DOP-C02 Resilient Cloud Solutions Practice Question
A company runs a critical web application on EC2 instances in an Auto Scaling group. The application uses an Application Load Balancer (ALB) with health checks pointing to /health. Recently, the application experienced intermittent failures where the ALB would mark instances as unhealthy and route traffic away, causing a reduction in capacity. The development team noticed that the /health endpoint occasionally returns HTTP 503 when the application is under heavy load, but the application can recover quickly. The team wants to avoid unnecessary instance replacements while ensuring availability. Which solution should the DevOps engineer implement?
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
✓
Increase the health check interval and increase the unhealthy threshold
Increasing the health check interval and increasing the unhealthy threshold makes the health check less sensitive to transient errors, such as occasional 503 responses under heavy load. This prevents unnecessary instance replacements while maintaining availability. Option A is incorrect because implementing a custom Lambda health check that ignores 503 responses would not leverage the built-in ALB health check tuning and adds complexity. Option B is incorrect because decreasing the unhealthy threshold would make instances more easily marked unhealthy, worsening the problem. Option D is incorrect because decreasing the health check interval increases the frequency of checks, which might cause more frequent detections of transient errors, and decreasing the healthy threshold does not address the issue of avoiding unnecessary replacements.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Implement a custom health check using Lambda that ignores 503 responses
Why it's wrong here
An ALB target group already offers the ability to tune health-check tolerance by adjusting the probe path, success-codes matcher, interval, and thresholds. Wrapping health checks in a Lambda that ignores 503s would mask genuine overload or dependency failures, preventing the load balancer from taking the instance out of rotation when it truly cannot accept traffic. It also adds asynchronous latency, cost, and complexity while doing nothing to smooth out transient errors better than simply increasing the unhealthy threshold or interval. A 503 is often an intentional, honest signal from the application that it is saturated, so it should never be silently disregarded.
- ✗
Decrease the unhealthy threshold to mark instances unhealthy faster
Why it's wrong here
Reducing the unhealthy threshold, for example from 3 to 2 consecutive failures, causes the ALB to declare an instance unhealthy almost immediately after a single probe fails. This speeds up failover for truly dead instances, but it also turns fleeting CPU spikes or one-off 503s into full lifecycle replacements, because a second failed check in a row can cross the threshold. The scenario explicitly asks for fewer replacements, not faster detection, and this change increases flapping and adds load to Auto Scaling processes. It also leaves the interval untouched, so it does not provide the wider observation window that transient-error tolerance requires.
- ✓
Increase the health check interval and increase the unhealthy threshold
Why this is correct
In a target group's health-check settings, increasing the interval spaces out probe requests, while increasing the unhealthy threshold demands more consecutive failures before an instance is marked unhealthy. Together, these settings build a longer smoothing window: sporadic errors such as brief 503s during a rolling deploy or dependency hiccup will not immediately cause a healthy instance to be replaced. The instance remains in service and in rotation until the failures are sustained over an extended period, which is exactly the desired behavior when the goal is to reduce replacement churn. The trade-off is that genuinely dead instances take longer to detect, but that is acceptable in many critical applications that favor stability over instantaneous failover.
- ✗
Decrease the health check interval and decrease the healthy threshold
Why it's wrong here
Decreasing the health-check interval makes the ALB send probes more frequently, and decreasing the healthy threshold means an instance can return to the healthy state after only one or two successful responses. This combination makes the system extremely jumpy: a short-lived error can quickly push the instance to unhealthy, and a quick recovery can pull it right back, causing flapping and repeated replacements. It also increases the volume of health-check requests, which becomes additional application traffic and can itself contribute to the load that triggers 503s. Since the stated problem is to decrease replacements, this is the opposite of the corrective action needed; it amplifies sensitivity rather than smoothing it out.
Quick reference
Cloud Service Model Comparison
| Model | You Manage | Provider Manages | Examples |
|---|---|---|---|
| IaaS | OS, runtime, apps, data | Hardware, hypervisor, networking | EC2, Azure VMs, GCP Compute Engine |
| PaaS | Apps and data | OS, runtime, middleware, hardware | Elastic Beanstalk, Azure App Service |
| SaaS | Data and settings only | Everything else | Microsoft 365, Salesforce, Workday |
| FaaS / Serverless | Function code only | Infra, scaling, runtime | Lambda, Azure Functions, Cloud Run |
| CaaS | Containers and apps | Kubernetes, OS, hardware | EKS, AKS, GKE |
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This DOP-C02 practice question is part of Courseiva's free Amazon Web Services 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 DOP-C02 exam.