I’ve talked a lot about these two terms before in the context of software development: radial and sequential. Usually, I frame these two words in the world of unit testing. A radial app has much more utility from unit tests compared to sequential apps. This doesn’t obliterate the value of unit tests, but when you’re a project leader, the decision to focus on unit tests or not is an important one. As theory-crafting engineers, it’s impossible for us to say “no, don’t unit test.” But as practical builders, it’s difficult to weigh the value of unit tests against more respectable measures of success, like agile sprint velocity, customer happiness, sales, etc.
Yeah, I know — “but unit tests prevent errors, and when there are fewer bugs, the customers are going to be happier,” you say. A customer is also quite happy if they get the utility they want out of their app as well. And sometimes they value that utility more than their own happiness. Understanding the customer is crucial here, of course. And it may entirely be the case that the customer values their happiness and overall sentiment more than just simple utility. The market and industry play important roles here too. But I want to focus on the headline here: radial vs. sequential, and how it fits into more than just software development. So just bear with me for a few minutes.
A radial app is something like Microsoft Word. I think about it in terms of how the app is being used. You walk into a building, and now you’re loaded into the app. Where can you go from here? With Microsoft Word, you can launch the app, and then it’s pretty guided to this point what you can do: either load an existing document or create a new one. But beyond that, the experience becomes very unguided. You can start typing, save whenever, insert a table of contents, launch spell checker — you can do so many different things, and in any order.
Ever tried to manage a great big state diagram for something like this? It becomes rapidly unfeasible really quickly. It’s difficult to test every permutation of states and the ways customers might use the app. That’s where unit tests gain their utility — so you don’t accidentally break a permutation that worked a week ago.
A sequential app, on the other hand, is on the opposite end of the spectrum. And do note that not every app is exactly black or white, just as it isn’t entirely sequential or radial. It’s somewhere on the spectrum. The sequential app is a more guided experience that often can be smoke-tested. Even Microsoft Word is sequential to a point, and thus, some of it is smoke-tested up to at least being able to open and save a document.
The sequential app is much more guided, even beyond standard launch, open, and save. It’s hard to think of open-ended, sort-of open-world experiences that are very sequential. But I’ve got one: a mortgage point-of-sale application, in which loan “files” are created or saved, much like a Word document. For the most part, many aspects of loan processing are very sequential. You need to run credit, then price out the loan, lock the rate, and then send out disclosures. Granted, it’s not entirely sequential — some electronic verification steps can be used instead of manual ones, and there’s often communication involved that makes each loan unique.
But still, much more than a Word document, the mortgage POS is very sequential. And this is where unit tests lose some of their utility. Anytime you need to fix bugs or add features to something like loan locking, it requires running credit and pricing a loan. The value of unit tests on pricing a loan is lower, because manual testing on locking necessitates manual testing on pricing. This isn’t to say the value of unit tests is moot, because permutations pervade all systems and edge cases always exist. It just means that the value is reduced, and so when gauging whether to focus on them, it’s harder to weigh against big-picture customer success metrics.
That’s enough of the software stuff though. Radial vs. Sequential spreads beyond just software, because these terms are more about experiences in general. Even construction projects, or any craftwork. Project developers for a new Costco have much to consider and even oblige in terms of safety and certifications. Inspections need to be done, and in a sense, inspections equate to unit tests — they represent work done by other agents.
In software development, unit tests are automated by cloud agents that run them prior to every new build/pull request. In construction, the unit tests are the inspectors, who are agents apart from the builders, coming by to give pass/fail grades across different areas. It’s actually nicer in construction because the Costco project developer just needs to order the inspections, and doesn’t really need to define what to inspect. In software, the developer must define the unit test, which makes it more expensive. The construction inspector has done his job many times before, so his experience is valuable, whereas that burden of experience falls on the software developer.
It’s fun to draw these parallels — and neater yet to understand that there are many similarities between construction and software development because they are all forms of development. As a software engineer myself, I’ve often been attuned to what I really love: to build things. It’s no surprise I’m an avid DIYer, and that passion flows into gardening too, which is another form of development. Although gardening is probably more focused on maintenance, drawing parallels there is fun too.
But what else? What about video games?
The Legend of Zelda on the NES — the very first release — that one is a very radial game. It features several dungeons where you must collect pieces of the Triforce, but it doesn’t restrict the order of your adventure. Granted, the first dungeon is much easier than the last, and that’s the only guidance. There are no mechanical guides or obstructions otherwise, save for a few — you need an arrow to kill some bosses, and you need a raft or bridge to access certain required areas. Considered one of the first “open world” games, Zelda is a radial game by design, much to the desires of its creator, who related the game to his own childhood exploration.
Mega Man is a game that mixes radial and sequential. The stage selection is entirely up to you, and that’s one of the things that makes the game an all-time great. There are inherently optimal orderings of stages, but it’s up to you to figure them out. After selecting a stage, however, the experience leading to its boss fight is entirely sequential and guided. When you defeat the boss of each stage, you gain their weapon powers, which are often optimal for defeating other bosses. Hence, the ordering can be important, but for the avid challenger, anything is possible. Mega Man blends radial and sequential beautifully, and it’s enjoyable because of that balance.
Now let’s venture to the sequential: Final Fantasy. The original NES game, and really almost every Final Fantasy that followed. These games are legends in the RPG genre, and they are fully sequential because they’re telling a story — much like a novel. And on that note, all books are sequential as well… except, of course, choose-your-own-adventure books, which are radial.
Back to Final Fantasy though — RPG fans love them for their storytelling, combined with the developmental nature of the progression. Players invest in their characters as they grow, level up, gear up, and fully immerse themselves in the protagonists. It mirrors the developmental aspects of what I love most, and what I think drives many engineers who also share passions for DIY, gardening, and RPG gaming.
Now, I’m sure sequential vs. radial isn’t an entirely unexplored topic. In fact, tropes like linear vs. non-linear appear more often in the literature. But I like to frame it my own way, pulling from gaming, because while the real world might be an unstructured, radial free-for-all where anything can happen, we can still escape into game worlds where things are more focused and guided.
Radial might represent the real world’s chaos of endless permutations and possibilities. Sequential helps control that chaos — and when possible, I see a lot of value in introducing sequential flow to make sense of it.
