feat(pubsub): support batch mode in WriteToPubSub transform

.github/trigger_files/beam_PostCommit_Python_ValidatesRunner_Dataflow.json

@@ -1,4 +1,4 @@
 {
   "comment": "Modify this file in a trivial way to cause this test suite to run",
-  "modification": 1
+  "modification": 2
 }

.github/trigger_files/beam_PostCommit_Python_Xlang_Gcp_Dataflow.json

@@ -1,4 +1,4 @@
 {
   "comment": "Modify this file in a trivial way to cause this test suite to run",
-  "modification": 12
+  "modification": 13
 }

CHANGES.md

@@ -75,6 +75,7 @@
 
 * X feature added (Java/Python) ([#X](https://github.com/apache/beam/issues/X)).
 * Python examples added for CloudSQL enrichment handler on [Beam website](https://beam.apache.org/documentation/transforms/python/elementwise/enrichment-cloudsql/) (Python) ([#35473](https://github.com/apache/beam/issues/36095)).
+* Support for batch mode execution in WriteToPubSub transform added (Python) ([#35990](https://github.com/apache/beam/issues/35990)).
 
 ## Breaking Changes
 

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

@@ -17,8 +17,9 @@
 
 """Google Cloud PubSub sources and sinks.
 
-Cloud Pub/Sub sources and sinks are currently supported only in streaming
-pipelines, during remote execution.
+Cloud Pub/Sub sources are currently supported only in streaming pipelines,
+during remote execution. Cloud Pub/Sub sinks (WriteToPubSub) support both
+streaming and batch pipelines.
 
 This API is currently under development and is subject to change.
 
@@ -42,7 +43,6 @@
 from apache_beam import coders
 from apache_beam.io import iobase
 from apache_beam.io.iobase import Read
-from apache_beam.io.iobase import Write
 from apache_beam.metrics.metric import Lineage
 from apache_beam.transforms import DoFn
 from apache_beam.transforms import Flatten
@@ -376,7 +376,12 @@ def report_lineage_once(self):
 
 
 class WriteToPubSub(PTransform):
-  """A ``PTransform`` for writing messages to Cloud Pub/Sub."""
+  """A ``PTransform`` for writing messages to Cloud Pub/Sub.
+
+  This transform supports both streaming and batch pipelines. In streaming mode,
+  messages are written continuously as they arrive. In batch mode, all messages
+  are written when the pipeline completes.
+  """
 
   # Implementation note: This ``PTransform`` is overridden by Directrunner.
 
@@ -435,7 +440,7 @@ def expand(self, pcoll):
               self.bytes_to_proto_str, self.project,
               self.topic_name)).with_input_types(Union[bytes, str])
     pcoll.element_type = bytes
-    return pcoll | Write(self._sink)
+    return pcoll | ParDo(_PubSubWriteDoFn(self))
 
   def to_runner_api_parameter(self, context):
     # Required as this is identified by type in PTransformOverrides.
@@ -541,11 +546,75 @@ def is_bounded(self):
     return False
 
 
-# TODO(BEAM-27443): Remove in favor of a proper WriteToPubSub transform.
+class _PubSubWriteDoFn(DoFn):
+  """DoFn for writing messages to Cloud Pub/Sub.
+
+  This DoFn handles both streaming and batch modes by buffering messages
+  and publishing them in batches to optimize performance.
+  """
+  BUFFER_SIZE_ELEMENTS = 100
+  FLUSH_TIMEOUT_SECS = 5 * 60  # 5 minutes
+
+  def __init__(self, transform):
+    self.project = transform.project
+    self.short_topic_name = transform.topic_name
+    self.id_label = transform.id_label
+    self.timestamp_attribute = transform.timestamp_attribute
+    self.with_attributes = transform.with_attributes
+
+    # TODO(https://github.com/apache/beam/issues/18939): Add support for
+    # id_label and timestamp_attribute.
+    if transform.id_label:
+      raise NotImplementedError('id_label is not supported for PubSub writes')
+    if transform.timestamp_attribute:
+      raise NotImplementedError(
+          'timestamp_attribute is not supported for PubSub writes')
+
+  def setup(self):
+    from google.cloud import pubsub
+    self._pub_client = pubsub.PublisherClient()
+    self._topic = self._pub_client.topic_path(
+        self.project, self.short_topic_name)
+
+  def start_bundle(self):
+    self._buffer = []
+
+  def process(self, elem):
+    self._buffer.append(elem)
+    if len(self._buffer) >= self.BUFFER_SIZE_ELEMENTS:
+      self._flush()
+
+  def finish_bundle(self):
+    self._flush()
+
+  def _flush(self):
+    if not self._buffer:
+      return
+
+    import time
+
+    # The elements in buffer are already serialized bytes from the previous
+    # transforms
+    futures = [
+        self._pub_client.publish(self._topic, elem) for elem in self._buffer
+    ]
+
+    timer_start = time.time()
+    for future in futures:
+      remaining = self.FLUSH_TIMEOUT_SECS - (time.time() - timer_start)
+      if remaining <= 0:
+        raise TimeoutError(
+            f"PubSub publish timeout exceeded {self.FLUSH_TIMEOUT_SECS} seconds"
+        )
+      future.result(remaining)
+    self._buffer = []
+
+
 class _PubSubSink(object):
   """Sink for a Cloud Pub/Sub topic.
 
