apache/beam · error · RuntimeError

Re-used coder id

Error message

Re-used coder id: %s
%s
%s

What it means

While merging the external transform's components into the pipeline, a coder id generated inside the external namespace collided with one already registered in the context; component ids must be unique, so the merge aborts rather than silently aliasing coders.

Solutions

1. Retry pipeline construction: the ids are UUID-based and collisions are effectively random
2. If reproducible, report to Beam devs with the coder protos from the message

When it happens

Trigger: Thrown at sdks/python/apache_beam/transforms/external.py:948 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of apache/beam@12126d8942 (2026-09-13). Data as JSON: /api/errors/0bf1b6e6c468dfcc. Report an issue: GitHub.

Appendix: source

Thrown at sdks/python/apache_beam/transforms/external.py:948

    for tag, pcoll in self._outputs.items():
      pcoll_renames[self._expanded_transform.outputs[tag]] = (
          context.pcollections.get_id(pcoll))

    def _equivalent(coder1, coder2):
      return coder1 == coder2 or _normalize(coder1) == _normalize(coder2)

    def _normalize(coder_proto):
      normalized = copy.copy(coder_proto)
      normalized.spec.environment_id = ''
      # TODO(robertwb): Normalize components as well.
      return normalized

    for id, proto in self._expanded_components.coders.items():
      if id.startswith(self._external_namespace):
        context.coders.put_proto(id, proto)
      elif id in context.coders:
        if not _equivalent(context.coders._id_to_proto[id], proto):
          raise RuntimeError(
              'Re-used coder id: %s\n%s\n%s' %
              (id, context.coders._id_to_proto[id], proto))
      else:
        context.coders.put_proto(id, proto)
    for id, proto in self._expanded_components.windowing_strategies.items():
      if id.startswith(self._external_namespace):
        context.windowing_strategies.put_proto(id, proto)
    for id, proto in self._expanded_components.environments.items():
      if id.startswith(self._external_namespace):
        context.environments.put_proto(id, proto)
    for id, proto in self._expanded_components.pcollections.items():
      id = pcoll_renames.get(id, id)
      if id not in context.pcollections._id_to_obj.keys():
        context.pcollections.put_proto(id, proto)

    for id, proto in self._expanded_components.transforms.items():
      if id.startswith(self._IMPULSE_PREFIX):
        # Our fake inputs.

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