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From "Alexandre Normand (JIRA)" <j...@apache.org>
Subject [jira] [Created] (AVRO-1145) Can't read union of null and primitive from value written with schema as primitive
Date Fri, 31 Aug 2012 17:27:07 GMT
Alexandre Normand created AVRO-1145:
---------------------------------------

             Summary: Can't read union of null and primitive from value written with schema
as primitive
                 Key: AVRO-1145
                 URL: https://issues.apache.org/jira/browse/AVRO-1145
             Project: Avro
          Issue Type: Bug
          Components: java
    Affects Versions: 1.7.0
            Reporter: Alexandre Normand


Using the its Java's generic representation API and I have a problem dealing with our current
case of schema evolution. The scenario we're dealing with here is making a primitive-type
field optional by changing the field to be a {{union}} of {{null}} and that primitive type.

I'm going to use a simple example. Basically, our schemas are:

Initial: A record with one field of type {{int}}
Second version: Same record, same field name but the type is now a union of {{null}} and {{int}}
According to the [schema resolution|http://avro.apache.org/docs/current/spec.html#Schema+Resolution]
chapter of Avro's spec, the resolution for such a case should be:

{code}
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 signalled.
{code}

My interpretation is that we should resolve data serialized with the initial schema properly
as int is part of the union in the reader's schema.

However, when running a test of reading back a record serialized with version 1 using the
version 2, I get

*{{org.apache.avro.AvroTypeException: Attempt to process a int when a union was expected.}}*

Here's a test that shows exactly this:
{code}
@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 } ]  }");

    // Writing a record using the initial schema with the 
    // test field defined as an int
    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());

    // We try reading it back using the second schema 
    // version where the test field is defined as a union of null and int
    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);

    // We should be able to assert that the value is 10 but it
    // fails on reading the record before getting here
    assertEquals(10, read.get("test"));
}
{code}


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