Dmitriy V. Ryaboy commented on PIG-924:

Owen -- I may not have made the intent clear; the idea is that when Pig is 
rewritten to use the future-proofed APIs, the shims will go away (presumably 
for 0.5).   Right now, pig is not using the new APIs, even the 20 patch posted 
by Olga uses the deprecated mapred calls. 

This is only to make life easier in the transitional period while Pig is using 
the old, mutating APIs.

Check out the pig user list archives for motivation of why these shims are 

> Make Pig work with multiple versions of Hadoop
> ----------------------------------------------
>                 Key: PIG-924
>                 URL: https://issues.apache.org/jira/browse/PIG-924
>             Project: Pig
>          Issue Type: Bug
>            Reporter: Dmitriy V. Ryaboy
>         Attachments: pig_924.2.patch, pig_924.3.patch, pig_924.patch
> The current Pig build scripts package hadoop and other dependencies into the 
> pig.jar file.
> This means that if users upgrade Hadoop, they also need to upgrade Pig.
> Pig has relatively few dependencies on Hadoop interfaces that changed between 
> 18, 19, and 20.  It is possibly to write a dynamic shim that allows Pig to 
> use the correct calls for any of the above versions of Hadoop. Unfortunately, 
> the building process precludes us from the ability to do this at runtime, and 
> forces an unnecessary Pig rebuild even if dynamic shims are created.

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