> Don't manage IaC declarations alongside code in a polyrepo

Can you elaborate on this one?

(And thanks for the interesting comments)

> The source code for services should not be coupled to their deployments when managed in SCM. The lifecycle of changes for infrastructure are different than the lifecycles of artifacts for services. Any artifact should be able to be configured and deployed into any (supported) infrastructure configuration. For example, running git revert on a service should be able to yield a deployable artifact regardless of how the infrastructure is configured. By coupling these, you tie changes to infrastructure to changes in services. A rollback for your service can also unintentionally rollback how that service gets deployed – and avoiding that requires an engineer hold both the context of the infrastructure and the context of the service in their head whenever they are manging git history. It becomes difficult to deploy an older version of a service for testing. It also breaks git-bisect since, now, searching for a regression in software also changes how that software is getting deployed. This is an extension of managing IaC as small composable modules. The source code for a service should, itself, be viewed as a composable module. That module may take on a different format than other pieces of IaC (i.e. a .tar.gz, a .deb, or a docker image instead of a terraform module or a terraform provider) – but it’s API contracts are still drawn around the unit of deployment and not the monolithic infrastructure stack that will be deployed with it.

This, of course, does not apply to projects using a monorepo.

how would one manage independent infra and services in a monorepo?

The philosophy is the same, but the implementation is different.

You still keep them separate but your monorepo tooling handles that separation. When multiple changes go out together, some being infra some being code, the tooling should be aware of those dependencies (just like any other) and handle resolving the infra first.

The muscle memory of devs in a monorepo tend to be different too. Folks are used to scoping their SCM changes to folders instead of working at the top level of the git repo (i.e. you usually don't find yourself doing a `git reset --hard HEAD~10` in a monorepo outside of a feature branch - other teams get grumpy when you blow away their changes on the mainline branch).

I make this distinction between polyrepos and monorepos for IaC because I've seen this advice result in folks splitting their monorepo into a birepo, or using IaC adoption as a driving reason to migrate their company to a polyrepo. There isn't anything wrong with the birepo approach, but it can be accomplished inside the monorepo all the same.