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Avro, mail # user - Schema resolution failure when the writer's schema is a primitive type and the reader's schema is a union


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Schema resolution failure when the writer's schema is a primitive type and the reader's schema is a union
Alexandre Normand 2012-08-17, 00:59
Hey,
I've been running into this case where I have a field of type int but I need to allow for null values. To do this, I now have a new schema that defines that field as a union of null and int such as:
type: [ "null", "int" ]
According to my interpretation of the spec, avro should resolve this correctly. For reference, this reads like this (from http://avro.apache.org/docs/current/spec.html#Schema+Resolution):
> if reader's is a union, but writer's is not
> The first schema in the reader's union that matches the writer's schema is recursively resolved against it. If none match, an error is signaled.)
>
However, when trying to do this, I get this:
org.apache.avro.AvroTypeException: Attempt to process a int when a union was expected.
I've written a simple test that illustrates what I'm saying:

    @Test
    public void testReadingUnionFromValueWrittenAsPrimitive() throws Exception {
        Schema writerSchema = new Schema.Parser().parse("{\n" +
                "    \"type\":\"record\",\n" +
                "    \"name\":\"NeighborComparisons\",\n" +
                "    \"fields\": [\n" +
                "      {\"name\": \"test\",\n" +
                "      \"type\": \"int\" }]} ");
        Schema readersSchema = new Schema.Parser().parse(" {\n" +
                "    \"type\":\"record\",\n" +
                "    \"name\":\"NeighborComparisons\",\n" +
                "    \"fields\": [ {\n" +
                "      \"name\": \"test\",\n" +
                "      \"type\": [\"null\", \"int\"],\n" +
                "      \"default\": null } ]  }");
        GenericData.Record record = new GenericData.Record(writerSchema);
        record.put("test", Integer.valueOf(10));
        ByteArrayOutputStream output = new ByteArrayOutputStream();
        JsonEncoder jsonEncoder = EncoderFactory.get().jsonEncoder(writerSchema, output);
        GenericDatumWriter<GenericData.Record> writer = new GenericDatumWriter<GenericData.Record>(writerSchema);
        writer.write(record, jsonEncoder);
        jsonEncoder.flush();
        output.flush();
        System.out.println(output.toString());
        JsonDecoder jsonDecoder = DecoderFactory.get().jsonDecoder(readersSchema, output.toString());
        GenericDatumReader<GenericData.Record> reader

                new GenericDatumReader<GenericData.Record>(writerSchema, readersSchema);
        GenericData.Record read = reader.read(null, jsonDecoder);
        
        assertEquals(10, read.get("test"));
    }

Am I misunderstanding how avro should handle such a case of schema resolution or is the problem in the implementation?
Cheers!
--
Alex