Google PCA Ensure solution and operations reliability Practice Question
A company runs a critical application on Compute Engine instances in a managed instance group (MIG) with autoscaling. During a traffic spike, some instances become unhealthy but are not automatically replaced. What is the most likely cause?
⚠ Common exam trap
Google Cloud often tests the distinction between the MIG's autohealing health check and the load balancer's health check, leading candidates to incorrectly attribute instance replacement failures to load balancer issues rather than the MIG's own health check configuration.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The autohealing health check is misconfigured.
The most likely cause is that the autohealing health check is misconfigured. In a managed instance group, autohealing relies on a health check to detect unhealthy instances and trigger replacement. If the health check is misconfigured (e.g., wrong port, path, or protocol), the MIG will not recognize instances as unhealthy and will not automatically replace them, even during a traffic spike.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
The MIG is regional and one zone failed.
Why it's wrong here
A zone failure triggers the regional MIG to recreate instances in surviving zones, so unhealthy instances would still be replaced. It is tempting because zone loss does cause disruption, and would be correct if the MIG were zonal rather than regional.
- ✓
The autohealing health check is misconfigured.
Why this is correct
Autohealing relies on a health check to detect and recreate unhealthy instances. If that health check is misconfigured — wrong path, port, or thresholds — the MIG never marks instances unhealthy, so no automatic replacement occurs despite the traffic spike.
- ✗
The instance template has a startup script error.
Why it's wrong here
A startup script error causes new instances to fail during initialisation, but existing unhealthy instances are still replaced by autohealing. It is tempting because script errors do cause instance failure, and would be the cause if replacement instances kept failing to become healthy.
- ✗
The HTTP load balancer's health check is failing.
Why it's wrong here
A failing load balancer health check marks instances unhealthy but does not stop the MIG's autohealing from recreating them; autohealing uses its own health check. It is tempting because health checks do govern traffic routing, and would be the cause if instances were removed from the backend pool.
Go deeper
Related to this question
Learn chapter
Google Kubernetes Engine (GKE)
Key term
Health check
A health check is an automated test that monitors whether a system, application, container, or network service is running correctly and can respond to requests.
Key term
Instance group
An instance group is a collection of virtual machine instances that are managed as a single unit for scaling, load balancing, and lifecycle management in cloud computing.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This PCA practice question is part of Courseiva's free Google Cloud certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the PCA exam.