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Small Design Up Front

February 13, 2021

At my current gig and several before that, the initial engineering design document is the Request for Comments (RFC), sometimes called the Request for Discussion (RFD).

If you've been reading the series on building a ZFS plugin for Nomad, you might have asked yourself if this kind of stumbling through the design is typical of the RFC documents I've written. But that series is really about all the design work that happens before the initial design is documented. It's brainstorming, hypothesizing, and exploration. The first draft of a RFC is the output of that work, so typically by the time anyone else has seen it hopefully the obviously dumb ideas and dead ends have been weeded out.

The RFC ends up being a good "sandbox" for a small up-front design process. I suspect it's especially valuable for system software where even a minimal experiment can be costly. And the structure discourages you from trying to come up with a rigid specification that's doomed to be invalid the moment you start implementing it.

In some sense you're writing a RFC to communicate to your peers what you've already figured out about the problem. Their time is a gift, and the most valuable feedback to get is that which you couldn't think of on your own. So you should invest the time to ensure they're not just going to tell what you should already know. In a healthy organization, writing is a way to collectively discover the design, rather than persuade the team.

The phrase "in a healthy organization" is doing a lot of work here. I've worked places where RFC discussions were more of a battle ground for interpersonal conflict and office politics than meaningful engineering discussion. In that environment you end up writing defensively to head off debate and hide implementation details that will trigger objections. These documents are better named Request for Permission. And if you're in this situation... well, writing RFCs ain't gonna save you.

I'm probably a weird outlier, but I even write RFCs for personal projects. Call it writing as structured thought experiment. It's a tool, and one that supplements rather than replaces a whiteboard diagram or a throwaway spike. I could easily throw it out as soon as it's done, but why not keep it?

There can be a few audiences for those artifacts. A project might get completed to the point where it could be open sourced, in which case having those early design documents would be valuable to users or contributors. But the most important audience is Future Me. My level of interest or volume of work on these projects ebbs and flows. I might take a project through the initial design, feel like I've explored the problem well enough to learn what I wanted to learn, and set it aside for months. The RFC is like a well-written commit message for the project as a whole.

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© Timothy Gross

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