beam-concepts

name	beam-concepts
description	Explains core Apache Beam programming model concepts including PCollections, PTransforms, Pipelines, and Runners. Use when learning Beam fundamentals or explaining pipeline concepts.

Apache Beam Core Concepts

The Beam Model

Evolved from Google's MapReduce, FlumeJava, and Millwheel projects. Originally called the "Dataflow Model."

Key Abstractions

Pipeline

A Pipeline encapsulates the entire data processing task, including reading, transforming, and writing data.

// Java
Pipeline p = Pipeline.create(options);
p.apply(...)
 .apply(...)
 .apply(...);
p.run().waitUntilFinish();

# Python
with beam.Pipeline(options=options) as p:
    (p | 'Read' >> beam.io.ReadFromText('input.txt')
       | 'Transform' >> beam.Map(process)
       | 'Write' >> beam.io.WriteToText('output'))

PCollection

A distributed dataset that can be bounded (batch) or unbounded (streaming).

Properties

• Immutable - Once created, cannot be modified
• Distributed - Elements processed in parallel
• May be bounded or unbounded
• Timestamped - Each element has an event timestamp
• Windowed - Elements assigned to windows

PTransform

A data processing operation that transforms PCollections.

// Java
PCollection<String> output = input.apply(MyTransform.create());

# Python
output = input | 'Name' >> beam.ParDo(MyDoFn())

Core Transforms

ParDo

General-purpose parallel processing.

// Java
input.apply(ParDo.of(new DoFn<String, Integer>() {
    @ProcessElement
    public void processElement(@Element String element, OutputReceiver<Integer> out) {
        out.output(element.length());
    }
}));

# Python
class LengthFn(beam.DoFn):
    def process(self, element):
        yield len(element)

input | beam.ParDo(LengthFn())
# Or simpler:
input | beam.Map(len)

GroupByKey

Groups elements by key.

PCollection<KV<String, Integer>> input = ...;
PCollection<KV<String, Iterable<Integer>>> grouped = input.apply(GroupByKey.create());

CoGroupByKey

Joins multiple PCollections by key.

Combine

Combines elements (sum, mean, etc.).

// Global combine
input.apply(Combine.globally(Sum.ofIntegers()));

// Per-key combine
input.apply(Combine.perKey(Sum.ofIntegers()));

Flatten

Merges multiple PCollections.

PCollectionList<String> collections = PCollectionList.of(pc1).and(pc2).and(pc3);
PCollection<String> merged = collections.apply(Flatten.pCollections());

Partition

Splits a PCollection into multiple PCollections.

Windowing

Types

• Fixed Windows - Regular, non-overlapping intervals
• Sliding Windows - Overlapping intervals
• Session Windows - Gaps of inactivity define boundaries
• Global Window - All elements in one window (default)

input.apply(Window.into(FixedWindows.of(Duration.standardMinutes(5))));

input | beam.WindowInto(beam.window.FixedWindows(300))

Triggers

Control when results are emitted.

input.apply(Window.<T>into(FixedWindows.of(Duration.standardMinutes(5)))
    .triggering(AfterWatermark.pastEndOfWindow()
        .withEarlyFirings(AfterProcessingTime.pastFirstElementInPane()
            .plusDelayOf(Duration.standardMinutes(1))))
    .withAllowedLateness(Duration.standardHours(1))
    .accumulatingFiredPanes());

Side Inputs

Additional inputs to ParDo.

PCollectionView<Map<String, String>> sideInput =
    lookupTable.apply(View.asMap());

mainInput.apply(ParDo.of(new DoFn<String, String>() {
    @ProcessElement
    public void processElement(ProcessContext c) {
        Map<String, String> lookup = c.sideInput(sideInput);
        // Use lookup...
    }
}).withSideInputs(sideInput));

Pipeline Options

Configure pipeline execution.

public interface MyOptions extends PipelineOptions {
    @Description("Input file")
    @Required
    String getInput();
    void setInput(String value);
}

MyOptions options = PipelineOptionsFactory.fromArgs(args).as(MyOptions.class);

Schema

Strongly-typed access to structured data.

@DefaultSchema(AutoValueSchema.class)
@AutoValue
public abstract class User {
    public abstract String getName();
    public abstract int getAge();
}

PCollection<User> users = ...;
PCollection<Row> rows = users.apply(Convert.toRows());

Error Handling

Dead Letter Queue Pattern

TupleTag<String> successTag = new TupleTag<>() {};
TupleTag<String> failureTag = new TupleTag<>() {};

PCollectionTuple results = input.apply(ParDo.of(new DoFn<String, String>() {
    @ProcessElement
    public void processElement(ProcessContext c) {
        try {
            c.output(process(c.element()));
        } catch (Exception e) {
            c.output(failureTag, c.element());
        }
    }
}).withOutputTags(successTag, TupleTagList.of(failureTag)));

results.get(successTag).apply(WriteToSuccess());
results.get(failureTag).apply(WriteToDeadLetter());

Cross-Language Pipelines

Use transforms from other SDKs.

# Use Java Kafka connector from Python
from apache_beam.io.kafka import ReadFromKafka

result = pipeline | ReadFromKafka(
    consumer_config={'bootstrap.servers': 'localhost:9092'},
    topics=['my-topic']
)

Best Practices

1. Prefer built-in transforms over custom DoFns
2. Use schemas for type-safe operations
3. Minimize side inputs for performance
4. Handle late data explicitly
5. Test with DirectRunner before deploying
6. Use TestPipeline for unit tests