Hi all,
I've been working with +Scott Lukas <slu...@google.com> on using the new
schema io interfaces [1] in cross-language. This means adding a
general-purpose ExternalTransformRegistrar [2,3] that will register all
SchemaIOProvider implementations via ServiceLoader.
We've run into an issue though - ExternalTransformRegistrar is supposed to
return a `Map<String, Class<? extends ExternalTransformBuilder>>`. This
makes it very challenging (impossible?) for us to create a general-purpose
ExternalTransformBuilder that defers to SchemaIOProvider. Ideally we would
instead return a Map<String, ExternalTransformBuilder> (i.e. a concrete
instance rather than a class object), so that we could just register
different instances of a class like:
class SchemaIOBuilder extends ExternalTransformBuilder {
private SchemaIOProvider provider;
PTransform<InputT, OutputT> buildExternal(ConfigT configuration) {
// Use provider to create PTransform
}
}
I think it would be possible to change the ExternalTransformRegistrar
interface so it has a single method, Map<String, ExternalTransformBuilder>
knownBuilders(). It could even be done in a backwards-compatible way if we
keep the old method and provide a default implementation of the new method
that builds instances.
However, I'm curious if there's some strong reason for using Class<?
extends ExternalTransformBuilder> as the return type for knownBuilders that
I'm missing. Does anyone know why we chose that?
Thanks,
Brian
[1] https://s.apache.org/beam-schema-io
[2]
https://github.com/apache/beam/blob/master/sdks/java/core/src/main/java/org/apache/beam/sdk/transforms/ExternalTransformRegistrar.java
[3]
https://github.com/apache/beam/blob/master/sdks/java/core/src/main/java/org/apache/beam/sdk/transforms/ExternalTransformBuilder.java
