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https://issues.apache.org/jira/browse/AVRO-1340?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16427629#comment-16427629
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Doug Cutting commented on AVRO-1340:
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That's right, defaults are used by readers, not writers. In general, schema
resolution is done at read time to adapt written data to a reader's schema.
Defaults are part of schema resolution.
(Defaults may also be used when creating instances programmatically. For
example, GenericRecordBuilder#build() will set unset fields to their default.)
As for validating an enum's default, program-generated schemas might reasonably
have "default":null and I see no harm in treating that the same as no default
specified. It might be best to abide by Postel's law here.
> 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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