>> On Fri, Jan 12, 2018 at 2:48 PM, Tzu-Li (Gordon) Tai <tzulitai@apache.org>
wrote:
>>>>>>>>> Hi Juho,
>>>>>>>>> 
>>>>>>>>>> Now that I think of it this seems like a bug to me:
why does the call to getSideOutput succeed if it doesn't provide _any_ input?
>>>>>>>>> 
>>>>>>>>> 
>>>>>>>>> With the way side outputs work, I don’t think this
is possible (or would make sense). An operator does not know whether or not it would ever
emit some element with a given tag.
>>>>>>>>> As far as I understand it, calling `getSideOutput` essentially
adds a virtual node to the result stream graph that listens to the specified tag from the
upstream input.
>>>>>>>>> 
>>>>>>>>> While I’m not aware whether or not your           
                                               observation is expected behavior, from an API
perspective, I can see why it is a bit confusing for you.
>>>>>>>>> Aljoscha would be the expert here, maybe he’ll have
more insights. I’ve looped him in cc’ed.
>>>>>>>>> 
>>>>>>>>> Cheers,
>>>>>>>>> Gordon
>>>>>>>>> 
>>>>>>>>> 
>>>>>>>>>> On 12 January 2018 at 4:05:13 PM, Juho Autio (juho.autio@rovio.com)
wrote:
>>>>>>>>>> 
>>>>>>>>>> When I run the code below (Flink 1.4.0 or 1.3.1),
only "a" is printed. If I switch the position of .process() & .filter() (ie. filter first,
then process), both "a" & "b" are printed, as expected.
>>>>>>>>>> 
>>>>>>>>>> I guess it's a bit hard to say what the side output
should include in this case: the stream before filtering or after it?
>>>>>>>>>> 
>>>>>>>>>> What I would suggest is Flink to protect against
this kind of a user error that is hard to debug. Would it be possible that Flink throws an
exception                                                           if one tries to call .getSideOutput()
on anything that doesn't actually provide that side output? Now that I think of it this seems
like a bug to me: why does the call to getSideOutput succeed if it doesn't provide _any_ input?
I would expect it to get the side output data stream _before_ applying .filter().
>>>>>>>>>> 
>>>>>>>>>> import org.apache.flink.api.common.functions.FilterFunction;
>>>>>>>>>> import org.apache.flink.streaming.api.datastream.DataStreamSource;
>>>>>>>>>> import org.apache.flink.streaming.api.datastream.SingleOutputStreamOperator;
>>>>>>>>>> import org.apache.flink.streaming.api.environment.StreamExecutionEnvironment;
>>>>>>>>>> import org.apache.flink.streaming.api.functions.ProcessFunction;
>>>>>>>>>> import org.apache.flink.util.Collector;
>>>>>>>>>> import org.apache.flink.util.OutputTag;
>>>>>>>>>> 
>>>>>>>>>> public class SideOutputProblem {
>>>>>>>>>> 
>>>>>>>>>>     public static void main(String[] args) throws
Exception {
>>>>>>>>>> 
>>>>>>>>>>         StreamExecutionEnvironment env = StreamExecutionEnvironment.getExecutionEnvironment();
>>>>>>>>>>         DataStreamSource<String> stream = env.fromElements("a",
"b");
>>>>>>>>>>         OutputTag<String> sideOutputTag = new
OutputTag<String>("side-output"){};
>>>>>>>>>> 
>>>>>>>>>>         SingleOutputStreamOperator<String>
processed = stream
>>>>>>>>>> 
>>>>>>>>>>                 .process(new ProcessFunction<String,
String>() {
>>>>>>>>>>                     @Override
>>>>>>>>>>                     public void processElement(String
s, Context context, Collector<String> collector) throws Exception {
>>>>>>>>>>                         if ("a".equals(s)) {
>>>>>>>>>>                             collector.collect(s);
>>>>>>>>>>                         } else {
>>>>>>>>>>                             context.output(sideOutputTag,
s);
>>>>>>>>>>                         }
>>>>>>>>>>                     }
>>>>>>>>>>                 })
>>>>>>>>>> 
>>>>>>>>>>                 .filter(new FilterFunction<String>()
{
>>>>>>>>>>                     @Override
>>>>>>>>>>                     public boolean filter(String
s) throws Exception {
>>>>>>>>>>                         return true;
>>>>>>>>>>                     }
>>>>>>>>>>                 });
>>>>>>>>>> 
>>>>>>>>>>         processed.getSideOutput(sideOutputTag).printToErr();
>>>>>>>>>> 
>>>>>>>>>>         processed.print();
>>>>>>>>>> 
>>>>>>>>>>         env.execute();
>>>>>>>>>>     }
>>>>>>>>>> 
>>>>>>>>>> }
>>>>>>>>>> 
>>>>>>>>>> Cheers,
>>>>>>>>>> Juho
>>>>>>>> 
>>>>>>>> 
>>>>>>> 
>>>>>> 
>>>>> 
>>> 
> 

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