The answer is a managed instance group with autoscaling and a health check, as this combination provides the auto-healing mechanism for unhealthy instances. When a health check detects a failed instance, the managed instance group automatically recreates it from the instance template, ensuring continuous availability without manual intervention. On the Google Professional Cloud Developer exam, this scenario tests your understanding of how MIGs decouple instance health from load balancing—a common trap is confusing the load balancer’s own health checks with the MIG’s autohealing feature, but only the MIG triggers instance replacement. Remember that autoscaling handles scale-up and scale-down based on load, while autohealing is the MIG’s dedicated process for replacing unhealthy instances. A useful memory tip: “MIG heals, LB steers”—the load balancer routes traffic away from unhealthy instances, but the MIG actually replaces them.
PCD Practice Question: Designing highly scalable, available, and reliable cloud-native applications
This PCD practice question tests your understanding of designing highly scalable, available, and reliable cloud-native applications. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A team runs a microservice on Compute Engine behind a regional external HTTP load balancer. They want to automatically replace unhealthy instances without manual intervention. Which feature should they use?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Managed instance group with autoscaling and health check
A managed instance group (MIG) with autoscaling and a health check is the correct choice because it automatically replaces unhealthy instances based on the health check results. The MIG uses the health check to detect failed instances, then automatically recreates them from the instance template, ensuring high availability without manual intervention. Autoscaling further adjusts the number of instances based on load, but the core replacement mechanism is driven by the MIG's health check and autohealing feature.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
Unmanaged instance group with health check
Why it's wrong here
Unmanaged instance groups do not automatically heal instances.
✗
Instance template with manual replacement
Why it's wrong here
Instance templates define configuration but do not automate replacement.
✗
Load balancer backend service health check only
Why it's wrong here
Health checks alone do not recreate instances; they only affect traffic routing.
✓
Managed instance group with autoscaling and health check
Why this is correct
Managed instance groups support autohealing, which automatically recreates instances based on health check results.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse the load balancer's health check (which only affects traffic routing) with the managed instance group's health check (which triggers automatic instance replacement), leading them to choose option C instead of D.
Detailed technical explanation
How to think about this question
Under the hood, a managed instance group uses a health check probe (e.g., HTTP, HTTPS, TCP, or SSL) to monitor instance health. When an instance fails the health check for a configurable number of consecutive failures (default is 2), the MIG's autohealing policy automatically deletes the unhealthy VM and creates a new one from the instance template. This behavior is distinct from autoscaling, which adds or removes instances based on load metrics; autohealing ensures the desired number of healthy instances is maintained regardless of load.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
An e-commerce site experiences heavy traffic on Black Friday and near-zero traffic during off-peak weeks. Rather than provisioning permanent large VMs, the team uses auto-scaling groups that add capacity automatically under load and reduce it overnight. Questions like this test whether you understand elasticity, availability zones, and cloud compute scaling patterns.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Designing highly scalable, available, and reliable cloud-native applications — This question tests Designing highly scalable, available, and reliable cloud-native applications — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Managed instance group with autoscaling and health check — A managed instance group (MIG) with autoscaling and a health check is the correct choice because it automatically replaces unhealthy instances based on the health check results. The MIG uses the health check to detect failed instances, then automatically recreates them from the instance template, ensuring high availability without manual intervention. Autoscaling further adjusts the number of instances based on load, but the core replacement mechanism is driven by the MIG's health check and autohealing feature.
What should I do if I get this PCD question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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