Design Your Pipeline

This page helps you design your Apache Beam pipeline. It includes information about how to determine your pipeline’s structure, how to choose which transforms to apply to your data, and how to determine your input and output methods.

Before reading this section, it is recommended that you become familiar with the information in the Beam programming guide.

What to consider when designing your pipeline

When designing your Beam pipeline, consider a few basic questions:

A basic pipeline

The simplest pipelines represent a linear flow of operations, as shown in figure 1.

A linear pipeline starts with one input collection, sequentially applies
  three transforms, and ends with one output collection.

Figure 1: A linear pipeline.

However, your pipeline can be significantly more complex. A pipeline represents a Directed Acyclic Graph of steps. It can have multiple input sources, multiple output sinks, and its operations (PTransforms) can both read and output multiple PCollections. The following examples show some of the different shapes your pipeline can take.

Branching PCollections

It’s important to understand that transforms do not consume PCollections; instead, they consider each individual element of a PCollection and create a new PCollection as output. This way, you can do different things to different elements in the same PCollection.

Multiple transforms process the same PCollection

You can use the same PCollection as input for multiple transforms without consuming the input or altering it.

The pipeline in figure 2 is a branching pipeline. The pipeline reads its input (first names represented as strings) from a database table and creates a PCollection of table rows. Then, the pipeline applies multiple transforms to the same PCollection. Transform A extracts all the names in that PCollection that start with the letter ‘A’, and Transform B extracts all the names in that PCollection that start with the letter ‘B’. Both transforms A and B have the same input PCollection.

The pipeline applies two transforms to a single input collection. Each
  transform produces an output collection.

Figure 2: A branching pipeline. Two transforms are applied to a single PCollection of database table rows.

The following example code applies two transforms to a single input collection.

PCollection<String> dbRowCollection = ...;

PCollection<String> aCollection = dbRowCollection.apply("aTrans", ParDo.of(new DoFn<String, String>(){
  @ProcessElement
  public void processElement(ProcessContext c) {
    if(c.element().startsWith("A")){
      c.output(c.element());
    }
  }
}));

PCollection<String> bCollection = dbRowCollection.apply("bTrans", ParDo.of(new DoFn<String, String>(){
  @ProcessElement
  public void processElement(ProcessContext c) {
    if(c.element().startsWith("B")){
      c.output(c.element());
    }
  }
}));

A single transform that produces multiple outputs

Another way to branch a pipeline is to have a single transform output to multiple PCollections by using tagged outputs. Transforms that produce more than one output process each element of the input once, and output to zero or more PCollections.

Figure 3 illustrates the same example described above, but with one transform that produces multiple outputs. Names that start with ‘A’ are added to the main output PCollection, and names that start with ‘B’ are added to an additional output PCollection.

The pipeline applies one transform that produces multiple output collections.

Figure 3: A pipeline with a transform that outputs multiple PCollections.

If we compare the pipelines in figure 2 and figure 3, you can see they perform the same operation in different ways. The pipeline in figure 2 contains two transforms that process the elements in the same input PCollection. One transform uses the following logic:

if (starts with 'A') { outputToPCollectionA }

while the other transform uses:

if (starts with 'B') { outputToPCollectionB }

Because each transform reads the entire input PCollection, each element in the input PCollection is processed twice.

The pipeline in figure 3 performs the same operation in a different way - with only one transform that uses the following logic:

if (starts with 'A') { outputToPCollectionA } else if (starts with 'B') { outputToPCollectionB }

where each element in the input PCollection is processed once.

The following example code applies one transform that processes each element once and outputs two collections.

// Define two TupleTags, one for each output.
final TupleTag<String> startsWithATag = new TupleTag<String>(){};
final TupleTag<String> startsWithBTag = new TupleTag<String>(){};

PCollectionTuple mixedCollection =
    dbRowCollection.apply(ParDo
        .of(new DoFn<String, String>() {
          @ProcessElement
          public void processElement(ProcessContext c) {
            if (c.element().startsWith("A")) {
              // Emit to main output, which is the output with tag startsWithATag.
              c.output(c.element());
            } else if(c.element().startsWith("B")) {
              // Emit to output with tag startsWithBTag.
              c.output(startsWithBTag, c.element());
            }
          }
        })
        // Specify main output. In this example, it is the output
        // with tag startsWithATag.
        .withOutputTags(startsWithATag,
        // Specify the output with tag startsWithBTag, as a TupleTagList.
                        TupleTagList.of(startsWithBTag)));

// Get subset of the output with tag startsWithATag.
mixedCollection.get(startsWithATag).apply(...);

// Get subset of the output with tag startsWithBTag.
mixedCollection.get(startsWithBTag).apply(...);

You can use either mechanism to produce multiple output PCollections. However, using additional outputs makes more sense if the transform’s computation per element is time-consuming.

Merging PCollections

Often, after you’ve branched your PCollection into multiple PCollections via multiple transforms, you’ll want to merge some or all of those resulting PCollections back together. You can do so by using one of the following:

The example in figure 4 is a continuation of the example in figure 2 in the section above. After branching into two PCollections, one with names that begin with ‘A’ and one with names that begin with ‘B’, the pipeline merges the two together into a single PCollection that now contains all names that begin with either ‘A’ or ‘B’. Here, it makes sense to use Flatten because the PCollections being merged both contain the same type.

The pipeline merges two collections into one collection with the Flatten transform.

Figure 4: A pipeline that merges two collections into one collection with the Flatten transform.

The following example code applies Flatten to merge two collections.

//merge the two PCollections with Flatten
PCollectionList<String> collectionList = PCollectionList.of(aCollection).and(bCollection);
PCollection<String> mergedCollectionWithFlatten = collectionList
    .apply(Flatten.<String>pCollections());

// continue with the new merged PCollection
mergedCollectionWithFlatten.apply(...);

Multiple sources

Your pipeline can read its input from one or more sources. If your pipeline reads from multiple sources and the data from those sources is related, it can be useful to join the inputs together. In the example illustrated in figure 5 below, the pipeline reads names and addresses from a database table, and names and order numbers from a Kafka topic. The pipeline then uses CoGroupByKey to join this information, where the key is the name; the resulting PCollection contains all the combinations of names, addresses, and orders.

The pipeline joins two input collections into one collection with the Join transform.

Figure 5: A pipeline that does a relational join of two input collections.

The following example code applies Join to join two input collections.

PCollection<KV<String, String>> userAddress = pipeline.apply(JdbcIO.<KV<String, String>>read()...);

PCollection<KV<String, String>> userOrder = pipeline.apply(KafkaIO.<String, String>read()...);

final TupleTag<String> addressTag = new TupleTag<String>();
final TupleTag<String> orderTag = new TupleTag<String>();

// Merge collection values into a CoGbkResult collection.
PCollection<KV<String, CoGbkResult>> joinedCollection =
  KeyedPCollectionTuple.of(addressTag, userAddress)
                       .and(orderTag, userOrder)
                       .apply(CoGroupByKey.<String>create());

coGbkResultCollection.apply(...);

What’s next