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From "Ryan Blue (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (AVRO-1568) Allow Java polymorphism in Avro for third-party code
Date Wed, 15 Apr 2015 17:56:59 GMT

    [ https://issues.apache.org/jira/browse/AVRO-1568?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14496597#comment-14496597
] 

Ryan Blue commented on AVRO-1568:
---------------------------------

Here's an example: say you have a 3rd party geo library for shapes. A shape has some internal
representation, but also an envelope (think bounding box) that is probably calculated and
stored. So Shape looks like roughly like this API:

{code:java}
class Shape:
  static Shape fromBytes(byte[] bytes)
  byte[] getShapeAsBytes()
  float getMinX()
  float getMinY()
  float getLength()
  float getWidth()
{code}

Because you don't control the implementation for Shape or its subclasses, Polygon and Path,
you don't know what you're going to get when you reflect their fields. I'm arguing that it
is much more reliable to serialize Shape using its public API. That way, you can use its compact
representation accessed through {{getShapeAsBytes}} and store information relevant for generic
use, its bounding box. Those might be derived values in the implementations, plus the internal
fields can change in ways that aren't compatible with Avro's schema evolution rules. Using
the public API instead of reflection is, I think, a better choice.

> Allow Java polymorphism in Avro for third-party code
> ----------------------------------------------------
>
>                 Key: AVRO-1568
>                 URL: https://issues.apache.org/jira/browse/AVRO-1568
>             Project: Avro
>          Issue Type: Improvement
>    Affects Versions: 1.7.6
>            Reporter: Sachin Goyal
>
> A large number of Java designs interacting with databases with Hibernate/Couchbase (perhaps,
even otherwise) have Java polymorphism of the form:
> {code:java}
> class Base 
> {
>    Integer a = 5;
> }
> class Derived extends Base
> {
>     String b = "Foo";
> }
> class PolymorphicDO
> {
>    Base b = new Derived();
> }
> {code}
> Jackson handles this kind of field by using annotations such as:
> {code}
> @JsonTypeInfo(use = JsonTypeInfo.Id.CLASS, include = JsonTypeInfo.As.PROPERTY, property
= "@class”)
> {code}
> If such a thing can be added to Avro, all those Java designs could become immediately
usable with Avro. They would also become Hadoop compatible due to AvroSerde.



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