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From "Segel, Mike" <mse...@navteq.com>
Subject RE: Custom Class Loader for Hadoop M/R jobs?
Date Wed, 14 Apr 2010 17:33:29 GMT
Scott,

While that may work for a quick fix. Its not a good long term solution and you then run in
to a problem where you upgrade your hadoop release and the removed jar is replaced or if you
replace the jar, it possible to get overwritten.

In this specific instance, the Jackson libraries are not that important and they can be replaced.
But that doesn't mean that this issue won't come up again and its something you can't easily
pop out and replace.

This is why I'm looking at custom class loading and trying to understand what can be accomplished
with the methods in the Configuration class. 

Thx

-Mike


-----Original Message-----
From: Scott Carey [mailto:scott@richrelevance.com] 
Sent: Wednesday, April 14, 2010 12:02 PM
To: general@hadoop.apache.org
Subject: Re: Custom Class Loader for Hadoop M/R jobs?

Depending on what the dependency is, you might be able to just remove it from hadoop's lib
directory on your cluster.

For me, Hadoop's later versions has jackson-1.0.1 in its lib directory and that breaks usage
of Avro in a M/R job among other things.  However, the feature that uses this library is unimportant
to me (configuration dump in JSON format) so I just removed the jar.

-Scott
 
On Apr 14, 2010, at 6:39 AM, Segel, Mike wrote:

> Hi,
> 
> Ok, here's a bit of a bizarre  issue...
> 
> How do you handle class collisions between Hadoop and your m/r job which calls other
3rd party classes.
> 
> An example: Hadoop has an older version of an open source jar in its /lib directory.
You're interfacing with a 3rd party OS tool that uses a later release of the same jar.
> 
> You can modify the classpath, and that might work. But the better way is to create a
Custom Class Loader. (Non-trivial)
> 
> Looking at the Configuration class, it looks like there are a couple of methods that
deal with loading a class in to the configuration so that the m/r jobs can have access to
them on each node.
> 
> Is this the correct intended use, or am I missing something?
> Has anyone done something like this?
> 
> Thx
> 
> -Mike
> 
> Michael Segel
> Architect,  R&D
> NAVTEQ
> 425 West Randolph Street
> Chicago, IL 60606
> (T)  +1 312-780-3432
> (C)  +1 312-952-8175
> www.navteq.com<http://www.navteq.com/>
> 
> 
> 
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The information contained in this communication may be CONFIDENTIAL and is intended only for
the use of the recipient(s) named above.  If you are not the intended recipient, you are hereby
notified that any dissemination, distribution, or copying of this communication, or any of
its contents, is strictly prohibited.  If you have received this communication in error, please
notify the sender and delete/destroy the original message and any copy of it from your computer
or paper files.

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