The answer is the load balancer's health check interval and timeout caused a delay in marking the backend instances as unhealthy and routing traffic to the new zone. This is correct because when a zonal failure occurs in a Compute Engine MIG, the load balancer does not instantly stop sending traffic to the failed zone; it relies on health checks to detect unresponsive instances, and the default interval and timeout settings create a window where requests are still directed to dead backends. On the Google Professional Cloud Developer exam, this scenario tests your understanding of how MIG zonal failure recovery interacts with load balancing health check configuration, and the common trap is assuming that MIG autoscaling alone guarantees seamless failover without considering the health check propagation delay. Remember the memory tip: health checks are the gatekeepers of traffic—if they are slow to close the gate, requests keep hitting a dead zone.
PCD Deploying applications Practice Question
This PCD practice question tests your understanding of deploying 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.
You are deploying a critical application on Compute Engine. The application requires high availability and must survive a zonal failure. You have created a managed instance group (MIG) with autoscaling across two zones. The application state is stored in a Cloud SQL instance with a read replica in another region. The application also uses a shared static IP address for client access. During a test, you simulate a failure of zone us-central1-a. You observe that the MIG automatically creates new instances in the remaining zone, but the application becomes unreachable for several minutes. What is the most likely cause of the downtime?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
The load balancer's health check interval and timeout caused a delay in marking the backend instances as unhealthy and routing traffic to the new zone.
Health checks need time to detect failure and update routing.
B
The static IP address was not configured to failover to the remaining zone.
Why wrong: Static IP is not tied to a zone in this context.
C
The Cloud SQL read replica did not promote to primary quickly enough.
Why wrong: The read replica is for reads, not writes; the primary may not be affected.
D
The managed instance group's autoscaler took too long to create new instances in the remaining zone.
Why wrong: MIGs typically create instances quickly.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The load balancer's health check interval and timeout caused a delay in marking the backend instances as unhealthy and routing traffic to the new zone.
The most likely cause is that the load balancer's health check interval and timeout delayed the detection of unhealthy instances in the failed zone, preventing traffic from being rerouted to the new instances in the remaining zone. Even though the MIG created new instances quickly, the load balancer continued sending requests to the failed zone until the health check marked those backends as unhealthy, causing the application to be unreachable during that window.
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.
✓
The load balancer's health check interval and timeout caused a delay in marking the backend instances as unhealthy and routing traffic to the new zone.
Why this is correct
Health checks need time to detect failure and update routing.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The static IP address was not configured to failover to the remaining zone.
Why it's wrong here
Static IP is not tied to a zone in this context.
✗
The Cloud SQL read replica did not promote to primary quickly enough.
Why it's wrong here
The read replica is for reads, not writes; the primary may not be affected.
✗
The managed instance group's autoscaler took too long to create new instances in the remaining zone.
Why it's wrong here
MIGs typically create instances quickly.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often assume the MIG's autoscaling speed is the bottleneck, but Cisco tests the understanding that the load balancer's health check configuration is the critical factor in traffic rerouting during a zonal failure.
Detailed technical explanation
How to think about this question
Under the hood, the load balancer uses health checks (e.g., HTTP, TCP, or SSL) with configurable interval (default 5 seconds) and timeout (default 5 seconds) to determine backend instance health. When a zone fails, the health check must fail for the configured number of consecutive failures (default 2) before the instance is marked unhealthy, causing a delay of up to 15 seconds or more. In real-world scenarios, this delay can be exacerbated by longer intervals or timeouts, leading to several minutes of downtime if the application's client connections are not resilient to transient failures.
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
A company's IT admin needs to give a contractor read-only access to production logs without sharing account credentials. Using role-based access control (RBAC) and temporary scoped permissions — not a permanent shared password — is the correct pattern. Questions like this test whether you can apply least-privilege access across cloud identity services.
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.
Deploying applications — This question tests Deploying applications — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The load balancer's health check interval and timeout caused a delay in marking the backend instances as unhealthy and routing traffic to the new zone. — The most likely cause is that the load balancer's health check interval and timeout delayed the detection of unhealthy instances in the failed zone, preventing traffic from being rerouted to the new instances in the remaining zone. Even though the MIG created new instances quickly, the load balancer continued sending requests to the failed zone until the health check marked those backends as unhealthy, causing the application to be unreachable during that window.
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.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
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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