GCP Professional Cloud Architect Practice Question

An e-commerce team is containerizing a Java API but will initially run it on Compute Engine virtual machines. The service must

stay available even during a complete zone outage,
scale out whenever either the average CPU load rises above 60 % or a Pub/Sub order-queue backlog grows,
support zero-downtime releases with an instant rollback path, and
automatically shrink to one VM during very low overnight traffic.

Which deployment strategy best satisfies these requirements?

Use an unmanaged instance group with an external HTTP(S) load balancer; trigger a Cloud Functions script to add or delete instances based on Pub/Sub backlog, and reboot instances in place for each release.
Create a regional managed instance group from an instance template, attach an autoscaler driven by CPU and a custom Cloud Monitoring metric for Pub/Sub backlog, and perform application releases with the SUBSTITUTE update method.
Create a single-zone managed instance group behind an internal load balancer, enable autoscaling on CPU utilization only, and handle releases with a blue-green approach by cloning the group and swapping backend services.
Deploy the container to Cloud Run, set minimum instances to 1, configure a Pub/Sub push subscription, and use traffic splitting for new revisions.

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Answer Description

A regional managed instance group (MIG) spreads its VMs across two or more zones, so it keeps serving if one zone fails. A Compute Engine autoscaler can attach to the MIG with two scaling policies-CPU utilization and a custom Cloud Monitoring metric that reflects the Pub/Sub backlog-so either signal triggers scale-out. For releases, the MIG's SUBSTITUTE update method keeps the existing instances serving until their replacements are healthy, providing true zero downtime and a simple rollback. An autoscaler minimum-size setting of 1 lets the group shrink to a single instance off-peak. The other options fail at least one requirement: a single-zone MIG cannot survive a zonal outage, unmanaged groups lack health and update management, and Cloud Run does not use the mandated Compute Engine VMs.

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