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https://issues.apache.org/jira/browse/AVRO-248?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12785041#action_12785041
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Philip Zeyliger commented on AVRO-248:
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BTW, I came up with another argument for named union branches while thinking
about the python implementation yesterday. In python, you're not supposed to
ever use instanceof. Say you have two record types, A, and B, with A having
fields a,b, c, and B having fields a,b,c,d. In pythonic theory, you're
supposed to take the object you're dealing with, and, to see if it's an
instance of A, just see if it has fields "a", "b", and "c" using getattr.
Voila, it's an instance of A. Of course, for all we know, it was actually an
instance of B. Because of this, you'd have to annotate every non-primitive
with it's avro type, and, moreover, you'd have to make sure you can always
distinguish between primitive types (string, unicode and bytes are the most
irksome here). It's do-able, but can lead to some confusing situations.
Just wanted to throw this out there. I still think both approaches have
disadvantages.
> 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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