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From "Doug Cutting (JIRA)" <j...@apache.org>
Subject [jira] Created: (AVRO-248) make unions a named type
Date Tue, 01 Dec 2009 19:37:20 GMT
make unions a named type
------------------------

                 Key: AVRO-248
                 URL: https://issues.apache.org/jira/browse/AVRO-248
             Project: Avro
          Issue Type: New Feature
          Components: spec
            Reporter: Doug Cutting
            Assignee: Doug Cutting
             Fix For: 1.3.0


Unions are currently anonymous.  However it might be convenient if they were named.  In particular:
 - when code is generated for a union, a class could be generated that includes an enum indicating
which branch of the union is taken, e.g., a union of string and int named Foo might cause
a Java class like {code}
public class Foo {
  public static enum Type {STRING, INT};
  private Type type;
  private Object datum;
  public Type getType();
  public String getString() { if (type==STRING) return (String)datum; else throw ... }
  public void setString(String s) { type = STRING;  datum = s; }
  ....
}
{code} Then Java applications can easily use a switch statement to process union values rather
than using instanceof.
 - when using reflection, an abstract class with a set of concrete implementations can be
represented as a union (AVRO-241).  However, if one wishes to create an array one must know
the name of the base class, which is not represented in the Avro schema.  One approach would
be to add an annotation to the reflected array schema (AVRO-242) noting the base class.  But
if the union itself were named, that could name the base class.  This would also make reflected
protocol interfaces more consise, since the base class name could be used in parameters return
types and fields.
 - Generalizing the above: Avro lacks class inheritance, unions are a way to model inheritance,
and this model is more useful if the union is named.

This would be an incompatible change to schemas.  If we go this way, we should probably rename
1.3 to 2.0.  Note that AVRO-160 proposes an incompatible change to data file formats, which
may also force a major release.

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