[adtac]

There is no standard for design docs at Google and everyone is free to experiment, but most commonly used templates usually share the same ideas. My favourite is to write a green doc and a blue doc. The green doc is typically a "one pager" (basically always 2 pages at Google) and it typically never discusses implementation details. It's mostly about why we need X and the goals and non-goals of the project. Once your green doc is reviewed by your teammates, if everyone's happy, you write a more in-depth blue doc where you'll probably design the protobufs and interfaces, draw the system architecture, make framework choices and trade-offs, design metrics for monitoring, think about scalability, alternative approaches, and maybe even outline a timeline for the project if you're feeling ambitious. This, too, will go through a few rounds of reviews until everyone is satisfied. The green doc is significantly less investment than the blue doc, but it's still some investment (say ~1 SWE-week of writing the first draft + reviews + future iterations), so this pattern only makes sense if your project is complex enough. If I'm just adding things to an existing project and I still need a design doc reviewed by other people, I'd probably just write one design doc.