Google App Engine retains previously deployed versions of an application. The standard and immediate method for rolling back a faulty deployment is to use the built-in traffic splitting feature. By navigating to the 'Versions' page for the specific App Engine service, you can re-route 100% of the user traffic from the new, buggy version to a previously known stable version. This action is instantaneous and effectively reverts the application to its prior state without requiring a new deployment, thus minimizing downtime and impact on users.

A. gcloud app restore is not a valid gcloud command for App Engine. The correct command-line tool for managing traffic is gcloud app services set-traffic.

B. There is no single "Revert" button at the application level in the Google Cloud Console. Reverting is a function of traffic management between different versions.

D. This approach is incorrect and inefficient. App Engine automatically keeps prior versions, so redeploying is unnecessary. Furthermore, traffic is split between versions of a service, not between separate applications.

1. Google Cloud Documentation

"Splitting Traffic": "If you need to roll back a version

you can easily do so by migrating traffic to one or more other versions." This page details the process of using the Google Cloud Console to select a version and allocate 100% of traffic to it

which is the procedure described in the correct answer. (See section: "Migrating traffic to a single version").

2. Google Cloud Documentation

"Managing Versions": "When you deploy changes to your app

a new version is created and traffic is routed to it. You can

however

choose to not route traffic to the new version automatically... If you discover a bug in a new version

you can quickly roll back to a previous version by routing traffic to it." This confirms that previous versions are retained and that traffic routing is the intended rollback mechanism.

3. Google Cloud SDK Documentation

gcloud app services set-traffic: This command-line reference demonstrates the programmatic way to manage traffic splits. The documentation states its purpose is to "[s]et the traffic splitting configuration for a service." This reinforces that traffic management is the core concept for version control

and it implicitly shows that a command like gcloud app restore does not exist for this purpose.