Document review apps setup for gitlab-{ce,ee}

Signed-off-by: Rémy Coutable <remy@rymai.me>
Signed-off-by: ddavison <ddavison@gitlab.com>

doc/development/testing_guide/index.md

@@ -59,6 +59,12 @@ parallelization, monitoring.
 
 ---
 
+## [Review apps](review_apps.md)
+
+How review apps are set up for GitLab CE/EE and how to use them.
+
+---
+
 ## [Testing Rake tasks](testing_rake_tasks.md)
 
 Everything you should know about how to test Rake tasks.

doc/development/testing_guide/review_apps.md

+# Review apps
+
+We currently have review apps available as a manual job in EE pipelines. Here is
+[the first implementation](https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/6259).
+
+That said, [the Quality team is working](https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/6665)
+on making Review Apps automatically deployed by each pipeline, both in CE and EE.
+
+## How does it work?
+
+1. On every EE [pipeline][gitlab-pipeline] during the `test` stage, you can
+  start the [`review` job][review-job]
+1. The `review` job [triggers a pipeline][cng-pipeline] in the
+  [`CNG-mirror`][cng-mirror] [^1] project
+1. The `CNG-mirror` pipeline creates the Docker images of each component (e.g. `gitlab-rails-ee`,
+  `gitlab-shell`, `gitaly` etc.) based on the commit from the
+  [GitLab pipeline][gitlab-pipeline] and store them in its
+  [registry][cng-mirror-registry]
+1. Once all images are built, the review app is deployed using
+  [the official GitLab Helm chart][helm-chart] [^2] to the
+  [`review-apps-ee` Kubernetes cluster on GCP][review-apps-ee]
+  - The actual scripts used to deploy the review app can be found at
+    [`scripts/review_apps/review-apps.sh`][review-apps.sh]
+  - These scripts are basically
+    [our official Auto DevOps scripts][Auto-DevOps.gitlab-ci.yml] where the
+    default CNG images are overriden with the images built and stored in the
+    [`CNG-mirror` project's registry][cng-mirror-registry]
+1. Once the `review` job succeeds, you should be able to use your review app
+  thanks to the direct link to it from the MR widget. The default username is
+  `root` and its password can be found in the 1Password secure note named
+  **gitlab-{ce,ee} review app's root password**.
+
+**Additional notes:**
+
+- The Kubernetes cluster is connected to the `gitlab-ee` project using [GitLab's
+  Kubernetes integration][gitlab-k8s-integration]. This basically allows to have
+  a link to the review app directly from the merge request widget.
+- The manual `stop_review` in the `post-cleanup` stage can be used to stop a
+  review app manually, and is also started by GitLab once a branch is deleted
+- [TBD] Review apps are cleaned up regularly using a pipeline schedule that runs
+  the [`scripts/review_apps/automated_cleanup.rb`][automated_cleanup.rb] script
+
+[^1]: We use the `CNG-mirror` project so that the `CNG`, (**C**loud **N**ative **G**itLab), project's registry is
+  not overloaded with a lot of transient Docker images.
+[^2]: Since we're using [the official GitLab Helm chart][helm-chart], this means
+  you get the a dedicated environment for your branch that's very close to what it
+  would look in production
+
+## Frequently Asked Questions
+
+**Will it be too much to trigger CNG image builds on every test run? This could create thousands of unused docker images.**
+
+  > We have to start somewhere and improve later. If we see this getting out of hand, we will revisit.
+
+**How big is the Kubernetes cluster?**
+
+  > The cluster is currently setup with a single pool of preemptible
+    nodes, with a minimum of 1 node and a maximum of 30 nodes.
+
+**What are the machine running on the cluster?**
+
+  > We're currently using `n1-standard-4` (4 vCPUs, 15 GB memory) machines.
+
+**How do we secure this from abuse? Apps are open to the world so we need to find a way to limit it to only us.**
+
+  > This won't work for forks. We will add a root password to 1password shared vault.
+
+[gitlab-pipeline]: https://gitlab.com/gitlab-org/gitlab-ee/pipelines/29302122
+[review-job]: https://gitlab.com/gitlab-org/gitlab-ee/-/jobs/94294136
+[cng-mirror]: https://gitlab.com/gitlab-org/build/CNG-mirror
+[cng-pipeline]: https://gitlab.com/gitlab-org/build/CNG-mirror/pipelines/29307727
+[cng-mirror-registry]: https://gitlab.com/gitlab-org/build/CNG-mirror/container_registry
+[helm-chart]: https://gitlab.com/charts/gitlab/
+[review-apps-ee]: https://console.cloud.google.com/kubernetes/clusters/details/us-central1-b/review-apps-ee?project=gitlab-review-apps
+[review-apps.sh]: https://gitlab.com/gitlab-org/gitlab-ee/blob/master/scripts/review_apps/review-apps.sh
+[automated_cleanup.rb]: https://gitlab.com/gitlab-org/gitlab-ee/blob/master/scripts/review_apps/automated_cleanup.rb
+[Auto-DevOps.gitlab-ci.yml]: https://gitlab.com/gitlab-org/gitlab-ci-yml/blob/master/Auto-DevOps.gitlab-ci.yml
+[gitlab-k8s-integration]: https://docs.gitlab.com/ee/user/project/clusters/index.html
+
+---
+
+[Return to Testing documentation](index.md)