Vertical slices and SOA


Even the term “vertical slice”, a common stumbling block in agile adoption, kinda implies a large-scale n-tier application. Modern architectures and agile can play nicer together than that!

“Story sizing”, decomposition, vertical slice of functionality, Minimally Marketable Feature (MMF), Minimally Viable Feature (MVF), and my personal least-favorite, Potentially Shippable Product Increment (POS*). I think it’s the biggest hurdle for orgs moving from not-agile to agile. I think many other problems with initial adoption (estimation, timebox sizing) boil down to this one.

Every dev team I see trying to get started with this initially tries exactly the same wrong thing, usually because it’s how they’ve organized their work in their not-agile process before: they want to split things up by architectural layers, and build, let’s say, all of the database and then all of the business layer and then all of the UI.

Any time I see a sentence with “do all of… and then all of… and then all of…”, what’s that remind me of? Oh yeah: waterfall. There are reasons we devs cling to this in spite of ourselves. Maybe another post another day.

The thing is, “vertical slices” aren’t satisfying either. Every single team I’ve worked with resists and/or struggles with this for basically the same reason: the users asked us for an epic-sized feature because that’s what they want. They don’t want a slice of a feature, they want a feature. One of the cornerstones of agile is that we’re doing these short iterations in order to get feedback from users. That’s hard to do when they’re inherently unsatisfied with these ugly proto-features they don’t want (and they’re deeply alarmed when someone calls them “potentially shippable”)!

I discovered an interesting thing at one of my customers recently, though. We struggled with “vertical slices” vs. Big Database Up Front for like two days, and only then did I find out how much they’ve worked to transition their legacy LOB apps into a SOA model: collections of beautifully loosely-coupled services and APIs with clean interfaces talking to each other to achieve some nice user-interfaced result.

Wow! This was exactly the hook I needed. Because what is a service or an API if not a neat encapsulation of a small logically-contained bit of functionality? I realized that even the term “vertical slice” implies a traditional n-tier architecture in kind of a large-scale sense. Today’s SOA (is that still what we call it?) has already broken down those giant tiers into little slices. The team didn’t even realize they were already doing it. Each service might have its own little n tiers, but on a much smaller scale. Small is exactly what we need!

My customer got stuck trying to decompose from the epic feature level, still thinking about all the little services they’d need to assemble (plus BDUF) in order to hook up a UI and show a “vertical slice” to the user. They didn’t see their services as value in themselves, but I think the value is right there. APIs don’t have a user interface, but, um, the “I” stands for “interface”. They encapsulate something someone finds useful, and they are independently testable. Better yet, they almost demand automated testing, a practice we already wanted to reinforce. Imagine: at the iteration review, sure, the team should demo UI mockups early and often to get feature-related feedback from users… but can’t they also “demo” individual APIs (that implement underlying business capabilities and algorithms that the users do care about) by reviewing the acceptance criteria for the service and showing off a suite of automated test results to prove that the logic works?

I guess my point is that, as it always has, agile practice goes hand-in-hand with what we know about how to architect high-quality, maintainable software. I was just pleased to understand this in a new (to me) way.

* j/k. But I do hate that term.

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