Talk about flink DataStream’s split Operation

  flink

Order

This article mainly studies the split operation of flink DataStream.

Example

SplitStream<Integer> split = someDataStream.split(new OutputSelector<Integer>() {
    @Override
    public Iterable<String> select(Integer value) {
        List<String> output = new ArrayList<String>();
        if (value % 2 == 0) {
            output.add("even");
        }
        else {
            output.add("odd");
        }
        return output;
    }
});
  • This example converts dataStream split into two dataStreams, one outputName is even and the other outputName is odd.

DataStream.split

flink-streaming-java_2.11-1.7.0-sources.jar! /org/apache/flink/streaming/api/datastream/DataStream.java

@Public
public class DataStream<T> {

    //......

    public SplitStream<T> split(OutputSelector<T> outputSelector) {
        return new SplitStream<>(this, clean(outputSelector));
    }

    //......
}
  • The split operation of DataStream receives the OutputSelector parameter and then creates and returns a SplitStream.

OutputSelector

flink-streaming-java_2.11-1.7.0-sources.jar! /org/apache/flink/streaming/api/collector/selector/OutputSelector.java

@PublicEvolving
public interface OutputSelector<OUT> extends Serializable {

    Iterable<String> select(OUT value);

}
  • OutputSelector defines the select method used to type outputNames on the element.

SplitStream

flink-streaming-java_2.11-1.7.0-sources.jar! /org/apache/flink/streaming/api/datastream/SplitStream.java

@PublicEvolving
public class SplitStream<OUT> extends DataStream<OUT> {

    protected SplitStream(DataStream<OUT> dataStream, OutputSelector<OUT> outputSelector) {
        super(dataStream.getExecutionEnvironment(), new SplitTransformation<OUT>(dataStream.getTransformation(), outputSelector));
    }

    public DataStream<OUT> select(String... outputNames) {
        return selectOutput(outputNames);
    }

    private DataStream<OUT> selectOutput(String[] outputNames) {
        for (String outName : outputNames) {
            if (outName == null) {
                throw new RuntimeException("Selected names must not be null");
            }
        }

        SelectTransformation<OUT> selectTransform = new SelectTransformation<OUT>(this.getTransformation(), Lists.newArrayList(outputNames));
        return new DataStream<OUT>(this.getExecutionEnvironment(), selectTransform);
    }

}
  • SplitStream inherits DataStream, which defines the select method and can be used to select the Split DataStream according to outputNames. The select method creates SelectTransformation.

StreamGraphGenerator

flink-streaming-java_2.11-1.7.0-sources.jar! /org/apache/flink/streaming/api/graph/StreamGraphGenerator.java

@Internal
public class StreamGraphGenerator {

    //......

    private Collection<Integer> transform(StreamTransformation<?> transform) {

        if (alreadyTransformed.containsKey(transform)) {
            return alreadyTransformed.get(transform);
        }

        LOG.debug("Transforming " + transform);

        if (transform.getMaxParallelism() <= 0) {

            // if the max parallelism hasn't been set, then first use the job wide max parallelism
            // from theExecutionConfig.
            int globalMaxParallelismFromConfig = env.getConfig().getMaxParallelism();
            if (globalMaxParallelismFromConfig > 0) {
                transform.setMaxParallelism(globalMaxParallelismFromConfig);
            }
        }

        // call at least once to trigger exceptions about MissingTypeInfo
        transform.getOutputType();

        Collection<Integer> transformedIds;
        if (transform instanceof OneInputTransformation<?, ?>) {
            transformedIds = transformOneInputTransform((OneInputTransformation<?, ?>) transform);
        } else if (transform instanceof TwoInputTransformation<?, ?, ?>) {
            transformedIds = transformTwoInputTransform((TwoInputTransformation<?, ?, ?>) transform);
        } else if (transform instanceof SourceTransformation<?>) {
            transformedIds = transformSource((SourceTransformation<?>) transform);
        } else if (transform instanceof SinkTransformation<?>) {
            transformedIds = transformSink((SinkTransformation<?>) transform);
        } else if (transform instanceof UnionTransformation<?>) {
            transformedIds = transformUnion((UnionTransformation<?>) transform);
        } else if (transform instanceof SplitTransformation<?>) {
            transformedIds = transformSplit((SplitTransformation<?>) transform);
        } else if (transform instanceof SelectTransformation<?>) {
            transformedIds = transformSelect((SelectTransformation<?>) transform);
        } else if (transform instanceof FeedbackTransformation<?>) {
            transformedIds = transformFeedback((FeedbackTransformation<?>) transform);
        } else if (transform instanceof CoFeedbackTransformation<?>) {
            transformedIds = transformCoFeedback((CoFeedbackTransformation<?>) transform);
        } else if (transform instanceof PartitionTransformation<?>) {
            transformedIds = transformPartition((PartitionTransformation<?>) transform);
        } else if (transform instanceof SideOutputTransformation<?>) {
            transformedIds = transformSideOutput((SideOutputTransformation<?>) transform);
        } else {
            throw new IllegalStateException("Unknown transformation: " + transform);
        }

        // need this check because the iterate transformation adds itself before
        // transforming the feedback edges
        if (!alreadyTransformed.containsKey(transform)) {
            alreadyTransformed.put(transform, transformedIds);
        }

        if (transform.getBufferTimeout() >= 0) {
            streamGraph.setBufferTimeout(transform.getId(), transform.getBufferTimeout());
        }
        if (transform.getUid() != null) {
            streamGraph.setTransformationUID(transform.getId(), transform.getUid());
        }
        if (transform.getUserProvidedNodeHash() != null) {
            streamGraph.setTransformationUserHash(transform.getId(), transform.getUserProvidedNodeHash());
        }

        if (transform.getMinResources() != null && transform.getPreferredResources() != null) {
            streamGraph.setResources(transform.getId(), transform.getMinResources(), transform.getPreferredResources());
        }

        return transformedIds;
    }

    private <T> Collection<Integer> transformSelect(SelectTransformation<T> select) {
        StreamTransformation<T> input = select.getInput();
        Collection<Integer> resultIds = transform(input);

        // the recursive transform might have already transformed this
        if (alreadyTransformed.containsKey(select)) {
            return alreadyTransformed.get(select);
        }

        List<Integer> virtualResultIds = new ArrayList<>();

        for (int inputId : resultIds) {
            int virtualId = StreamTransformation.getNewNodeId();
            streamGraph.addVirtualSelectNode(inputId, virtualId, select.getSelectedNames());
            virtualResultIds.add(virtualId);
        }
        return virtualResultIds;
    }

    private <T> Collection<Integer> transformSplit(SplitTransformation<T> split) {

        StreamTransformation<T> input = split.getInput();
        Collection<Integer> resultIds = transform(input);

        // the recursive transform call might have transformed this already
        if (alreadyTransformed.containsKey(split)) {
            return alreadyTransformed.get(split);
        }

        for (int inputId : resultIds) {
            streamGraph.addOutputSelector(inputId, split.getOutputSelector());
        }

        return resultIds;
    }

    //......
}
  • The transformation in StreamGraphGenerator will process the SelectTransformation and SplitterTransformation accordingly.
  • The transformSelect method addVirtualSelectNode according to select.getSelectedNames ()
  • The transformSplit method addOutputSelector () according to split.getOutputSelector ()

Summary

  • The split operation of DataStream receives the OutputSelector parameter and then creates and returns a SplitStream.
  • OutputSelector defines the select method used to type outputNames on the element.
  • SplitStream inherits DataStream, which defines the select method and can be used to select the dataStream split according to outputNames.

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