PTransform[In, Out] is an operation (function) with an input of type In and an output of type Out.

PTransform: In => Out

A directed acyclic graph of PTransforms and PCollections is a Pipeline.

Root PTransform

Root PTransform (Source PTransform) is a PTransform with PBegin as the In type (PTransform<PBegin, PCollection<…​>>).

Root PTransform takes no input (conventionally marked as PBegin), e.g. TextIO.Read or Create.

Sink PTransform

Sink PTransform (Output PTransform) is a PTransform<PCollection<…​>, PDone> that has no output (conventionally marked as PDone output type), e.g. TextIO.Write.