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From "Todd Lipcon (JIRA)" <j...@apache.org>
Subject [jira] Commented: (AVRO-248) make unions a named type
Date Wed, 23 Dec 2009 23:29:29 GMT

    [ https://issues.apache.org/jira/browse/AVRO-248?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12794273#action_12794273

Todd Lipcon commented on AVRO-248:

I am strongly pro-naming. AVRO-266 (object reuse for deserializing unions) is another reason
that having names for unions makes sense.

As for nullability, I agree that we definitely don't want to force type names on all nullable
fields. Anonymous unions are one solution, but special-casing nullability in schemas doesn't
seem entirely wrong to me either...

As for naming other types, is a typedef construct useful? This would solve the union-of-arrays
issue as well as some others. To give a concrete example, imagine an MR job where we want
to aggregate over both users and products. Users and products are both represented by their
database IDs. I'd want to write:

{"type": "union", "branches": [{"name": "user_id", "type": "int"}, {"name": "product_id",
"type": "int"}]}

or with typedefs:
{"type": "typedef", "name": "UserId", "is_type": "int"},
{"type": "typedef", "name": "ProductId", "is_type": "int"}
and then use ["UserId", "ProductId"] with some way to distinguish between the two.

> 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
>             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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