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https://issues.apache.org/jira/browse/AVRO-1340?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16421841#comment-16421841
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Bridger Howell commented on AVRO-1340:
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# The current PR looks like it has the parser treat anything that is not a
textual value as if nothing were specified.
So an enum schema with {{"default": null}} or {{"default": 2}} would be
successfully parsed, but would have no effect.
Otherwise, if the default is a string, it is only successfully parsed if it is
contained in the enum symbols.
Given that only string values make sense for an enum fallback, I could
definitely see an argument for raising a parse error in those cases.
# That seems correct according my understanding of schema resolution and
reading how {{SchemaCompatibility}} and {{ResolvingGrammarGenerator}} are
implemented. The idea that only the reader can add support for new writers
makes sense to me, and it __ should be preserved with the addition of enum
defaults.
(I'm not completely confident on that answer, however. Any correction or
confirmation from somebody who has a more comprehensive understanding of schema
resolution would be helpful.)
> 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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