Example showing failure when not calling get_side_output on a pyflink DataStream immediately after the ProcessFunction that emits to a side output

pyflink_sideout_fail_word_count_example.py

import argparse
import logging
import sys

from pyflink.common import WatermarkStrategy, Encoder, Types
from pyflink.datastream import StreamExecutionEnvironment, RuntimeExecutionMode, ProcessFunction, OutputTag
from pyflink.datastream.connectors.file_system import FileSource, StreamFormat, FileSink, OutputFileConfig, RollingPolicy


word_count_data = ["To be, or not to be,--that is the question:--",
                   "Whether 'tis nobler in the mind to suffer",
                   "The slings and arrows of outrageous fortune",
                   "Or to take arms against a sea of troubles,",
                   "And by opposing end them?--To die,--to sleep,--",
                   "No more; and by a sleep to say we end",
                   "The heartache, and the thousand natural shocks",
                   "That flesh is heir to,--'tis a consummation",
                   "Devoutly to be wish'd. To die,--to sleep;--",
                   "To sleep! perchance to dream:--ay, there's the rub;",
                   "For in that sleep of death what dreams may come,",
                   "When we have shuffled off this mortal coil,",
                   "Must give us pause: there's the respect",
                   "That makes calamity of so long life;",
                   "For who would bear the whips and scorns of time,",
                   "The oppressor's wrong, the proud man's contumely,",
                   "The pangs of despis'd love, the law's delay,",
                   "The insolence of office, and the spurns",
                   "That patient merit of the unworthy takes,",
                   "When he himself might his quietus make",
                   "With a bare bodkin? who would these fardels bear,",
                   "To grunt and sweat under a weary life,",
                   "But that the dread of something after death,--",
                   "The undiscover'd country, from whose bourn",
                   "No traveller returns,--puzzles the will,",
                   "And makes us rather bear those ills we have",
                   "Than fly to others that we know not of?",
                   "Thus conscience does make cowards of us all;",
                   "And thus the native hue of resolution",
                   "Is sicklied o'er with the pale cast of thought;",
                   "And enterprises of great pith and moment,",
                   "With this regard, their currents turn awry,",
                   "And lose the name of action.--Soft you now!",
                   "The fair Ophelia!--Nymph, in thy orisons",
                   "Be all my sins remember'd."]


output_tag = OutputTag("side-output", Types.STRING())


class LinesWithTheToSideOutputProcessFunction(ProcessFunction):
    """
    Any line that starts with "The" will be emitted to output_tag side output.
    """
    def process_element(self, line: str, ctx: ProcessFunction.Context):
        if line.startswith("The"):
            yield output_tag, "sideout-line: " + line
        else:
            # emit data to regular output
            yield line


def word_count(input_path, output_path):
    env = StreamExecutionEnvironment.get_execution_environment()
    env.set_runtime_mode(RuntimeExecutionMode.BATCH)
    # write all the data to one file
    env.set_parallelism(1)

    # define the source
    if input_path is not None:
        ds = env.from_source(
            source=FileSource.for_record_stream_format(StreamFormat.text_line_format(),
                                                       input_path)
                .process_static_file_set().build(),
            watermark_strategy=WatermarkStrategy.for_monotonous_timestamps(),
            source_name="file_source"
        )
    else:
        print("Executing word_count example with default input data set.")
        print("Use --input to specify file input.")
        ds = env.from_collection(word_count_data)

    def split(line):
        yield from line.split()

    processed_ds = ds.process(LinesWithTheToSideOutputProcessFunction())

    side_output_ds = None
    # If we don't call processed_dc.get_side_output, the following pipeline will fail.
    # Apparently, we must call processed_dc.get_side_output immediately after the
    # ProcessFunction that emits records to side output, otherwise those
    # records will remain in the main DataStream as a Tuple(OutputTag, value).
    # If you uncomment this line, things will work as expected.
    #side_output_ds = processed_ds.get_side_output(output_tag)

    # compute word count
    ds = processed_ds.flat_map(split) \
        .map(lambda i: (i, 1), output_type=Types.TUPLE([Types.STRING(), Types.INT()])) \
        .key_by(lambda i: i[0]) \
        .reduce(lambda i, j: (i[0], i[1] + j[1]))

    # define the sink
    if output_path is not None:
        ds.sink_to(
            sink=FileSink.for_row_format(
                base_path=output_path,
                encoder=Encoder.simple_string_encoder())
                .with_output_file_config(
                OutputFileConfig.builder()
                    .with_part_prefix("prefix")
                    .with_part_suffix(".ext")
                    .build())
                .with_rolling_policy(RollingPolicy.default_rolling_policy())
                .build()
        )
    else:
        print("Printing result to stdout. Use --output to specify output path.")
        ds.print()

    if side_output_ds:
        print("printing side output to stdout")
        side_output_ds.print()

    # submit for execution
    env.execute()


if __name__ == '__main__':
    logging.basicConfig(stream=sys.stdout, level=logging.INFO, format="%(message)s")

    parser = argparse.ArgumentParser()
    parser.add_argument(
        '--input',
        dest='input',
        required=False,
        help='Input file to process.')
    parser.add_argument(
        '--output',
        dest='output',
        required=False,
        help='Output file to write results to.')

    argv = sys.argv[1:]
    known_args, _ = parser.parse_known_args(argv)

    word_count(known_args.input, known_args.output)