Saturday, January 22, 2011

JRuby and Hadoop, Notes From a Non-Java Programmer

So I spent a fair deal of time playing with JRuby this weekend. Here's my notes/conclusions so far. Disclaimer: My experience with java overall is limited, so most of this might be obvious to a java ninja but maybe not to a ruby one...


The whole point here is that it sure would be nice to only write a few lines of ruby code into a file called something like 'rb_mapreduce.rb' and run it by saying: ./rb_mapreduce.rb. No compiling, no awkward syntax. Pure ruby, plain and simple.


I created a 'WukongPlusMapper' that subclassed 'org.apache.hadoop.mapreduce.Mapper' and implemented the important methods, namely 'map'. Then I setup and launched a job from inside jruby using this jruby mapper ('WukongPlusMapper') as the map class.

The job launched and ran just fine. But...

Problems and Lessons Learned:

  • It is possible (in fact extremely easy) to setup and launch a Hadoop job with pure jruby

  • It is not possible, that I can tell so far, to use an uncompiled jruby class as either the mapper or the reducer for a Hadoop job. It doesn't throw an error (so long as you've subclassed a proper java mapper) but actually just uses the superclass's definition instead. I believe the reason is that each map task must have access to the full class definition for its mapper (only sensible) and has no idea what to do with my transient 'WukongPlusMapper' class. Obviously the same would apply to the reducer

  • It is possible to compile a jruby class ahead-of-time, stuff it into the job jar, and then launch the job with ordinary means. There are a couple somewhat obvious drawbacks with this method:

    • You've got to specify 'java_signatures' for each of your methods that are going to be called inside java

    • Messy logic+code for compiling thyself, stuffing thyself into a jar, shipping thyself with MR job. Might as well just write java at this point. radoop has some logic for doing that pretty well laid out.

  • It is possible to define and create an object in jruby that subclasses a java class or implements a java interface. Then you can simply overwrite the methods you want to overwrite. It's possible to pass instances of this class to a java runtime that only knows about the superclass and the subclass's methods (at least the ones that have the signatures defined in the superclass) will work just fine. Unfortunately, (and plainly obvious in hindsight) this does NOT work with Hadoop since these instances all show up in java as 'proxy classes' and are only accessible to the launching jvm

  • On another note there is the option of using the scripting engine which, as far as I can tell, is what both jruby-on-hadoop and radoop are using. Something of concern though is that neither of these two projects seem to have much traction. However, it may be that the scripting engine is the only way to reasonably make this work, at least 2 people vote yes ...

So, implementation complexity aside, it looks like all one would have to do is come up with some way of making JRuby's in-memory class definitions available to all of the spawned mappers and reducers. Probably not something I want to delve into at the moment.

Script engine it is.


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