The legacy Source API and the Read transform

Before the creation of the splittable DoFn object, Beam used the Source API and its associated Read transform. Although this transform is currently deprecated and should not be used for implementing new sources, it is still supported. On some runners and under specific conditions, using the deprecated Read transform might still be preferred. We have already seen examples of this – for example, the use_deprecated_read flag passed when using the --experiments flag for Python's ReadFromKafka transform.

The Read transform accepts a single parameter: either an object of the BoundedSource type or the UnboundedSource type. Whether the source is bounded or unbounded then determines if the resulting PCollection object is bounded or unbounded.

We apply the Read transform as follows:

Pipeline p = ...;
p.apply(Read.from(new MyUnboundedSource());

We will not go into the details of BoundedSource or UnboundedSource, mostly because...