Locking & the apply gate

No colliding applies; review first.

When multiple PRs touch the same Terraform, concurrent plans and applies would race on shared state. Atlantis prevents this with locking: when a PR plans a project, Atlantis locks that project (directory + workspace), and any other PR trying to plan or apply it is blocked with a comment pointing at the PR that holds the lock. The lock releases when the PR is applied and merged, or when you manually unlock it. One project changes through one PR at a time.

This project is currently locked by an unapplied plan from PR #142.
To continue, apply and merge PR #142, or unlock it via the Atlantis UI.
# your plan will run once the lock is released

The apply gate

Locking pairs with the apply gate to make the review meaningful. Atlantis can require that a PR be approved (and that its plan is current) before atlantis apply is allowed, and restrict who may apply. Combined, these enforce: only one change to a project in flight, only after review, only by authorized people. That is the governance the whole tool exists to provide — serialize, review, authorize — enforced automatically rather than by convention.

# require approval + a mergeable PR before apply is permitted
--require-approval          # (or per-repo: apply_requirements: [approved, mergeable])
# restrict who can run apply via VCS permissions + team membership