Based on Javadocs and the blog post at https://beam.apache.org/blog/2017/02/13/stateful-processing.html, I tried using a simple de-duplication example using 2.0.0-beta-2 SDK which reads a file from GCS (containing a list of jsons each with a user_id field) and then running it through a pipeline as explained below.
The input data contains about 146K events of which only 50 events are unique. The entire input is about 50MB which should be processable in considerably less time than the 2 min Fixed window. I just placed a window there to make sure the per-key-per-window semantics hold without using a GlobalWindow. I run the windowed data through 3 parallel stages to compare the results, each of which are explained below.
- just copies the contents into a new file on GCS - this ensures all the events were being processed as expected and I verified the contents are exactly the same as input
- Combine.PerKey on the user_id and pick only the first element from the Iterable - this essentially should deduplicate the data and it works as expected. The resulting file has the exact number of unique items from the original list of events - 50 elements
- stateful ParDo which checks if the key has been seen already and emits an output only when its not. Ideally, the result from this should match the deduped data as [2] but all I am seeing is only 3 unique events. These 3 unique events always point to the same 3 user_ids in a few runs I did.
Interestingly, when I just switch from the DataflowRunner to the DirectRunner running this whole process locally, I see that the output from [3] matches [2] having only 50 unique elements as expected. So, I am doubting if there are any issues with the DataflowRunner for the Stateful ParDo.
public class StatefulParDoSample {
private static Logger logger = LoggerFactory.getLogger(StatefulParDoSample.class.getName());
static class StatefulDoFn extends DoFn<KV<String, String>, String> {
final Aggregator<Long, Long> processedElements = createAggregator("processed", Sum.ofLongs());
final Aggregator<Long, Long> skippedElements = createAggregator("skipped", Sum.ofLongs());
@StateId("keyTracker")
private final StateSpec<Object, ValueState<Integer>> keyTrackerSpec =
StateSpecs.value(VarIntCoder.of());
@ProcessElement
public void processElement(
ProcessContext context,
@StateId("keyTracker") ValueState<Integer> keyTracker) {
processedElements.addValue(1l);
final String userId = context.element().getKey();
int wasSeen = firstNonNull(keyTracker.read(), 0);
if (wasSeen == 0) {
keyTracker.write( 1);
context.output(context.element().getValue());
} else {
keyTracker.write(wasSeen + 1);
skippedElements.addValue(1l);
}
}
}
public static void main(String[] args) {
DataflowPipelineOptions pipelineOptions = PipelineOptionsFactory.create().as(DataflowPipelineOptions.class);
pipelineOptions.setRunner(DataflowRunner.class);
pipelineOptions.setProject("project-name");
pipelineOptions.setStagingLocation(GCS_STAGING_LOCATION);
pipelineOptions.setStreaming(false);
pipelineOptions.setAppName("deduper");
Pipeline p = Pipeline.create(pipelineOptions);
final ObjectMapper mapper = new ObjectMapper();
PCollection<KV<String, String>> keyedEvents =
p
.apply(TextIO.Read.from(GCS_SAMPLE_INPUT_FILE_PATH))
.apply(WithKeys.of(new SerializableFunction<String, String>() {
@Override
public String apply(String input) {
try {
Map<String, Object> eventJson =
mapper.readValue(input, Map.class);
return (String) eventJson.get("user_id");
} catch (Exception e) {
}
return "";
}
}))
.apply(
Window.into(
FixedWindows.of(Duration.standardMinutes(2))
)
);
keyedEvents
.apply(ParDo.of(new StatefulDoFn()))
.apply(TextIO.Write.to(GCS_SAMPLE_OUTPUT_FILE_PATH).withNumShards(1));
keyedEvents
.apply(Values.create())
.apply(TextIO.Write.to(GCS_SAMPLE_COPY_FILE_PATH).withNumShards(1));
keyedEvents
.apply(Combine.perKey(new SerializableFunction<Iterable<String>, String>() {
@Override
public String apply(Iterable<String> input) {
return !input.iterator().hasNext() ? "empty" : input.iterator().next();
}
}))
.apply(Values.create())
.apply(TextIO.Write.to(GCS_SAMPLE_COMBINE_FILE_PATH).withNumShards(1));
PipelineResult result = p.run();
result.waitUntilFinish();
}
}
