Improve your web app backends connections

I've talked about this previously, but let me state it again before I introduce the tools.

One of the most common issue I witness in web applications out there is their inability to cope with backend issues.

For example, if you are using a MySQL database, a simple test to do to see how your web app behaves is to shutdown your MySQL cluster or server and play with the app.

Of course it'll be all broken. But how broken ? can I still browse pages that are not doing SQL Queries ?

What happens when you restart MySQL ? Is your web application back on its feet or do you have broken connectors ?

How a simple MySQL restart impacts your app ?

You get the idea: we should test these scenarios, before they happen in production -- because they will happen.

This topic is basically what I've been working on at Mozilla for the last month, and we built a tool to help us testing this: Vaurien

Setting up the proxy

Shutting down MySQL is a good test to do, but you should also ask yourself how your application behaves when MySQL works but is very slow.

Vaurien is a TCP proxy that can simulate these kind of behaviors.

To set it up, just run:

$ pip install vaurien

Since Vaurien uses Gevent, you will need libevent. Under Windows I preconise to use Gevent's binary distributions.

Now let's run a proxy in front of your MySQL server. Vaurien supports several protocols including MySQL.

To launch the proxy, open a shell and run:

$ vaurien --protocol mysql --proxy --backend --http

This line will open the 3307 socket on your local host and relay all calls to, supposing your MySQL server runs there.

The --http flag opens a web service to drive the proxy from the command line or from some APIs calls.

From there, change your application settings so it uses as the MySQL server, and double check that the proxy works as expected.


By default Vaurien runs its web service on port 8080. In case you already use that port, change it with --http-port.

Breaking things

The first test you can do is to fake a shut down. Open another shell and just call the vaurienctl command line:

$ vaurienctl set-behavior blackout

By default, vaurienctl knows the proxy can be reached on the local host on port 8080. If you ran it on another port, be sure to adapt the call with the --http-port option.

Once the call has been made, play with your application and see how it behaves :)

Make sure you put the proxy back to being a transparent proxy with:

$ vaurienctl set-behavior dummy

And see if you application is back to normal.

Another thing you can try is to make MySQL just hangs on any call. It's different from a black out because it just keeps the socket open and hang there. Usually, if your application does not handle timeouts correctly, it will hang too - and that's something to fix.

$ vaurienctl set-behavior hang

More breaking

With Vaurien, you can also simulare errors and delays.

The full list of behaviors is here:

It also support a good range of protocols: and the TCP one should work out of the box with any protocol we did not specifically implement.

Instead of using the vaurienctl command line (or even Curl), you can also use the API we provide.

For example you could write functional tests that launch the proxy and drive it:

import unittest
from vaurien import Client, start_proxy, stop_proxy

class MyTest(unittest.TestCase):

    def setUp(self):
        self.proxy_pid = start_proxy(port=8080)

    def tearDown(self):

    def test_one(self):
        client = Client()
        options = {'inject': True}

        with client.with_behavior('error', **options):
            # do something...

        # we're back to normal here

The real fun is to start writing your own protocols and behaviors, since Vaurien is based on a plugins system.


The code repository & bug tracker are located at Don’t hesitate to send us pull requests or open issues.