SideOutput doesn't receive anything if filter is applied after the process function

[Juho Autio]

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

[Tzu-Li (Gordon) Tai]

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 ([hidden email]) 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

[Juho Autio]

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 <[hidden email]> 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 ([hidden email]) 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

[Chen Qin]

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 <[hidden email]> 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 <[hidden email]> 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 ([hidden email]) 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

[Juho Autio]

> 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 <[hidden email]> 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 <[hidden email]> 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 <[hidden email]> 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 ([hidden email]) 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

[Chesnay Schepler]

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 <[hidden email]> 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 <[hidden email]> 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 <[hidden email]> 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 ([hidden email]) 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

[Juho Autio]

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 <[hidden email]> 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 <[hidden email]> 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 <[hidden email]> 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 <[hidden email]> 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 ([hidden email]) 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

[Chesnay Schepler]

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 <[hidden email]> 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 <[hidden email]> 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 <[hidden email]> 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 <[hidden email]> 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 ([hidden email]) 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

[Juho Autio]

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 <[hidden email]> 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 <[hidden email]> 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 <[hidden email]> 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 <[hidden email]> 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 <[hidden email]> 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 ([hidden email]) 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

[Chesnay Schepler]

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 <[hidden email]> 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 <[hidden email]> 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 <[hidden email]> 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 <[hidden email]> 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 <[hidden email]> 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 ([hidden email]) 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

[Chen Qin]

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 <[hidden email]> 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 <[hidden email]> 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 <[hidden email]> 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 <[hidden email]> 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 <[hidden email]> 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 <[hidden email]> 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 ([hidden email]) 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