Vaurien, the Chaos TCP Proxy

Ever heard of the Chaos Monkey?


It’s a project at Netflix to enhance the infrastructure tolerance. The Chaos Monkey will randomly shut down some servers or block some network connections, and the system is supposed to survive to these events. It’s a way to verify the high availability and tolerance of the system.

Besides a redundant infrastructure, if you think about reliability at the level of your web applications there are many questions that often remain unanswered:

  • What happens if the MYSQL server is restarted? Are your connectors able to survive this event and continue to work properly afterwards?
  • Is your web application still working in degraded mode when Membase is down?
  • Are you sending back the right 503s when postgresql times out ?

Of course you can – and should – try out all these scenarios on stage while your application is getting a realistic load.

But testing these scenarios while you are building your code is also a good practice, and having automated functional tests for this is preferable.

That’s where Vaurien is useful.

Vaurien is basically a Chaos Monkey for your TCP connections. Vaurien acts as a proxy between your application and any backend.

You can use it in your functional tests or even on a real deployment through the command-line.

Installing Vaurien

You can install Vaurien directly from PyPI. The best way to do so is via pip:

$ pip install vaurien


Vaurien is a TCP proxy that simply reads data sent to it and pass it to a backend, and vice-versa.

It has built-in protocols: TCP, HTTP, Redis, SMTP, MySQL & Memcache. The TCP protocol is the default one and just sucks data on both sides and pass it along.

Having higher-level protocols is mandatory in some cases, when Vaurien needs to read a specific amount of data in the sockets, or when you need to be aware of the kind of response you’re waiting for, and so on.

Vaurien also has behaviors. A behavior is a class that’s going to be invoked everytime Vaurien proxies a request. That’s how you can impact the behavior of the proxy. For instance, adding a delay or degrading the response can be implemented in a behavior.

Both protocols and behaviors are plugins, allowing you to extend Vaurien by adding new ones.

Last (but not least), Vaurien provides a couple of APIs you can use to change the behavior of the proxy live. That’s handy when you are doing functional tests against your server: you can for instance start to add big delays and see how your web application reacts.

Using Vaurien from the command-line

Vaurien is a command-line tool.

Let’s say you want to add a delay for 20% of the HTTP requests made on

$ vaurien --protocol http --proxy localhost:8000 --backend \
        --behavior 20:delay

With this set up, Vaurien will stream all the traffic to by using the http protocol, and will add delays 20% of the time.

You can find a description of all built-in protocols here: Protocols.

You can pass options to the behavior using –behavior-NAME-OPTION options:

$ vaurien --protocol http --proxy localhost:8000 --backend \
    --behavior 20:delay \
    --behavior-delay-sleep 2

Passing all options through the command-line can be tedious, so you can also create an ini file for this:

backend =
proxy = localhost:8000
protocol = http
behavior = 20:delay

sleep = 2

You can find a description of all built-in behaviors here: Behaviors.

You can also find some usage examples here: Examples.

Controlling Vaurien live

Vaurien provides an HTTP server with an API, which can be used to control the proxy and change its behavior on the fly.

To activate it, use the –http option:

$ vaurien --http

By default the server runs on locahost:8080 but you can change it with the –http-host and –http-port options.

See APIs for a full list of APIs.

Controlling Vaurien from your code

If you want to run and drive a Vaurien proxy from your code, the project provides a few helpers for this.

For example, if you want to write a test your backend service that runs on host:port using Vaurien proxy, you can write:

import unittest
from vaurien.util import start_proxy
from vaurien.util import stop_proxy
from vaurienclient import Client

class MyTest(unittest.TestCase):

    def setUp(self):
        # by default the HTTP service used for controlling vaurien
        # runs on localhost:8080, can be made to run on a different
        # host and port by using `http_host` and `http_port` as
        # argument to start_proxy.
        # by default the proxy is bound to localhost:8000, can be bound
        # to on a different host and port by using `proxy_host` and
        # `proxy_port` as argument to start_proxy.

        self.proxy_pid = start_proxy(
            backend_host=host, # host where your backend service runs
            backend_port=port, # port where your backend service runs
            protocol='http' # :ref:`protocols`

    def tearDown(self):

    def test_one(self):
        # client that connects to the HTTP server which controls vaurien
        client = Client(host='localhost', port=8080)

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

        # we're back to normal here

In this test, the proxy is started and stopped before and after the test, and the Client class will let you drive its behavior.

Within the with block, the proxy will error out any call by using the errors behavior, so you can verify that your application is behaving as expected when it happens.

Extending Vaurien

Vaurien comes with a handful of useful Behaviors and Protocols, but you can create your own ones and plug them in a configuration file.

In fact, that’s the best way to create realistic issues: imagine you have a very specific type of error on your LDAP server everytime your infrastructure is under heavy load. You can reproduce this issue in your behavior and make sure your web application behaves as it should.

Creating new behaviors and protocols is done by implementing classes with specific signatures.

For example if you want to create a “super” behavior, you just have to write a class with two special methods: on_before_handle and on_after_handle.

Once the class is ready, you can register it with Behavior.register:

from vaurien.behaviors import Behavior

class MySuperBehavior(object):

    name = 'super'
    options = {}

    def on_before_handle(self, protocol, source, dest, to_backend):
        # do something here
        return True

     def on_after_handle(self, protocol, source, dest, to_backend):
        # do something else
        return True


You will find a full tutorial in Extending Vaurien.


The code repository & bug tracker are located at

Don’t hesitate to send us pull requests or open issues!