Mozilla Services / Week 10/11

What's this ? read this post.

What happened

Well.. Firefox 4 is out and the number of downloads is impressive. Check this page: It's almost 7 millions right now !

Back to my Services dev topics :D

It's been a while since I've posted a bi-weekly status update, because I was busy traveling to Pycon, then to Vancouver to work with the messaging team on F1. If you've never heard of it, check Bryan's post. Then go there, install the Add-on and enjoy it.

The work there was quite interesting as we've started to think about how to scale F1 so it can be used by millions of users. On server-side --the part I am the most interested in-- they have built a Pylons application that acts like a oauth proxy, used by the Add-On to send tweets, e-mails etc to third-party services like Twitter, GMail or Facebook.

The choice of Pylons made sense back when the project started because there was a database that stored some user data, and a few panels that are displayed when you want to manage your settings. But that database has been removed from the server and the code has shrinked to a few static pages and an oauth lib used to communicate with the various services.

One thing we will do is to move to a lighter web application and just use WebOb and Routes. Pylons became overkill for that application, and has now joined the cemetery of deprecated frameworks (Pyramid is the new thing).

On the scaling part, we're still working hard to come up with the best design, and we're trying to keep this wiki page up-to-date with that work: Note that this is a work in progress, so nothing is settled in stone. The most interesting part is to decide if we do a synchronous architecture or asynchronous one. I personally think a synchronous architecture is simpler to start with, since we already have a working application that does what we want, and since we can always move to an async model later.

Another interesting part I am working on is how to do functional tests against our application, knowing that most APIs will call in turn a third-party server like Twitter or GMail. We need to mock those but in the meantime find a smart way to replay real sessions. I've worked on this topic in the past and blogged about it. I had created an initial version of a small mock tool but not really used it until this week.

Mark Hammond has been working on the topic as well and we need to synchronize our efforts next, but the general idea we've discussed and we'll use is to record a session that occurs with the real servers, dump it in a file, then allow the tests to reuse the recorded session instead of calling the real server again. This also needs to be done through a real TCP call to a third party server, and not mocked in Python, for a realistic behavior.

My first take on this is a simple proxy called recproxy which is a new version of my previous proxy. You can use it like this:
class TestSendController(unittest.TestCase):

    @recproxy('https', '', 'test_send.rec')

    def test_send(self):


This proxy does the following:
- A proxy server is launched on the localhost - Any call via urrlib2 which destination is is intercepted and sent to the local proxy - The local proxy reads the test_send.rec file that contains request/response pairs. If a request in that file corresponds to the request, the response is returned.

Of course, this means that the test_send.rec has to be created first. To do it, all you have to do is to run the tests with a PROXY environment variable set to 1. This will call the proxy server, but instead of returning values found in the file, it will proxy the requests to the real server and record the responses into the file.

Since we want to share the recorded files and obfuscate sensitive data, you can replace those data by XXX in the file. The proxy will match the incoming requests using a regular expression that will replace XXX by anything.

Here's an example of such a rec file:
POST /statuses/update.json HTTP/1.1

Accept-Encoding: identity

Connection: close

Content-Length: XXX

Content-Type: application/x-www-form-urlencoded

Host: localhost:65535

User-Agent: Python-urllib/2.6



200 OK

Content-Type: application/json; charset=UTF-8

Content-Length: 11

{"id": 123}


This is basically a mock of a successful status update. And since the proxy works as a test function decorator, you can create records for bad behaviors as well, when you are fixing a bug that happens when there's a unexpected answer from the third party server.

You can also mock several servers (like and twitter) by simply adding more decorators to the function.

The recproxy code is here. It's still an early version and will probably evolve a lot.

What's planned

In the next weeks we will continue to work on the architecture for F1, and also add more tests with Mark & Shane.

I am looking forward to synchronize my testing work with what Mark has done. Also, I'll probably make the proxy tool a lib, because that's what we want to use in other projects.