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From "Zoltan Farkas (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (AVRO-1340) use default to allow old readers to specify default enum value when encountering new enum symbols
Date Thu, 13 Oct 2016 10:47:20 GMT

    [ https://issues.apache.org/jira/browse/AVRO-1340?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15571571#comment-15571571
] 

Zoltan Farkas commented on AVRO-1340:
-------------------------------------

Agree, fallback and alias are different concepts. Alias implies that it is the same thing
with a different name.

The more I think, I think using the default value declared for the field is the better way.

for example in v1:

{code}
enum Suit {
  UNKNOWN,
  CLUBS,
  HEARTS,
}
...
Suit field = UNKNOWN;
...
{code}

And v2:

{code}
enum Suit {
  UNKNOWN,
  CLUBS,
  HEARTS,
  SPADES,
  DIAMONDS
}
...
Suit field = UNKNOWN;
...
{code}

Originally I though using something specific for the fallback, but thinking through all the
use cases you always end up having with an enum with a "UNKNOWN" value which is always the
right default value...

Aliases should be used for the same purpose as field aliases. (improving names...)



> use default to allow old readers to specify default enum value when encountering new
enum symbols
> -------------------------------------------------------------------------------------------------
>
>                 Key: AVRO-1340
>                 URL: https://issues.apache.org/jira/browse/AVRO-1340
>             Project: Avro
>          Issue Type: Improvement
>          Components: spec
>         Environment: N/A
>            Reporter: Jim Donofrio
>            Priority: Minor
>
> The schema resolution page says:
> > if both are enums:
> > if the writer's symbol is not present in the reader's enum, then an
> error is signalled.
> This makes it difficult to use enum's because you can never add a enum value and keep
old reader's compatible. Why not use the default option to refer to one of enum values so
that when a old reader encounters a enum ordinal it does not recognize, it can default to
the optional schema provided one. If the old schema does not provide a default then the older
reader can continue to fail as it does today.



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