apache/beam · error · RuntimeException

Unable to automatically infer a Coder for the Kafka…

Error message

Unable to automatically infer a Coder for the Kafka Deserializer %s: no coder registered for type %s

What it means

LocalDeserializerProvider.getNullableCoder infers the Beam Coder by reading the Deserializer<T> generic type parameter and asking the CoderRegistry for a coder of that class. When no coder is registered for the deserializer's target type, this RuntimeException fires — Beam cannot serialize the deserialized records without an explicit coder.

Solutions

1. Register a Coder for the deserializer's output type in the CoderRegistry.
2. Set the coder explicitly on the transform instead of relying on inference.
3. Use a deserializer whose generic type has a built-in coder (primitives, String, Avro, etc.).

When it happens

Trigger: Thrown at sdks/java/io/kafka/src/main/java/org/apache/beam/sdk/io/kafka/LocalDeserializerProvider.java:93 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/8cd30aa331c8409a. Report an issue: GitHub.

Appendix: source

Thrown at sdks/java/io/kafka/src/main/java/org/apache/beam/sdk/io/kafka/LocalDeserializerProvider.java:93

    for (Type type : deserializer.getGenericInterfaces()) {
      if (!(type instanceof ParameterizedType)) {
        continue;
      }

      // This does not recurse: we will not infer from a class that extends
      // a class that extends Deserializer<T>.
      ParameterizedType parameterizedType = (ParameterizedType) type;

      if (parameterizedType.getRawType() == Deserializer.class) {
        Type parameter = parameterizedType.getActualTypeArguments()[0];

        @SuppressWarnings("unchecked")
        Class<T> clazz = (Class<T>) parameter;

        try {
          return NullableCoder.of(coderRegistry.getCoder(clazz));
        } catch (CannotProvideCoderException e) {
          throw new RuntimeException(
              String.format(
                  "Unable to automatically infer a Coder for "
                      + "the Kafka Deserializer %s: no coder registered for type %s",
                  deserializer, clazz));
        }
      }
    }
    throw new RuntimeException(
        String.format("Could not extract the Kafka Deserializer type from %s", deserializer));
  }
}

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