Avro schema doesn't honor backward compatibilty

Question

I have this avro schema

{
 "namespace": "xx.xxxx.xxxxx.xxxxx",
 "type": "record",
 "name": "MyPayLoad",
 "fields": [
     {"name": "filed1",  "type": "string"},
     {"name": "filed2",     "type": "long"},
     {"name": "filed3",  "type": "boolean"},
     {
          "name" : "metrics",
          "type": 
          {
             "type" : "array", 
             "items": 
             { 
                 "name": "MyRecord", 
                 "type": "record", 
                 "fields" : 
                     [                         
                       {"name": "min", "type": "long"}, 
                       {"name": "max", "type": "long"}, 
                       {"name": "sum", "type": "long"}, 
                       {"name": "count", "type": "long"}
                     ]
             } 
          }
     }
  ]
}

Here is the code which we use to parse the data

public static final MyPayLoad parseBinaryPayload(byte[] payload) {
        DatumReader<MyPayLoad> payloadReader = new SpecificDatumReader<>(MyPayLoad.class);
        Decoder decoder = DecoderFactory.get().binaryDecoder(payload, null);
        MyPayLoad myPayLoad = null;
        try {
            myPayLoad = payloadReader.read(null, decoder);
        } catch (IOException e) {
            logger.log(Level.SEVERE, e.getMessage(), e);
        }

        return myPayLoad;
    }

Now i want to add one more field int the schema so the schema looks like below

 {
 "namespace": "xx.xxxx.xxxxx.xxxxx",
 "type": "record",
 "name": "MyPayLoad",
 "fields": [
     {"name": "filed1",  "type": "string"},
     {"name": "filed2",     "type": "long"},
     {"name": "filed3",  "type": "boolean"},
     {
          "name" : "metrics",
          "type": 
          {
             "type" : "array", 
             "items": 
             { 
                 "name": "MyRecord", 
                 "type": "record", 
                 "fields" : 
                     [                         
                       {"name": "min", "type": "long"}, 
                       {"name": "max", "type": "long"}, 
                       {"name": "sum", "type": "long"}, 
                       {"name": "count", "type": "long"}
                     ]
             } 
          }
     }
     {"name": "agentType",  "type": ["null", "string"], "default": "APP_AGENT"}
  ]
}

Note the filed added and also the default is defined. The problem is that if we receive the data which was written using the older schema i get this error

java.io.EOFException: null
    at org.apache.avro.io.BinaryDecoder.ensureBounds(BinaryDecoder.java:473) ~[avro-1.7.4.jar:1.7.4]
    at org.apache.avro.io.BinaryDecoder.readInt(BinaryDecoder.java:128) ~[avro-1.7.4.jar:1.7.4]
    at org.apache.avro.io.BinaryDecoder.readIndex(BinaryDecoder.java:423) ~[avro-1.7.4.jar:1.7.4]
    at org.apache.avro.io.ResolvingDecoder.doAction(ResolvingDecoder.java:229) ~[avro-1.7.4.jar:1.7.4]
    at org.apache.avro.io.parsing.Parser.advance(Parser.java:88) ~[avro-1.7.4.jar:1.7.4]
    at org.apache.avro.io.ResolvingDecoder.readIndex(ResolvingDecoder.java:206) ~[avro-1.7.4.jar:1.7.4]
    at org.apache.avro.generic.GenericDatumReader.read(GenericDatumReader.java:152) ~[avro-1.7.4.jar:1.7.4]
    at org.apache.avro.generic.GenericDatumReader.readRecord(GenericDatumReader.java:177) ~[avro-1.7.4.jar:1.7.4]
    at org.apache.avro.generic.GenericDatumReader.read(GenericDatumReader.java:148) ~[avro-1.7.4.jar:1.7.4]
    at org.apache.avro.generic.GenericDatumReader.read(GenericDatumReader.java:139) ~[avro-1.7.4.jar:1.7.4]
    at com.appdynamics.blitz.shared.util.XXXXXXXXXXXXX.parseBinaryPayload(BlitzAvroSharedUtil.java:38) ~[blitz-shared.jar:na]

What i understood from this document that this should have been backward compatible but somehow that doesn't seem to be the case. Any idea what i am doing wrong?

Answer 1

finally i got this working. I need to give both the schemas in the SpecificDatumReader So i modified the parsing like this where i passed both the old and new schema in the reader and it worked like a charm

public static final MyPayLoad parseBinaryPayload(byte[] payload) {
        DatumReader<MyPayLoad> payloadReader = new SpecificDatumReader<>(SCHEMA_V1, SCHEMA_V2);
        Decoder decoder = DecoderFactory.get().binaryDecoder(payload, null);
        MyPayLoad myPayLoad = null;
        try {
            myPayLoad = payloadReader.read(null, decoder);
        } catch (IOException e) {
            logger.log(Level.SEVERE, e.getMessage(), e);
        }

        return myPayLoad;
    }