Using side inputs

We have already seen how to use side outputs, and side inputs are analogous to them. Besides the single main input, a ParDo transform can have multiple additional side inputs, as shown in the following figure:

Figure 3.13 – Side inputs

We have multiple ways of declaring a side input to a ParDo object. For instance, consider the following example:

ParDo.of(new MyDoFn())
Analogous to side outputs is also the way how we declare a side input – we must provide it to the ParDo by call to withSideInput as follows:
input.apply(ParDo.of(new MyDoFn())
    .withSideInput("side-input", sideInput));

Because we may have multiple side inputs, we need a way to distinguish them – if we assign a name to the side input, we can later access it easily in DoFn using a @SideInput annotation:

@ProcessElement
public void processElement(
    @Element .. element,
    ...