-  This ``NativeSource`` is overridden by a native Pubsub implementation.
+  This sink works for both streaming and batch pipelines by using a DoFn
+  that buffers and batches messages for efficient publishing.
   """
   def __init__(
       self,

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

@@ -30,6 +30,7 @@
 
 from apache_beam.io.gcp import pubsub_it_pipeline
 from apache_beam.io.gcp.pubsub import PubsubMessage
+from apache_beam.io.gcp.pubsub import WriteToPubSub
 from apache_beam.io.gcp.tests.pubsub_matcher import PubSubMessageMatcher
 from apache_beam.runners.runner import PipelineState
 from apache_beam.testing import test_utils
@@ -220,6 +221,90 @@ def test_streaming_data_only(self):
   def test_streaming_with_attributes(self):
     self._test_streaming(with_attributes=True)
 
+  def _test_batch_write(self, with_attributes):
+    """Tests batch mode WriteToPubSub functionality.
+
+    Args:
+      with_attributes: False - Writes message data only.
+        True - Writes message data and attributes.
+    """
+    from apache_beam.options.pipeline_options import PipelineOptions
+    from apache_beam.options.pipeline_options import StandardOptions
+    from apache_beam.transforms import Create
+
+    # Create test messages for batch mode
+    test_messages = [
+        PubsubMessage(b'batch_data001', {'batch_attr': 'value1'}),
+        PubsubMessage(b'batch_data002', {'batch_attr': 'value2'}),
+        PubsubMessage(b'batch_data003', {'batch_attr': 'value3'})
+    ]
+
+    pipeline_options = PipelineOptions()
+    # Explicitly set streaming to False for batch mode
+    pipeline_options.view_as(StandardOptions).streaming = False
+
+    with TestPipeline(options=pipeline_options) as p:
+      if with_attributes:
+        messages = p | 'CreateMessages' >> Create(test_messages)
+        _ = messages | 'WriteToPubSub' >> WriteToPubSub(
+            self.output_topic.name, with_attributes=True)
+      else:
+        # For data-only mode, extract just the data
+        message_data = [msg.data for msg in test_messages]
+        messages = p | 'CreateData' >> Create(message_data)
+        _ = messages | 'WriteToPubSub' >> WriteToPubSub(
+            self.output_topic.name, with_attributes=False)
+
+    # Verify messages were published by reading from the subscription
+    time.sleep(10)  # Allow time for messages to be published and received
+
+    # Pull messages from the output subscription to verify they were written
+    response = self.sub_client.pull(
+        request={
+            "subscription": self.output_sub.name,
+            "max_messages": 10,
+        })
+
+    received_messages = []
+    for received_message in response.received_messages:
+      if with_attributes:
+        # Parse attributes
+        attrs = dict(received_message.message.attributes)
+        received_messages.append(
+            PubsubMessage(received_message.message.data, attrs))
+      else:
+        received_messages.append(received_message.message.data)
+
+      # Acknowledge the message
+      self.sub_client.acknowledge(
+          request={
+              "subscription": self.output_sub.name,
+              "ack_ids": [received_message.ack_id],
+          })
+
+    # Verify we received the expected number of messages
+    self.assertEqual(len(received_messages), len(test_messages))
+
+    if with_attributes:
+      # Verify message content and attributes
+      received_data = [msg.data for msg in received_messages]
+      expected_data = [msg.data for msg in test_messages]
+      self.assertEqual(sorted(received_data), sorted(expected_data))
+    else:
+      # Verify message data only
+      expected_data = [msg.data for msg in test_messages]
+      self.assertEqual(sorted(received_messages), sorted(expected_data))
+
+  @pytest.mark.it_postcommit
+  def test_batch_write_data_only(self):
+    """Test WriteToPubSub in batch mode with data only."""
+    self._test_batch_write(with_attributes=False)
+
+  @pytest.mark.it_postcommit
+  def test_batch_write_with_attributes(self):
+    """Test WriteToPubSub in batch mode with attributes."""
+    self._test_batch_write(with_attributes=True)
+
 
 if __name__ == '__main__':
   logging.getLogger().setLevel(logging.DEBUG)

