There's words I use a lot at work. One of them is capability. What's a capability?

It's something your system can do, kind of like a skill or feature. If you're building a content management system, and that content management system can display blocks of HTML, and not only rich text, that's a capability. It's also a feature. So what's the difference? Probably there's a formally correct definition, but I like to think of capabilities as features that you can reuse. It's kind of like the product version of abstractions. You can use that HTML block for one specific thing, like embedding a YouTube-Video, but you can also use it for plenty of things down the road that you might not even think or know about at the moment.

When you're an engineering lead, you're also always wearing some sort of product hat – even if it's just being the sidekick to your product counterpart, helping to solve product problems in the right way. One of the ways in which I like to live this is by finding ways to not build only features, but to building capabilities. A feature is always a more risky investment if you can't reuse the mechanic for something else.

Imagine a feature where you need to run a query periodically, and as a result send out an email to some customers – something along the lines of an abandoned cart, where you want to remind people that they can still order their items. There's plenty of ways on how to approach this, but one that focuses on capabilities would try to break down this problem into a few small chunks. Those chunks could be running a query, running something periodically and sending emails from within your system. By building those three capabilities (and making them reusable), you're creating something that solves the problem at hand, but also augments your code base by adding things that can be used at a later point, in different contexts and for different problems.

Capabilities are not abstractions. They are an entirely different concept – you might have the capability to store something, it's not necessarily very interesting how or where you store things. You might have a KV Store, but you might not really care what specific technology is behind that. Abstractions are a different, yet also important angle to consider for some problems, but they're not relevant in this discussion here. You ideally only care about being able to do something, not how something is very specifically done.

Good systems are, from some angle, like something built from Lego. Incredibly robust, but composed of reusable and flexible units that can be moved and applied to a number of challenges. Capabilities are one helpful mental model that I regularly apply when looking at a solution to problems, and maybe it even makes sense to you.