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From "Grant Overby (groverby)" <grove...@cisco.com>
Subject Re: Union resolution in dynamic languages
Date Thu, 05 Jun 2014 18:00:01 GMT
Sure, but that is kind of an unbounded question. Can you be more specific as to what you’re
looking for?

Here’s a shot at an answer:
Polymorphism is a weak spot for Avro; unions help get around that short coming. We have unions
which contain multiple record specifications. The reference that has a union datatype in the
schema could point to an instance of one of many classes at runtime with which class that
is being known only at runtime.


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Grant Overby
Software Engineer
Cisco.com
groverby@cisco.com<mailto:groverby@cisco.com>
Mobile: 865 724 4910






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From: Wai Yip Tung <wy@tungwaiyip.info<mailto:wy@tungwaiyip.info>>
Reply-To: "user@avro.apache.org<mailto:user@avro.apache.org>" <user@avro.apache.org<mailto:user@avro.apache.org>>
Date: Thursday, June 5, 2014 at 1:40 PM
To: "user@avro.apache.org<mailto:user@avro.apache.org>" <user@avro.apache.org<mailto:user@avro.apache.org>>
Subject: Re: Union resolution in dynamic languages

That's good to know. Would you mind sharing your use case with us?

Wai Yip

[cid:part1.00000904.07040903@tungwaiyip.info]
Grant Overby (groverby)<mailto:groverby@cisco.com>
Thursday, June 05, 2014 6:46 AM
Disallowing multiple named types within a union would break our use cases.

We have a similar problem. With two record types in a union, the Python driver doesn’t choose
well.

We solved this problem by adding a pseudo-reserved key to the dict to indicate which named
type to use. I started the process of open sourcing that patch a few days ago. It’s definitely
a hack, but I’m hoping the community will accept it.

Our patch doesn’t change the time complexity. From a brief glance , choosing within the
union seems to typically be O(n) as the recursion short circuits. For named types, the complexity
could be O(1). Achieving O(1) for non named types seems achievable too. How many projects
are impacted by this ‘wasted’ complexity? Simpler code might be better than faster code.

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Grant Overby
Software Engineer
Cisco.com
groverby@cisco.com<mailto:groverby@cisco.com>
Mobile: 865 724 4910






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From: Wai Yip Tung <wy@tungwaiyip.info<mailto:wy@tungwaiyip.info>>
Reply-To: "user@avro.apache.org<mailto:user@avro.apache.org>" <user@avro.apache.org<mailto:user@avro.apache.org>>
Date: Wednesday, June 4, 2014 at 9:34 PM
To: "user@avro.apache.org<mailto:user@avro.apache.org>" <user@avro.apache.org<mailto:user@avro.apache.org>>
Subject: Re: Union resolution in dynamic languages

Also I ask about this in the context of building an optimized encoder. For this implementation,
the resolution will be much simpler if we limit union to not support two records, similar
to the spec do not allow two array or two map types. I wonder if this limit breaks any significant
use case.

Wai Yip
[cid:part2.05020606.06070201@tungwaiyip.info]
Wai Yip Tung<mailto:wy@tungwaiyip.info>
Wednesday, June 04, 2014 6:34 PM
Also I ask about this in the context of building an optimized encoder. For this implementation,
the resolution will be much simpler if we limit union to not support two records, similar
to the spec do not allow two array or two map types. I wonder if this limit breaks any significant
use case.

Wai Yip
[cid:part2.05020606.06070201@tungwaiyip.info]
Wai Yip Tung<mailto:wy@tungwaiyip.info>
Wednesday, June 04, 2014 4:40 PM
For encoding data of union type, the Avro specification do not say a lot which one of the
type in the union is used. So far I am mostly using union so that I can write null or another
simple type. In these cases, it is fairly obvious for the encoding to distinguish null from
other types.

However a union can also be any named types. So they can be two records. Let say a Manger
record and a NonManager record. I think with strongly typed languages, the suitable type in
the union can be selected by introspection. But for dynamic languages, these might just be
a represented as maps without any notion of type. In some case, we may find that the object
has all the attributes of a NonManager but not the Manager. So we can conclude NonManager
is the proper schema to use. But this can get complicated with nested data structure where
the attribute that can disambiguate thing appear in a deeper level. Or you can think of valid
scenario where inspecting the content of the obj cannot unambiguously resolve the union branch.

I notice that the Python implementation use two pass recursive validation possible for the
reason of for resolving the union choice.

I am wonder if there are much consideration about are potentially complex, indirectly nested
union types that might be difficult to resolve? Thus adding complexity to the implementation
of the encoders? Are there use case in practice that involve complex union decision?

Wai Yip


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