Elliot West created AVRO-2895:
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Summary: Use default to allow old readers to specify default union
value when encountering new union branches
Key: AVRO-2895
URL: https://issues.apache.org/jira/browse/AVRO-2895
Project: Apache Avro
Issue Type: Improvement
Components: spec
Reporter: Elliot West
The schema resolution page says:
{quote}*if both are unions:*
The first schema in the reader's union that matches the selected writer's
union schema is recursively resolved against it. if none match, an error is
signalled.
{quote}
This makes it difficult to use unions because you can never add a new type to a
union and keep old reader's compatible. Can we not follow the approach provided
by other Avro extensible types ({{record}}, {{enum}}) and use a default value
so that when an old reader encounters a union branch it does not recognise, 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.
Clearly, the provided default value must be valid with respect to one of the
types declared in the reader's union schema.
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