robertwb commented on a change in pull request #12185:
URL: https://github.com/apache/beam/pull/12185#discussion_r476702785



##########
File path: 
sdks/python/apache_beam/runners/portability/fn_api_runner/translations.py
##########
@@ -690,6 +692,200 @@ def fix_side_input_pcoll_coders(stages, pipeline_context):
   return stages
 
 
+def pack_combiners(stages, context):
+  # type: (Iterable[Stage], TransformContext) -> Iterator[Stage]
+  """Packs sibling CombinePerKey stages into a single CombinePerKey.
+
+  If CombinePerKey stages have a common input, one input each, and one output
+  each, pack the stages into a single stage that runs all CombinePerKeys and
+  outputs resulting tuples to a new PCollection. A subsequent stage unpacks
+  tuples from this PCollection and sends them to the original output
+  PCollections.
+  """
+
+  class _UnpackFn(core.DoFn):
+    """A DoFn that unpacks a packed to multiple tagged outputs.
+
+    Example:
+      tags = (T1, T2, ...)
+      input = (K, (V1, V2, ...))
+      output = TaggedOutput(T1, (K, V1)), TaggedOutput(T2, (K, V1)), ...
+    """
+
+    def __init__(self, tags):
+      self._tags = tags
+
+    def process(self, element):
+      key, values = element
+      return [
+          core.pvalue.TaggedOutput(tag, (key, value))
+          for tag, value in zip(self._tags, values)
+      ]
+
+  def _get_fallback_coder_id():
+    return context.add_or_get_coder_id(
+        coders.registry.get_coder(object).to_runner_api(None))
+
+  def _get_component_coder_id_from_kv_coder(coder, index):
+    assert index < 2
+    if coder.spec.urn == common_urns.coders.KV.urn and len(
+        coder.component_coder_ids) == 2:
+      return coder.component_coder_ids[index]
+    return _get_fallback_coder_id()
+
+  def _get_key_coder_id_from_kv_coder(coder):
+    return _get_component_coder_id_from_kv_coder(coder, 0)
+
+  def _get_value_coder_id_from_kv_coder(coder):
+    return _get_component_coder_id_from_kv_coder(coder, 1)
+
+  def _try_fuse_stages(a, b):
+    if a.can_fuse(b, context):
+      return a.fuse(b)
+    else:
+      raise ValueError
+
+  # Group stages by parent and environment, yielding ineligible stages.
+  combine_stages_by_input_pcoll_id = collections.defaultdict(list)
+  for stage in stages:
+    is_packable_combine = False
+
+    if (len(stage.transforms) == 1 and
+        stage.environment is not None and
+        python_urns.PACKED_COMBINE_FN in
+        context.components.environments[stage.environment].capabilities):
+      transform = only_transform(stage.transforms)
+      if (transform.spec.urn == common_urns.composites.COMBINE_PER_KEY.urn and
+          len(transform.inputs) == 1 and
+          len(transform.outputs) == 1):
+        combine_payload = proto_utils.parse_Bytes(
+            transform.spec.payload, beam_runner_api_pb2.CombinePayload)
+        if combine_payload.combine_fn.urn == python_urns.PICKLED_COMBINE_FN:
+          is_packable_combine = True
+
+    if is_packable_combine:
+      input_pcoll_id = only_element(transform.inputs.values())
+      stage_key = (input_pcoll_id, stage.environment)
+      combine_stages_by_input_pcoll_id[stage_key].append(stage)
+    else:
+      yield stage
+
+  for stage_key, packable_stages in combine_stages_by_input_pcoll_id.items():
+    input_pcoll_id, _ = stage_key
+    try:
+      if not len(packable_stages) > 1:
+        raise ValueError('Only one stage in this group: Skipping stage 
packing')
+      # Fused stage is used as template and is not yielded.
+      fused_stage = functools.reduce(_try_fuse_stages, packable_stages)
+    except ValueError:
+      # Skip packing stages in this group.
+      # Yield the stages unmodified, and then continue to the next group.
+      for stage in packable_stages:
+        yield stage
+      continue
+
+    transforms = [only_transform(stage.transforms) for stage in 
packable_stages]
+    combine_payloads = [
+        proto_utils.parse_Bytes(transform.spec.payload,
+                                beam_runner_api_pb2.CombinePayload)
+        for transform in transforms
+    ]
+    output_pcoll_ids = [
+        only_element(transform.outputs.values()) for transform in transforms
+    ]
+
+    # Build accumulator coder for (acc1, acc2, ...)
+    accumulator_coder_ids = [
+        combine_payload.accumulator_coder_id
+        for combine_payload in combine_payloads
+    ]
+    tuple_accumulator_coder_id = context.add_or_get_coder_id(
+        beam_runner_api_pb2.Coder(
+            spec=beam_runner_api_pb2.FunctionSpec(
+                urn=common_urns.coders.KV.urn),

Review comment:
       Now use `python_urns.TUPLE_CODER` here rather than the KV one.

##########
File path: 
sdks/python/apache_beam/runners/portability/fn_api_runner/translations.py
##########
@@ -818,7 +819,7 @@ def _try_fuse_stages(a, b):
         for output_kv_coder_id in output_kv_coder_ids
     ]
     pack_output_value_coder = beam_runner_api_pb2.Coder(
-        spec=beam_runner_api_pb2.FunctionSpec(urn=common_urns.coders.KV.urn),
+        spec=beam_runner_api_pb2.FunctionSpec(urn=python_urns.tuple.KV.urn),

Review comment:
       Shouldn't this be `python_urns.TUPLE_CODER`?




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