After Beam Summit EU I was curious about the External transform. I was interested on the scenario of using it to call python code in the middle of a Java pipeline. This is a potentially useful scenario for example to evaluate models from python ML frameworks on Java pipelines. In my example I did a transform to classify elements in a simple Python ParDo and tried to connect it via the Java External transform.
I found that the ExternalTransform code was added into `runners/core-construction-java` as part of BEAM-6747 [1]. However this code is not exposed currently as part of the Beam Java SDK, so end users won’t be able to find it easily. I found this weird and thought well it will be as simple as to move it into the Java SDK and voila! But of course this could not be so easy because this transform calls the Expansion service via gRPC and Java SDK does not have (and probably should not have) gRPC in its dependencies. So my second reflex was to add it into Java SDK and translate it a generic expansion all the runners, but this may not make sense because the External transform is not part of the runner translation since this is part of the Pipeline construction process (as pointed to me by Max in a slack discussion). So the question is: How do you think this should be exposed to the end users? 1. Should we add gRPC with all its deps to SDKs Java core? (this of course it is not nice because we will leak our vendored gRPC and friends into users classpath). 2. Should we do the dynamic loading of classes only an runtime if the transform is used to avoid the big extra compile dependency (and add runners/core-construction-java) as a runtime dependency. 3. Should we create a ‘shim’ module to hide the gRPC dependency and load the gRPC classes dynamically on it when the External transform is part of the pipeline. 4. Should we pack it as an extension (with the same issue of needing to leak the dependencies, but with less impact for users who do not use External) ? 5. Other? The ‘purist’ me thinks we should have External in sdks/java/core but maybe it is better not to. Any other opinions or ideas? Thanks, Ismaël [1] https://issues.apache.org/jira/browse/BEAM-6747
