[jira] [Comment Edited] (AVRO-248) make unions a named type

["Elliot West (Jira)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/AVRO-248?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17089535#comment-17089535 ] 

Elliot West edited comment on AVRO-248 at 4/22/20, 10:33 AM:
-------------------------------------------------------------

I see this issue is quite old, but I am wondering if there would be any interest in adding this to the specification and implementing it? Specifically, I'm thinking about this kind of construct as described previously by [~cutting]:
{code:java}
{
  "type": "union",
  "name": "Foo",
  "branches": [
    "string",
    "Bar",
    ...
  ]
}{code}
The reason I ask is that I believe that there are new use-cases that could greatly benefit from this feature, specifically those that currently require [multi-typed streams in Kafka|https://www.confluent.io/blog/put-several-event-types-kafka-topic/] or indeed any streaming platform. There is already [an alternative implementation for this|https://github.com/confluentinc/schema-registry/pull/680#issuecomment-511796090] for this functionality, but this sits outside of Avro and in my opinion a sub-optimal work-around with [a number of significant issues|https://github.com/confluentinc/schema-registry/pull/680#issuecomment-511796090].

I would suggest that by implementing this feature in Avro, we can fully satisfy multi-typed stream use-cases in a clean, simple, and elegant manner, without needing to build out external implementations that attempt to work around this absent Avro feature.

 


was (Author: teabot):
I see this issue is quite old, but I am wondering if there would be any interest in adding this to the specification and implementing it? Specifically, I'm thinking about this kind of construct as described previously by [~cutting]:

 
{code:java}
{
  "type": "union",
  "name": "Foo",
  "branches": [
    "string",
    "Bar",
    ...
  ]
}{code}
 

The reason I ask is that I believe that there are new use-cases that could greatly benefit from this feature, specifically those that currently require [multi-typed streams in Kafka|https://www.confluent.io/blog/put-several-event-types-kafka-topic/] or indeed any streaming platform. There is already [an alternative implementation for this|https://github.com/confluentinc/schema-registry/pull/680#issuecomment-511796090] for this functionality, but this sits outside of Avro and in my opinion a sub-optimal work-around with [a number of significant issues|https://github.com/confluentinc/schema-registry/pull/680#issuecomment-511796090].

I would suggest that by implementing this feature in Avro, we can fully satisfy multi-typed stream use-cases in a clean, simple, and elegant manner, without needing to build out external implementations that attempt to work around this absent Avro feature.

 

> make unions a named type
> ------------------------
>
>                 Key: AVRO-248
>                 URL: https://issues.apache.org/jira/browse/AVRO-248
>             Project: Apache Avro
>          Issue Type: New Feature
>          Components: spec
>            Reporter: Doug Cutting
>            Priority: Major
>
> 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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