Lua Log #7: Routing Requests

Any web framework that wants to be useful needs some kind of mechanism to take HTTP request paths and map those requests to some kind of code that can return a response. That’s the functionality that I added to Atlas this week.

Generally, in my experience with web frameworks, this concept of connecting HTTP requests to the right handling code is called routing.

I’ve seen a couple of big ways to handle routing. Let’s consider the ways, then I’ll talk about my choice.

One option is to create a tree structure. In this model, there is a root node that maps to the / path, and child nodes can handle more detailed paths. This this model a request walks through this tree structure until it encounters the node that maps to the path. From there, the node can delegate to some code that can handle the request and generate a response.

Another option is to create a flat list of objects that match to paths, often using regular expressions to do pattern matching. A primary difference in this model is that path segments don’t need to match to individual nodes. Stated differently, in this model, a path like /some/long/path/to/something can match to a single object instead of traversing six nodes corresponding to /, some, long, path, to, and something.

Of those two options, I picked the latter path.

An Atlas app will look something like this:

-- app/main.lua

local Application = require "atlas.application"
local Route = require "atlas.route"

local controllers = require "app.controllers"

local routes = {
  Route("/", controllers.home),
  Route("/about", controllers.about),
app = Application(routes)

return {app = app}

The app object handles the LASGI (Lua AGSI) interface that I implemented recently. In the context of a single HTTP request, the Atlas app will loop through each of the Route instances, and try to find a match. If no match is found, the app will return a 404 Not Found response.

This example is very simplistic because it tries to match one of two literal routes. There are some other core features that I’ve built into the Route class. A common activity in routing is to extract information from the path itself.

For instance, maybe you have public profiles for users on a website. What if you want to support something like /users/mblayman? In this case, mblayman is a bit of dynamic data that the handling code will need to use to look something up in a database or some other data storage.

Atlas routes can use converters to extra this data.

local Route = require "atlas.route"

local controllers = require "app.controllers"

Route("/users/{username:string}", controllers.user_profile),

With this Route definition, a string value can be pulled from a request and passed to the handling code (which I’m calling a controller, named after the Model View Controller design pattern that I’m attempting to follow). The controller will receive that string as an argument.

-- app/controllers.lua

local function user_profile(request, username)
  -- do stuff

return {user_profile = user_profile}

Currently, I have added two types of converters for routes: string and int. That seemed like a enough to make routes generally useful, and I can create more converters over time.

Under the hood, the route path of /users/{username:string} is converted into a Lua pattern. That Lua pattern would be


The collection of routes gets processed by a router. The router has the job of

  • Receiving a request path
  • Iterating over the routes to find a match
  • Invoking the route to run with the request data

All of the core routing functionality is in place and operational! I have a more that I could do with routing, but I want to keep moving for now. The most notable missing thing is a way to group a bunch of related routes. Grouping will be a nice feature as I try to create a more modular design in the future.

My next goal is to put into place the Request and Response interfaces that will be crucial abstractions when working with HTTP. When I complete those, I’ll have a minimally viable application system that can receive requests, route a request to a controller, and return a response.

After that, I’ll have to turn my attention back to the web server side to process raw incoming requests to transform the network traffic into events that can be used by the LASGI interface.

Before finishing, I want to shout out the Starlette project, which is hugely inspirational for all this work. I may have made routing work for Lua, but the Starlette authors did all the hard work of thinking of clear ways to abstract this problem space. Much of my design is merely a translation of ideas from the Starlett project. Good artists copy. Great artists steal.

Thanks for reading! Have questions? Let me know on Twitter at @mblayman.