Skip to main content

Clustering: Hadoop

Google wrote a white paper called MapReduce: Simplified Data Processing on Large Clusters. It's a simple way to write software that works on a cluster of computers. Google also wrote a white paper on The Google File System.
Hadoop is a framework for running applications on large clusters of commodity hardware. The Hadoop framework transparently provides applications both reliability and data motion. Hadoop implements a computational paradigm named map/reduce, where the application is divided into many small fragments of work, each of which may be executed or reexecuted on any node in the cluster. In addition, it provides a distributed file system that stores data on the compute nodes, providing very high aggregate bandwidth across the cluster. Both map/reduce and the distributed file system are designed so that node failures are automatically handled by the framework.
Put simply, Hadoop is an open-source implementation of Google's map/reduce and distributed file system written in Java.

I needed something like that, so I decided to give it a whirl. I prefer to code in Python, so it's fortunate that Hadoop can "shell out" to Python on each of the remote systems. Shelling out once per system has negligible overhead, so that's fine.

You'll need to read the whitepaper to fully understand map/reduce, but let's look at some code. First, let's look at my input. It's a file:
Now, here's my mapper:
#!/usr/bin/env python

"""Figure out whether each number is even or odd."""

import sys

for line in sys.stdin:
num, _ignored = line[:-1].split("\t")
is_odd = int(num) % 2
print "%s\t%s" % (is_odd, num)
Here's my reducer:
#!/usr/bin/env python

"""Count and sum the even and odd numbers."""

import sys

counts = {0: 0, 1: 0}
sums = counts.copy()
for line in sys.stdin:
is_odd, num = map(int, line[:-1].split("\t"))
counts[is_odd] += 1
sums[is_odd] += num
for i in range(2):
name = {0: "even", 1: "odd"}[i]
print "%s\tcount:%s sum:%s" % (name, counts[i], sums[i])
This resulted in a single file:
even count:500 sum:249500
odd count:500 sum:250000
Once Hadoop is installed, executing this job is done at the shell via:
hadoop jar /usr/local/hadoop-install/hadoop/build/hadoop-streaming.jar \
-mapper -reducer -input input.txt -output out-dir
This was the first time I had ever written software for a cluster, and all in all, it was pretty easy. Too bad I didn't actually have a couple thousand machines to run this on ;)

(By the way, during installation, I ran into a couple issues which I was able to work around easily. I won't bother repeating them here. You can find my workarounds on the mailing list. You may need to wait for the archive to be updated since I just posted them earlier today.)


jjinux said…
I was pleased overall with Hadoop. My biggest comment / complaint was that it's built for massive data crunching, whereas I need something for lightning quick responses. They're really different use cases. For instance, I think I need to have the available slave instances already connected on the other end of a TCP/IP socket, with the code and data already loaded and ready to go. Hadoop makes more sense as a backend for a spider--which is what it was designed for ;)
Doug Cutting said…
Too bad I didn't actually have a couple thousand machines to run this on ;)

You can rent a cluster by the hour from Amazon.

Popular posts from this blog

Drawing Sierpinski's Triangle in Minecraft Using Python

In his keynote at PyCon, Eben Upton, the Executive Director of the Rasberry Pi Foundation, mentioned that not only has Minecraft been ported to the Rasberry Pi, but you can even control it with Python. Since four of my kids are avid Minecraft fans, I figured this might be a good time to teach them to program using Python. So I started yesterday with the goal of programming something cool for Minecraft and then showing it off at the San Francisco Python Meetup in the evening.

The first problem that I faced was that I didn't have a Rasberry Pi. You can't hack Minecraft by just installing the Minecraft client. Speaking of which, I didn't have the Minecraft client installed either ;) My kids always play it on their Nexus 7s. I found an open source Minecraft server called Bukkit that "provides the means to extend the popular Minecraft multiplayer server." Then I found a plugin called RaspberryJuice that implements a subset of the Minecraft Pi modding API for Bukkit s…

Apple: iPad and Emacs

Someone asked my boss's buddy Art Medlar if he was going to buy an iPad. He said, "I figure as soon as it runs Emacs, that will be the sign to buy." I think he was just trying to be funny, but his statement is actually fairly profound.

It's well known that submitting iPhone and iPad applications for sale on Apple's store is a huge pain--even if they're free and open source. Apple is acting as a gatekeeper for what is and isn't allowed on your device. I heard that Apple would never allow a scripting language to be installed on your iPad because it would allow end users to run code that they hadn't verified. (I don't have a reference for this, but if you do, please post it below.) Emacs is mostly written in Emacs Lisp. Per Apple's policy, I don't think it'll ever be possible to run Emacs on the iPad.

Emacs was written by Richard Stallman, and it practically defines the Free Software movement (in a manner of speaking at least). Stal…

ERNOS: Erlang Networked Operating System

I've been reading Dreaming in Code lately, and I really like it. If you're not a dreamer, you may safely skip the rest of this post ;)

In Chapter 10, "Engineers and Artists", Alan Kay, John Backus, and Jaron Lanier really got me thinking. I've also been thinking a lot about Minix 3, Erlang, and the original Lisp machine. The ideas are beginning to synthesize into something cohesive--more than just the sum of their parts.

Now, I'm sure that many of these ideas have already been envisioned within, LLVM, Microsoft's Singularity project, or in some other place that I haven't managed to discover or fully read, but I'm going to blog them anyway.

Rather than wax philosophical, let me just dump out some ideas:Start with Minix 3. It's a new microkernel, and it's meant for real use, unlike the original Minix. "This new OS is extremely small, with the part that runs in kernel mode under 4000 lines of executable code." I bet it&…