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From: Aaron Kimball <akimball83@gmail.com>
Date: Wed, 30 Jan 2013 15:17:52 -0800
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 <CA+0G+5oKnoVbN6un5WboV3ENB_6POkAp2SatdVS9j2SgiGpfZg@mail.gmail.com>
Subject: static schema validation
To: user@avro.apache.org
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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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<div dir=3D"ltr">Does Avro have an API to allow you to tell whether two sch=
emas are a match, statically?<div><br></div><div style>i.e., schema1.canRea=
d(schema2) /** return true iff schema1 can be used as a reader schema for s=
chema2 */</div>

<div style><br></div><div style>From my (admittedly cursorary) scan of the =
docs + source, it seems like there isn&#39;t something quite that concise, =
though maybe this can be accomplished using ResolvingGrammarGenerator?<br>

</div><div style><br></div><div style>I&#39;m pessimistic because of the fo=
llowing quote from the spec [1]</div><div style><br></div><div style><div><=
b>[matching] if both are unions:</b></div><div>The first schema in the read=
er&#39;s union that matches the selected writer&#39;s union schema is recur=
sively resolved against it. if none match, an error is signalled.</div>

<div><br></div><div style>That sentence makes me think it&#39;s context dep=
endent; I interpret &quot;the selected writer&#39;s union schema&quot; as &=
quot;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&quot;. i.e., =
you can only tell if schema R can be a reader for some other schema W in te=
rms of a literal record written by W, and cannot be deduced statically for =
all possible records that can be encoded with schema W. =A0Is this interpre=
tation correct? If so, does anyone have any ideas how to ensure the best bo=
unds on statically-guaranteed backward compatibility between a given reader=
 and writer?</div>

<div><br></div></div><div style>Thanks,<br></div><div style>- Aaron</div><d=
iv style><br></div><div style>[1]=A0<a href=3D"http://avro.apache.org/docs/=
current/spec.html#Schema+Resolution">http://avro.apache.org/docs/current/sp=
ec.html#Schema+Resolution</a></div>

</div>

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