Preview environments per branch: one deployed copy per pull request, torn down on merge

Goal

Every pull request gets a running, reachable copy of the application at its own URL, created by the pipeline and removed when the branch is merged or closed, so that reviewers, testers and agents can exercise the change instead of imagining it from the diff.

Prerequisites

A deployable artefact per commit (container image or static bundle), infrastructure that can host many small copies cheaply, and a naming scheme from pull-request number to hostname. Hosted platforms provide this for static and serverless sites: Netlify describes deploy previews as deploying pull or merge requests to a unique URL with a deploy-preview-<number> prefix. For services on a cluster, Kubernetes namespaces provide a mechanism for isolating groups of resources within a single cluster, which makes one namespace per pull request the natural unit.

Steps

1. Build once per commit and tag the artefact with the commit SHA; the preview deploys that artefact, never a rebuild.
2. Derive the environment name from the pull-request number (pr-1234), not from the branch name, which may contain characters unfit for hostnames and may be renamed.
3. Provision the copy: a namespace or stack, a database filled from the local seed (see the related article), configuration pointing at mock or sandbox versions of third parties, and a wildcard DNS record with a matching certificate so no per-preview DNS change is needed.
4. Post the URL back to the pull request and register it as a deployment. GitHub environments carry deployment protection rules and environment-scoped secrets, so previews can get their own, weaker credentials.
5. Tear down on merge, close or after an inactivity timeout, deleting artefacts and data with it; run a nightly sweep for orphans.
6. Cap concurrent previews and resources per preview; a queue is better than a cluster that falls over.
7. Point the end-to-end smoke test at the preview URL so the pipeline verifies that the copy actually starts.

Expected result

A reviewer opens the link from the pull request, tries the change against seeded data, and the environment disappears after merge without anyone remembering to delete it.

Limits and test basis

Previews that share a stateful backend (one database or queue for all of them) leak changes between branches; stateful services need a copy or a schema per preview. Migrations that are hard to roll back sit badly with tearing down and re-creating. Cost scales with open pull requests. The procedure is a synthesis of the cited platform documentation; no throughput or cost figures are claimed.