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From Aaron Kimball <akimbal...@gmail.com>
Subject static schema validation
Date Wed, 30 Jan 2013 23:17:52 GMT
Does Avro have an API to allow you to tell whether two schemas are a match,
statically?

i.e., schema1.canRead(schema2) /** return true iff schema1 can be used as a
reader schema for schema2 */

>From my (admittedly cursorary) scan of the docs + source, it seems like
there isn't something quite that concise, though maybe this can be
accomplished using ResolvingGrammarGenerator?

I'm pessimistic because of the following quote from the spec [1]

*[matching] 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.

That sentence makes me think it's context dependent; I interpret "the
selected writer's union schema" as "the schema of the actual thing written
in a data buffer, which is one of the possible schemas the writer declared
in her union type". i.e., you can only tell if schema R can be a reader for
some other schema W in terms of a literal record written by W, and cannot
be deduced statically for all possible records that can be encoded with
schema W.  Is this interpretation correct? If so, does anyone have any
ideas how to ensure the best bounds on statically-guaranteed backward
compatibility between a given reader and writer?

Thanks,
- Aaron

[1] http://avro.apache.org/docs/current/spec.html#Schema+Resolution

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