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From "Doug Cutting (JIRA)" <j...@apache.org>
Subject [jira] Commented: (AVRO-248) make unions a named type
Date Tue, 01 Dec 2009 21:07:20 GMT

    [ https://issues.apache.org/jira/browse/AVRO-248?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12784427#action_12784427
] 

Doug Cutting commented on AVRO-248:
-----------------------------------

The nullable field use case for unions makes me want to continue to permit anonymous unions.
 An implementation could mostly ignore union names, except in resolving references to them
while parsing schemas and protocols.  But an implementation might, if a name is provided for
a union, represent it as an explicit type, or not (if it's anonymous).  So Java's specific
implementation would only generate a class if a union is named, and use runtime typing for
anonymous unions.  Does that sound reasonable?

> 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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