12.1What Ephemeral Environments Are

Ephemeral environments are disposable copies of your entire infrastructure stack, spun up for each pull request and torn down after merge. Instead of sharing a single staging environment across all developers (with its inevitable "who broke staging?" moments), each PR gets its own isolated world.

The developer pushes a PR. CI creates a complete environment -- VPC, database, queues, app containers, API gateway. Tests run against this real infrastructure at a unique preview URL. The PR comment shows test results and a link to the preview. When the PR merges or closes, everything is destroyed.