It's useful to take a step back and abstract what A/B testing is trying to do before diving into the code. What exactly will we need to conduct a test?
- A Goal that has a Condition
- At least two distinct Paths to meet the Goal's Condition
- A system for sending viewers down one of the Paths
- A system for recording the Results of the test
With this in mind let's think about implementation.
The Goal
When we think about a Goal on the web usually we mean that a user reaches a certain page or that they complete a specific action, for example successfully registering as a user or getting to the checkout page.
In Django we could model that in a couple of ways - perhaps naively inside a view, calling a function whenever a Goal has been reached:
def checkout(request):
a_b_goal_complete(request)
...
But that doesn't help because we'll have to add that code everywhere we need it - plus if we're using any pluggable apps we'd prefer not to edit their code to add our A/B test.
How can we introduce A/B Goals without directly editing view code? What about a Middleware?
class ABMiddleware:
def process_request(self, request):
if a_b_goal_conditions_met(request):
a_b_goal_complete(request)
That would allow us to track A/B Goals anywhere on the site.
How do we know that a Goal's conditions has been met? For ease of implementation I'll suggest that we know a Goal has had it's conditions met when a user reaches a specific URL path. As a bonus we can measure this without getting our hands dirty inside a view. To go back to our example of registering a user we could say that this goal has been met when the user reaches the URL path:
/registration/complete
So we define a_b_goal_conditions_met:
a_b_goal_conditions_met(request):
return request.path == "/registration/complete":
Paths
When thinking about Paths in Django it's natural to jump to the idea of using different templates. Whether there is another way remains to be explored. In A/B testing you make small differences between two pages and measure the results. Therefore it should be a best practice to define a single base Path template from which all Paths to the Goal should extend.
How should render these templates? A decorator is probably a good start - it's a best practice in Django to include a parameter template_name to your views a decorator could alter this parameter at runtime.
@a_b
def registration(request, extra_context=None, template_name="reg/reg.html"):
...
You could see this decorator either introspecting the wrapped function and modifying the template_name argument or looking up the correct templates from somewhere (like a Model). If we didn't want to add the decorator to every function we could implement this as part of our ABMiddleware:
class ABMiddleware:
...
def process_view(self, request, view_func, view_args, view_kwargs):
if should_do_a_b_test(...) and "template_name" in view_kwargs:
# Modify the template name to one of our Path templates
view_kwargs["template_name"] = get_a_b_path_for_view(view_func)
response = view_func(view_args, view_kwargs)
return response
We'd need also need to add some way to keep track of which views have A/B tests running etc.
A system for sending viewers down a Path
In theory this is easy but there are lot of different implementations so it's not clear which one is best. We know a good system should divide users evenly down the path - Some hash method must be used - Maybe you could use the modulus of memcache counter divided by the number of Paths - maybe there is a better way.
A system for recording the Results of the Test
We need to record how many users went down what Path - we'll also need access to this information when the user reaches the goal (we need to be able to say what Path they came down to met the Condition of the Goal) - we'll use some kind of Model(s) to record the data and either Django Sessions or Cookies to persist the Path information until the user meets the Goal condition.
Closing Thoughts
I've given a lot of pseudo code for implementing A/B testing in Django - the above is by no means a complete solution but a good start towards creating a reusable framework for A/B testing in Django.
For reference you may want to look at Paul Mar's Seven Minute A/Bs on GitHub - it's the ROR version of the above!
http://github.com/paulmars/seven_minute_abs/tree/master
Update
On further reflection and investigation of Google Website Optimizer it's apparent that there are gaping holes in the above logic. By using different templates to represent Paths you break all caching on the view (or if the view is cached it will always serve the same path!). Instead, of using Paths, I would instead steal GWO terminology and use the idea of Combinations - that is one specific part of a template changing - for instance, changing the <h1> tag of a site.
The solution would involve template tags which would render down to JavaScript. When the page is loaded in the browser the JavaScript makes a request to your server which fetches one of the possible Combinations.
This way you can test multiple combinations per page while preserving caching!
Update
There still is room for template switching - say for example you introduce an entirely new homepage and want to test it's performance against the old homepage - you'd still want to use the template switching technique. The thing to keep in mind is your going to have to figure out some way to switch between X number of cached versions of the page. To do this you'd need to override the standard cached middleware to see if their is a A/B test running on the requested URL. Then it could choose the correct cached version to show!!!
Update
Using the ideas described above I've implemented a pluggable app for basic A/B testing Django. You can get it off Github:
http://github.com/johnboxall/django-ab/tree/master
