Add Dataflow warning in WriteToPubSub.expand() for ordering key support

Author: Nikita Grover

sdks/python/apache_beam/io/gcp/pubsub.py

@@ -430,7 +430,17 @@ def bytes_to_proto_str(element: Union[bytes, str]) -> bytes:
   def expand(self, pcoll):
     # Store pipeline options for use in DoFn
     self.pipeline_options = pcoll.pipeline.options if pcoll.pipeline else None
-
+    # Warn Dataflow users to use the XLang path for ordering key support,
+    # since _PubSubWriteDoFn._flush() is not used by Dataflow's implementation.
+    import logging
+    runner = self.pipeline_options.get_all_options().get(
+        'runner', '') if self.pipeline_options else ''
+    if 'Dataflow' in str(runner):
+      logging.warning(
+          'WriteToPubSub ordering_key support is not available on Dataflow '
+          'via this transform. Use the XLang WriteToPubSub path instead: '
+          'apache_beam.io.external.gcp.pubsub.WriteToPubSub with '
+          'publish_with_ordering_key=True.')
     if self.with_attributes:
       pcoll = pcoll | 'ToProtobufX' >> ParDo(
           _AddMetricsAndMap(