Skip to main content

Walking Skeletons and TODO Outlines

These days, applications are so complicated and contain so many layers that it's difficult to know where to start. Should you work bottom up or top down? How much should you work on one layer before starting to work on the next layer? How can you ensure that the layers work properly together? Building a walking skeleton and managing a TODO document in outline format are two techniques that work well together to conquer complex problems, even ones involving multiple layers. Best of all, you don’t have to worry about trying to get things right the first time or getting lost along the way.
A Walking Skeleton is a tiny implementation of the system that performs a small end-to-end function. It need not use the final architecture, but it should link together the main architectural components. The architecture and the functionality can then evolve in parallel. -- Alistair Cockburn
Building a walking skeleton is a great way to handle the complexity of dealing with multiple layers. Start with the simplest possible feature, and implement it top down. Ideally, the feature should force you to work your way all the way down the stack. The goal is to make sure the layers work together.

As you’re building the walking skeleton, you may think of many things that you need to add, test, or in general worry about. It's helpful to maintain a TODO document in outline format so that you can organize and plan your attack, especially when you’re working with multiple layers at the same time. Eventually, each TODO item can be transferred into a test, a piece of code, an issue in the bug tracking system, or perhaps just an email to someone else.

Once you’ve built a walking skeleton, should you go back to developing one layer at a time? For most applications where the cost of change is low, probably not. Actively building one layer at a time is frequently very inefficient. A more efficient approach is to focus on one feature at a time. Sketch out the feature using a set of TODOs and build it top-down, managing the TODOs as you go. If you focus on one feature at a time instead of one layer at a time, you won’t end up building a lot of code in different layers that never actually gets used. The time saved by only building what you need and only building it when you have all the information you need more than compensates for the refactoring time.

Thanks go to Chris Lopez for his help with this post.

Comments

Sam Rushing said…
I use that technique for pretty much any large project, but it works especially well with compilers... usually you start with a really small, toy version of the language, then put just enough meat on it to follow all the way through to the end. Not only does it give you a place to start hacking, and a framework, but more importantly it lets you find and fix any misunderstandings you may have earlier rather than later.
jjinux said…
Yep. I've done a couple small interpreters, and I can't imagine doing it any other way.
killy said…
I've known that technique from the gaming industry, where it's called a "vertical slice". The name is taken from a piece of cake - when you get a slice of cake it contains a little bit of every layer for you to taste :)

Popular posts from this blog

Ubuntu 20.04 on a 2015 15" MacBook Pro

I decided to give Ubuntu 20.04 a try on my 2015 15" MacBook Pro. I didn't actually install it; I just live booted from a USB thumb drive which was enough to try out everything I wanted. In summary, it's not perfect, and issues with my camera would prevent me from switching, but given the right hardware, I think it's a really viable option. The first thing I wanted to try was what would happen if I plugged in a non-HiDPI screen given that my laptop has a HiDPI screen. Without sub-pixel scaling, whatever scale rate I picked for one screen would apply to the other. However, once I turned on sub-pixel scaling, I was able to pick different scale rates for the internal and external displays. That looked ok. I tried plugging in and unplugging multiple times, and it didn't crash. I doubt it'd work with my Thunderbolt display at work, but it worked fine for my HDMI displays at home. I even plugged it into my TV, and it stuck to the 100% scaling I picked for the othe

Drawing Sierpinski's Triangle in Minecraft Using Python

In his keynote at PyCon, Eben Upton, the Executive Director of the Rasberry Pi Foundation, mentioned that not only has Minecraft been ported to the Rasberry Pi, but you can even control it with Python . Since four of my kids are avid Minecraft fans, I figured this might be a good time to teach them to program using Python. So I started yesterday with the goal of programming something cool for Minecraft and then showing it off at the San Francisco Python Meetup in the evening. The first problem that I faced was that I didn't have a Rasberry Pi. You can't hack Minecraft by just installing the Minecraft client. Speaking of which, I didn't have the Minecraft client installed either ;) My kids always play it on their Nexus 7s. I found an open source Minecraft server called Bukkit that "provides the means to extend the popular Minecraft multiplayer server." Then I found a plugin called RaspberryJuice that implements a subset of the Minecraft Pi modding API for B

Creating Windows 10 Boot Media for a Lenovo Thinkpad T410 Using Only a Mac and a Linux Machine

TL;DR: Giovanni and I struggled trying to get Windows 10 installed on the Lenovo Thinkpad T410. We struggled a lot trying to create the installation media because we only had a Mac and a Linux machine to work with. Everytime we tried to boot the USB thumb drive, it just showed us a blinking cursor. At the end, we finally realized that Windows 10 wasn't supported on this laptop :-/ I've heard that it took Thomas Edison 100 tries to figure out the right material to use as a lightbulb filament. Well, I'm no Thomas Edison, but I thought it might be noteworthy to document our attempts at getting it to boot off a USB thumb drive: Download the ISO. Attempt 1: Use Etcher. Etcher says it doesn't work for Windows. Attempt 2: Use Boot Camp Assistant. It doesn't have that feature anymore. Attempt 3: Use Disk Utility on a Mac. Erase a USB thumb drive: Format: ExFAT Scheme: GUID Partition Map Mount the ISO. Copy everything from