You want to deploy a new version of an App Engine standard environment application using the `gcloud` CLI, but you want to test it before routing all…

The gcloud app deploy command is used to deploy application source code to App Engine. To meet the requirement of testing a new version before it serves live traffic, two specific flags are necessary. The --version flag allows you to assign a unique identifier to the new deployment (e.g., v2). The --no-promote flag is crucial; it instructs App Engine to deploy the new version but not to automatically route any traffic to it. This allows you to access and test the new version directly via its version-specific URL (e.g., v2-dot-YOURPROJECTID.appspot.com) before manually migrating traffic.

A. gcloud app services update is used to modify existing services, primarily for migrating traffic between versions, not for deploying new application code.

B. gcloud app create is used only once per project to initialize the App Engine application and set its region; it does not deploy code versions.

C. gcloud compute is the command group for managing Compute Engine resources like virtual machines, not for deploying App Engine applications.

1. Google Cloud SDK Documentation, gcloud app deploy: The official documentation for the command explicitly defines the --no-promote flag: "Do not promote the deployed version to receive traffic. The default is to promote the version and stop the previous version." This directly supports the chosen answer.

Source: Google Cloud, gcloud app deploy reference, Flags section. https://cloud.google.com/sdk/gcloud/reference/app/deploy

2. Google Cloud App Engine Documentation, Splitting Traffic: This guide explains the deployment workflow and the use of the --no-promote flag for testing purposes. It states, "When you deploy a new version, you can choose to prevent traffic from being automatically routed to that new version by using the --no-promote flag."

Source: Google Cloud, App Engine Standard Environment, "Splitting Traffic", Deploying new versions section. https://cloud.google.com/appengine/docs/standard/python3/splitting-traffic

3. Google Cloud App Engine Documentation, Managing Versions: This document outlines how versions are managed and clarifies that deploying a new version without promoting it is a standard practice for testing before a full rollout.

Source: Google Cloud, App Engine, "How Versions are Managed". https://cloud.google.com/appengine/docs/standard/python3/versions