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From Doug Cutting <cutt...@gmail.com>
Subject Re: Avro union compatibility mode enhancement proposal
Date Thu, 21 Apr 2016 17:52:12 GMT
On Wed, Apr 20, 2016 at 9:09 PM, Ryan Blue <rblue@netflix.com.invalid> wrote:
> Making the default a property of an
> inner schema makes me think that we will have to deal with multiple schemas
> with such a label at some point.

On Thu, Apr 21, 2016 at 6:54 AM, Matthieu Monsch <monsch@alum.mit.edu> wrote:
> Delegating default selection to the branches themselves is a great idea but it
> will be tricky to handle reference branches smoothly. More minor but it also
> doesn’t feel intuitive to not have the union “own” its default attribute.

If I understand your concerns correctly, I attempted to address this above:

"Note however that, when using a record as the default branch, one
could not then
use that same record as a non-default branch in another union.  To
ameliorate that, we might permit multiple default branches in a union
to be specified as default with the convention that the first such is
used."

Does that make sense?

This isn't ideal syntax, but it's not terrible, and it doesn't change
schema syntax incompatibly, which seems important, especially when its
unlikely that all implementations would implement such a syntax change
in a synchronized manner.

Alternately, one might annotate each derived record with the name of
its base record, then one wouldn't need to alter union definitions.
This would work like an alias.  If a record doesn't exist in the
reader's schema, then an alias to the missing record would be added in
the reader's schema to the base record it names in the writer's
schema.  Aliases work by rewriting the writer's schema at read-time,
updating names, including those in unions.  Might that work?  It seems
like perhaps a more elegant approach.  It has compatible syntax and
only alters behavior of a case that fails today.

Doug

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