kennknowles commented on issue #19239:
URL: https://github.com/apache/beam/issues/19239#issuecomment-1156885794

   The PTransforms in a graph contain "user code" in the form of Java and 
Python DoFns (things like BoundedSource and UnboundedSource also are DoFns when 
you get to execution). A connector like BigQueryIO is "just" a PTransform, in 
other words a small subgraph that performance the data reading.
   
   For most connectors, we expect to have just one implementation. Usually this 
will probably be Java. Other SDKs will use the connector via Beam's portability 
APIs. But some connectors are implemented more than once, in different SDKs. 
For example FileIO is implemented in all SDKs largely to get the SDK started. 
Another example is BigQueryIO which is implemented in both Java and Python. So 
the DoFns that are executed in each case are different. The Python 
implementation of BigQueryIO is also missing features that are available in 
Java.
   
   Hope that helps!


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