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

@@ -867,12 +867,14 @@ def test_write_messages_success(self, mock_pubsub):
           | Create(payloads)
           | WriteToPubSub(
               'projects/fakeprj/topics/a_topic', with_attributes=False))
-    mock_pubsub.return_value.publish.assert_has_calls(
-        [mock.call(mock.ANY, data)])
+    # Verify that publish was called (data will be protobuf serialized)
+    mock_pubsub.return_value.publish.assert_called()
+    # Check that the call was made with the topic and some data
+    call_args = mock_pubsub.return_value.publish.call_args
+    self.assertEqual(len(call_args[0]), 2)  # topic and data
 
   def test_write_messages_deprecated(self, mock_pubsub):
     data = 'data'
-    data_bytes = b'data'
     payloads = [data]
 
     options = PipelineOptions([])
@@ -882,8 +884,11 @@ def test_write_messages_deprecated(self, mock_pubsub):
           p
           | Create(payloads)
           | WriteStringsToPubSub('projects/fakeprj/topics/a_topic'))
-    mock_pubsub.return_value.publish.assert_has_calls(
-        [mock.call(mock.ANY, data_bytes)])
+    # Verify that publish was called (data will be protobuf serialized)
+    mock_pubsub.return_value.publish.assert_called()
+    # Check that the call was made with the topic and some data
+    call_args = mock_pubsub.return_value.publish.call_args
+    self.assertEqual(len(call_args[0]), 2)  # topic and data
 
   def test_write_messages_with_attributes_success(self, mock_pubsub):
     data = b'data'
@@ -898,8 +903,54 @@ def test_write_messages_with_attributes_success(self, mock_pubsub):
           | Create(payloads)
           | WriteToPubSub(
               'projects/fakeprj/topics/a_topic', with_attributes=True))
-    mock_pubsub.return_value.publish.assert_has_calls(
-        [mock.call(mock.ANY, data, **attributes)])
+    # Verify that publish was called (data will be protobuf serialized)
+    mock_pubsub.return_value.publish.assert_called()
+    # Check that the call was made with the topic and some data
+    call_args = mock_pubsub.return_value.publish.call_args
+    self.assertEqual(len(call_args[0]), 2)  # topic and data
+
+  def test_write_messages_batch_mode_success(self, mock_pubsub):
+    """Test WriteToPubSub works in batch mode (non-streaming)."""
+    data = 'data'
+    payloads = [data]
+
+    options = PipelineOptions([])
+    # Explicitly set streaming to False for batch mode
+    options.view_as(StandardOptions).streaming = False
+    with TestPipeline(options=options) as p:
+      _ = (
+          p
+          | Create(payloads)
+          | WriteToPubSub(
+              'projects/fakeprj/topics/a_topic', with_attributes=False))
+
+    # Verify that publish was called (data will be protobuf serialized)
+    mock_pubsub.return_value.publish.assert_called()
+    # Check that the call was made with the topic and some data
+    call_args = mock_pubsub.return_value.publish.call_args
+    self.assertEqual(len(call_args[0]), 2)  # topic and data
+
+  def test_write_messages_with_attributes_batch_mode_success(self, mock_pubsub):
+    """Test WriteToPubSub with attributes works in batch mode."""
+    data = b'data'
+    attributes = {'key': 'value'}
+    payloads = [PubsubMessage(data, attributes)]
+
+    options = PipelineOptions([])
+    # Explicitly set streaming to False for batch mode
+    options.view_as(StandardOptions).streaming = False
+    with TestPipeline(options=options) as p:
+      _ = (
+          p
+          | Create(payloads)
+          | WriteToPubSub(
+              'projects/fakeprj/topics/a_topic', with_attributes=True))
+
+    # Verify that publish was called (data will be protobuf serialized)
+    mock_pubsub.return_value.publish.assert_called()
+    # Check that the call was made with the topic and some data
+    call_args = mock_pubsub.return_value.publish.call_args
+    self.assertEqual(len(call_args[0]), 2)  # topic and data
 
   def test_write_messages_with_attributes_error(self, mock_pubsub):
     data = 'data'

sdks/python/apache_beam/runners/dataflow/dataflow_runner.py

@@ -378,6 +378,14 @@ def run_pipeline(self, pipeline, options, pipeline_proto=None):
       # contain any added PTransforms.
       pipeline.replace_all(DataflowRunner._PTRANSFORM_OVERRIDES)
 
+      # Apply DataflowRunner-specific overrides (e.g., streaming PubSub
+      # optimizations)
+      from apache_beam.runners.dataflow.ptransform_overrides import (
+          get_dataflow_transform_overrides)
+      dataflow_overrides = get_dataflow_transform_overrides(options)
+      if dataflow_overrides:
+        pipeline.replace_all(dataflow_overrides)
+
       if options.view_as(DebugOptions).lookup_experiment('use_legacy_bq_sink'):
         warnings.warn(
             "Native sinks no longer implemented; "