Welcome to Vaurien’s documentation!

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

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 requests done on google.com:

$ vaurien --local localhost:8000 --distant google.com:80 --behavior 20:delay

Vaurien will stream all the traffic to google.com but will add delays 20% of the time. You can pass options to the handler using –handlers.NAME.OPTION options:

$ vaurien --local localhost:8000 --distant google.com:80 --behavior 20:delay \
    --handlers.delay.sleep 2

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

distant = google.com:80
local = localhost:8000
behavior = 20:delay

sleep = 2

Each behavior applied on the request or response going through Vaurien is called a handler, and the ini file gets one section per handler.

You can find a descriptions of all built-in handlers here: Handlers.

Controlling Vaurien live

Sometimes, it is useful to control live the proxy, so you can change its behavior live between client calls.

Vaurien provides an HTTP server with a few APIs, which can be used to control the proxy.

To activate it, use the –http option:

$ vaurien --http

By default the server runs on port 8080 while the proxy runs on 8000

Once it runs, you can call it using cURL or any HTTP client. See the APIs.

Controlling Vaurien in 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 that uses a Vaurien proxy, you can write:

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_handler('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 handler, so you can verify that your application is behaving as expected when it happens.

Extending Vaurien

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

In fact that’s the best way to create realistic issues. Imagine that 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 handler and make sure your web application behaves as it should.

Creating new handlers is done by implementing a class with a specific signature.

You can inherit from the base class Vaurien provides and just implement the __call__ method:

from vaurien.handlers import BaseHandler

class MySuperHandler(BaseHandler):

    name = 'super'
    options = {}

    def __call__(self, client_sock, backend_sock, to_backend):
        # do something here

More about this in Writing Handlers.

Code repository

If you’re interested to look at the code, it’s there: https://github.com/mozilla-services/vaurien

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

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