[
https://issues.apache.org/jira/browse/AVRO-248?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12784604#action_12784604
]
Thiruvalluvan M. G. commented on AVRO-248:
------------------------------------------
Talking about names, the current specification that records, enums and fixed
(and now unions) are named seems somewhat arbitrary. Names serve two main
purposes:
- Named entities can be reused elsewhere in the schema
- Names are used to differentiate branches in unions
Strictly speaking names are not required if things if these situations do not
occur.
The third use of name is in code generation. If we can somehow handle the code
generation part, I'd propose that we make names completely optional.
Also, one should be able to name the other non-primitive types - arrays and
maps. The names for arrays and maps are of not much use for reuse, but very
useful in unions. Today, one cannot have a union of int arrays and string
arrays. One could argue that the same effect can be achieved by having an array
of unions of int and string. But they are not the same. Array of unions is
actually an heterogeneous array - some elements can be ints and some other
strings.
In summary, I propose we make all compound types named, but make names optional
for all of them.
I like Doug's new syntax for unions. The earlier way to implicitly specifying
unions by a JSON array was not intuitive. If we make names optional and support
both old and new syntax for unions, the change will not break the old schemas.
But I suggest we withdraw support for the old syntax to keep the specification
clean.
> 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.
--
This message is automatically generated by JIRA.
-
You can reply to this email to add a comment to the issue online.
