Shopify Engineering 09/04/2013

# Building a Rack Middleware

I'm Chris Saunders, one of Shopify's developers. I like to keep journal entries about the problems I run into while working on the various codebases within the company.

Recently we ran into a issue with authentication in one of our applications and as a result I ended up learning a bit about Rack middleware. I feel that the experience was worth sharing with the world at large so here's is a rough transcription of my entry. Enjoy!

I'm looking at invalid form submissions for users who were trying to log in via their Shopify stores. The issue was actually at a middleware level, since we were passing invalid data off to OmniAuth which would then choke because it was dealing with invalid URIs.

The bug in particular was we were generating the shop URL based on the data that the user was submitting. Normally we'd be expecting something like mystore.myshopify.com or simply mystore, but of course forms can be confusing and people put stuff in there like http://mystore.myshopify.com or even worse my store. We'd build up a URL and end up passing something like https://http::/mystore.myshopify.com.myshopify.com and cause an exception to get raised.

Another caveat is that we aren't able to even sanitize the input before passing it off to OmniAuth, unless we were to add more code to the lambda that we pass into the setup initializer.

Adding more code to an initializer is definitely less than optimal, so we figured that we could implement this in a better way: adding a middleware to run before OmniAuth such that we could attempt to recover the bad form data, or simply kill the request before we get too deep.

We took a bit of time to learn about how Rack middlewares work, and looked to the OmniAuth code for inspiration since it provides a lot of pluggability and is what I'd call a good example of how to build out easily extendable code.

We decided that our middleware would be initialized with a series of routes to run a bunch of sanitization strategies on. Based on how OmniAuth works, I gleaned that the arguments after config.use MyMiddleWare would be passed into the middleware during the initialization phase - perfect! We whiteboarded a solution that would work as follows:

Now that we had a goal we just had to implement it. We started off by building out the strategies since that was extremely easy to test. The interface we decided upon was the following:

We decided that the actions would be destructive, so instead of creating a new Rack::Request at the end of our strategies call, we'd change values on the object directly. It simplifies things a little bit but we need to be aware that order of operations might set some of our keys to nil and we'd have to anticipate that.

The simplest of sanitizers we'd need is one that cleans up our whitespace. Because we are building these for .myshopify.com domains we know the convention they follow: dashes are used as separators between words if the shop was created with spaces. For example, if I signed up with my super awesome store when creating a shop, that would be converted into my-super-awesome-store. So if a user accidentally put in my super awesome store we can totally recover that!

Now that we have a sanitization strategy written up, let's work on our actual middleware implementation.

According to the Rack spec, all we really need to do is ensure that we return the expected result: an array that consists of the following three things: A response code, a hash of headers and an iterable that represents the content body. An example of the most basic Rack response is:

Per the Rack spec, middlewares are always initialized where the first object is a Rack app, and whatever else afterwards. So let's get to the actual implementation:

That's pretty much it! We've written up a really simple middleware that takes care of cleaning up some bad user input that necessarily isn't a bad thing. People make mistakes and we should try as much as possible to react to this data in a way that isn't jarring to the users of our software.

You can check out our implementation on Github and install it via RubyGems. Happy hacking!