apache/druid · error · QueryDriver.RequestError

Could not write value

Error message

Could not write value [%s] to field [%s]

What it means

ProtobufWriter.writeRowField uses reflection to call Builder.setField, and the reflective invocation failed (IllegalAccessException or the underlying setter threw, e.g. type mismatch). The value produced for a column cannot be written into the corresponding protobuf field, so a QueryDriver.RequestError is raised to surface a result-encoding failure to the client.

Solutions

1. Inspect the wrapped InvocationTargetException for the setter's real failure — usually a value/field type mismatch.
2. Ensure the SQL column types are compatible with the declared .proto field types (e.g. int64 vs int32, string vs bytes).
3. Handle nulls and unsupported Java types before invoking setField.
4. Catch QueryDriver.RequestError at the server layer to return a descriptive gRPC error status.

When it happens

Trigger: Thrown at extensions-contrib/grpc-query/src/main/java/org/apache/druid/grpc/server/ProtobufWriter.java:120 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of apache/druid@9b90983fd2 (2026-09-07). Data as JSON: /api/errors/1bb374eef6a3b2fa. Report an issue: GitHub.

Appendix: source

Thrown at extensions-contrib/grpc-query/src/main/java/org/apache/druid/grpc/server/ProtobufWriter.java:120

          message.getClass()
      );
    }
    final Method method = methods.computeIfAbsent("setField", k -> {
      try {
        return rowBuilder
            .getClass()
            .getMethod(
                "setField", new Class<?>[]{Descriptors.FieldDescriptor.class, Object.class});
      }
      catch (NoSuchMethodException e) {
        throw new RuntimeException(e);
      }
    });
    try {
      method.invoke(rowBuilder, fieldDescriptor, value);
    }
    catch (IllegalAccessException | InvocationTargetException e) {
      throw new QueryDriver.RequestError(
          "Could not write value [%s] to field [%s]",
          value,
          name
      );
    }
  }

  @Override
  public void writeRowEnd() throws IOException
  {
    Message rowMessage = rowBuilder.build();
    rowMessage.writeDelimitedTo(outputStream);
  }

  @Override
  public void writeResponseEnd()
  {
  }

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