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From Echo <echo...@gmail.com>
Subject RE: How to deserialize avro file with union/many schemas?
Date Thu, 24 Jul 2014 14:23:00 GMT
Hi Sachin 

I didn't write the code to create the schema, I just need to use the svro file. The avro library
can't read the file with that 'union' schema, so I wonder:

- what's the right way to define the union schema so avro lib can deserislize?

- if I have to read the avro file schema defined like described in my last email, I guess
I have to write code to parse it, any idea how it can be done?


-----Original Message-----
From: "Sachin Goyal" <sgoyal@walmartlabs.com>
Sent: ‎7/‎23/‎2014 8:43 PM
To: "user@avro.apache.org" <user@avro.apache.org>
Subject: Re: How to deserialize avro file with union/many schemas?

Hi Echo,

Can you share the code that you used to create the below schema?
How are you appending the schemas into one object?
And how is the data being appended to the same object?

Wouldn’t it be simpler to segregate the objects for different schemas such that
one group of objects contains only one schema and its related data objects?


From: Echo Li <echolql@gmail.com<mailto:echolql@gmail.com>>
Reply-To: "user@avro.apache.org<mailto:user@avro.apache.org>" <user@avro.apache.org<mailto:user@avro.apache.org>>
Date: Wednesday, July 23, 2014 at 7:50 PM
To: "user@avro.apache.org<mailto:user@avro.apache.org>" <user@avro.apache.org<mailto:user@avro.apache.org>>
Subject: Re: How to deserialize avro file with union/many schemas?

thanks Sachin,

My schema more like:
[ { schema-one with type="record"}{schema-two with type="record"}...]

and followed by datums and each pertaining to one of the schemas, and each schema will map
to one class.

On Wed, Jul 23, 2014 at 3:42 PM, Sachin Goyal <sgoyal@walmartlabs.com<mailto:sgoyal@walmartlabs.com>>

To see a union schema, do the following:
System.out.println (ReflectData.AllowNull.get().getSchema(YourClass.class));

And then do the following:
System.out.println (ReflectData.get().getSchema(YourClass.class));

Diff the two outputs.
First one generates a UNION of each and every field with a null.

Hope that helps.

From: Echo Li <echolql@gmail.com<mailto:echolql@gmail.com><mailto:echolql@gmail.com<mailto:echolql@gmail.com>>>
Reply-To: "user@avro.apache.org<mailto:user@avro.apache.org><mailto:user@avro.apache.org<mailto:user@avro.apache.org>>"
Date: Wednesday, July 23, 2014 at 3:09 PM
To: "user@avro.apache.org<mailto:user@avro.apache.org><mailto:user@avro.apache.org<mailto:user@avro.apache.org>>"
Subject: Re: How to deserialize avro file with union/many schemas?

Hi Mike,

I read through most of the doc on avro site, don't see anything about the "union schema",
Mike, would you mind give me some example here how the union schma is defined? also what package/method
can retrieve the master schema from avro file? is that "getschema()" should work? and how
to read in each Avro datums whithout knowing their corresponding schema?....

very much appreciate your help!

On Tue, Jul 22, 2014 at 10:25 PM, Michael Pigott <mpigott.subscriptions@gmail.com<mailto:mpigott.subscriptions@gmail.com><mailto:mpigott.subscriptions@gmail.com<mailto:mpigott.subscriptions@gmail.com>>>

It's just a regular Union :-) http://avro.apache.org/docs/1.7.6/spec.html#Unions


On Jul 23, 2014 1:22 AM, "Echo" <echolql@gmail.com<mailto:echolql@gmail.com><mailto:echolql@gmail.com<mailto:echolql@gmail.com>>>
Thanks Mike, it sounds make sense, is there any doc I can read about union schema?

On Jul 22, 2014, at 2:32 PM, Michael Pigott <mpigott.subscriptions@gmail.com<mailto:mpigott.subscriptions@gmail.com><mailto:mpigott.subscriptions@gmail.com<mailto:mpigott.subscriptions@gmail.com>>>

    Just to make sure I understand you correctly - do you have a file with multiple Avro datums
in it, each one following a separate schema?  And are all of these schemas unioned together
in a file-level "master schema?"  (As far as I know, Avro file readers and writers only support
one schema per file, so this is the only way your question makes sense to me.)
    If that's the case, then you can get the file's "master schema" and determine what all
of the different types are:

List<Schema> allTypes = masterSchema.getTypes(); // Assumes masterSchema is of Type.UNION

Then when you read each Avro datum in the file, you can check which of the schemas it conforms
to, and write a new file with just that sub-schema and the one datum in it.

Does that make sense?

On Tue, Jul 22, 2014 at 3:22 PM, Lewis John Mcgibbney <lewis.mcgibbney@gmail.com<mailto:lewis.mcgibbney@gmail.com><mailto:lewis.mcgibbney@gmail.com<mailto:lewis.mcgibbney@gmail.com>>>
For the purpose of others on this list, can ytou please provide an example of your schema?

On Tue, Jul 22, 2014 at 12:06 PM, Echo Li <echolql@gmail.com<mailto:echolql@gmail.com><mailto:echolql@gmail.com<mailto:echolql@gmail.com>>>

I'm new here, hope I can get help from you guys. Basically I have an avro file with union/many
schemas and mixed records. I will need to split it to many avro file, one schema per file.
All the stuff I've been reading is about serializing and deserializing avro file with one
schema, which is pretty straightforward, but in my case I have no clue, any ideas?


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