Domain-Driven Infrastructure: Organize Your Terraform by Reason to Change

One morning, a new engineer on the team asked me a simple question. "The Lambda for the new notification feature — does it go under modules/ , or somewhere else?" I didn't have a good answer. We had a modules/lambda/ directory, so the obvious move was to put it there, and I nearly said so before something stopped me. The notification feature was part of the order workflow. Was this a reusable part, or a piece of the order domain? Two different questions were hiding inside one "where does it go?", and our directory structure couldn't tell them apart. The conversation ended the way these conversations always end. "Let's just put it in modules/lambda/ for now." The layout everyone uses You've probably seen this structure. Most Terraform repositories look like it: ├── modules/ │ ├── vpc/ │ ├── ecs/ │ ├── rds/ │ ├── iam/ │ └── lambda/ └── environments/ ├── dev/ └── prod/ It works. It plans, it applies, it looks organized. Nothing about it is wrong until the business asks for something. "Ship the new feature." "Traffic doubled, scale it up." "Compliance changed, revisit the permissions." Each request is one business change. And each one sends you into vpc/ , ecs/ , rds/ , iam/ , secrets/ , cloudwatch/ . Different requests, same sprawl. One reason to change, six directories to touch. Back when I worked this way, review time didn't go where you'd expect. Whether the change was correct was the easy part. The hard question was whether it was safe to apply, and nobody could answer that from the diff, so we asked whoever remembered what else depended on the security group being edited. Software design has a word for this: low cohesion. Things that change together are stored apart. We'd never accept this in application code. We learned — from decades of work on cohesion, coupling, and separation of concerns — to keep things that change together in one place. Somehow that vocabulary never made it down to our infrastructure repositories. This is not a Terraform problem. It is a de