Load testing with Tsung

One of the big issues with building scalable software is making tests scale along with the application. A high performance web application should be tested under heavy loads, preferably to the breaking point. Of course, now you need a second application that can generate lots of traffic. You could use something simple like httperf; however, this doesn't work so well with complex systems, since you're only hitting one URL at a time.

Enter Tsung. Tsung is a load testing tool written in Erlang (everybody's favorite scalable language) that can not only generate large amounts of traffic, but it can parametrize requests based on data returned by your web application or with data pulled from external files. It also can generate very nice HTML reports using gnuplot.

Here's how we're running Tsung on Ubuntu in EC2:

1. Start a new instance. We're using an Ubuntu Hardy instance build by Alestic.


2. Download, configure, compile, and install Erlang.


3. Get and install the Tsung dependencies: gnuplot and perl5.
   
4. Download, configure, compile, and install Tsung.


5. Install your favorite web server. I prefer Apache HTTPD...others in this office perfer nginx. If you want to be really Erlang-y, install Yaws or Mochiweb.


6. Configure your ~/.tsung/tsung.xml configuration file for your test. The Tsung user manual has pretty good documentation about how to do this. Note that you do NOT want to use vm-transport for heavy loads, as this prevents Erlang from spawning additional virtual machines, which limits the number of requests you can use at a time. This does require you to set up passwordless ssh access to localhost.


7. Point your web server at "~/.tsung/log/". Each test you run will log the results in a subdirectory of this location.


8. Start your test with the "tsung start" command.


9. Set the report-generating script /usr/lib/tsung/bin/tsung_stats.pl to run in the appropriate log directory every 10 seconds. You can do this via crons or simply having a "watch" command running in the background.

Now, you can just browse over to your machine to view the latest test report. Tsung exposes all of the statistics you would expect (req/sec, throughput, latency, etc) both in numerical and graphical form. All of the graphs can be downloaded as high quality postscript graphs, too.

If you want to generate truly large amounts of traffic, Tsung supports distributed testing environments (as you might expect from an Erlang testing tool). Just make sure that you have passwordless SSH set up between your test machines and configure the client list in your tsung.xml file appropriately.