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?

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.
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 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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