The correct answer is that the ALB health check configuration is incorrect, specifically the health check path or port. When an Application Load Balancer returns a 503 error due to health check failure, it means the ALB considers all registered targets unhealthy, even though the EC2 instances pass system status checks and are running. The HealthyHostCount metric drops to zero because the ALB’s health checks are not receiving a successful response from the application on the configured path or port—for example, a 200 OK from /health or /index.html. On the AWS Certified SysOps Administrator Associate SOA-C02 exam, this scenario tests your understanding that ALB health checks are application-level, not instance-level; a common trap is assuming a running instance with passed EC2 checks guarantees application availability. The administrator should verify the health check path and port in the target group settings, ensuring the application responds correctly. Memory tip: “HealthyHostCount zero? Check the path, not the instance.”
SOA-C02 Reliability and Business Continuity Practice Question
This SOA-C02 practice question tests your understanding of reliability and business continuity. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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 company runs a web application on EC2 instances in an Auto Scaling group across two Availability Zones. The application uses an Application Load Balancer. The SysOps administrator receives an alert that the application is returning 503 errors. The administrator checks the CloudWatch metrics and sees that the ALB's RequestCount is normal, but the HealthyHostCount is zero. The EC2 instances are in running state and pass the EC2 status checks. What is the MOST likely cause and what should the administrator do to resolve the issue?
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.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The ALB health check configuration is incorrect; verify the health check path and port.
Option C is correct. The ALB health checks are failing. The most likely cause is that the health check path is wrong or the application is not responding on the configured port. The administrator should verify the health check settings. Option A is wrong because the instances are running and passing EC2 checks. Option B is wrong because security group rules would affect connectivity, but the instances are running. Option D is wrong because scaling policies do not affect health checks.
Key principle: NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
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 ALB health check configuration is incorrect; verify the health check path and port.
Why this is correct
Health checks fail, causing HealthyHostCount to be zero.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Static NAT maps one inside address to one outside address.
✗
The instances are out of memory; increase the instance size.
Why it's wrong here
Instances pass EC2 status checks, so they are not out of memory.
✗
The Auto Scaling group's scaling policy is not launching new instances; update the policy.
Why it's wrong here
Instances are already running.
✗
The security group on the instances is blocking traffic from the ALB; update the security group.
Why it's wrong here
If security group was blocking, instances would not receive traffic, but they are running.
Common exam traps
Common exam trap: NAT rules depend on direction and matching traffic
NAT is not only about the public address. The inside/outside interface roles and the ACL or rule that matches traffic are just as important.
Detailed technical explanation
How to think about this question
NAT questions usually test address translation, overload/PAT behaviour, static mappings and whether the right traffic is being translated. Read the interface direction and address terms carefully.
KKey Concepts to Remember
Static NAT maps one inside address to one outside address.
PAT allows many inside hosts to share one public address using ports.
Inside local and inside global describe the private and translated addresses.
NAT ACLs identify traffic for translation, not always security filtering.
TExam Day Tips
→Identify inside and outside interfaces first.
→Check whether the scenario needs static NAT, dynamic NAT or PAT.
→Do not confuse NAT matching ACLs with normal packet-filtering intent.
Key takeaway
NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
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.
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related SOA-C02 NAT questions on configuration and troubleshooting.
Reliability and Business Continuity — This question tests Reliability and Business Continuity — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: The ALB health check configuration is incorrect; verify the health check path and port. — Option C is correct. The ALB health checks are failing. The most likely cause is that the health check path is wrong or the application is not responding on the configured port. The administrator should verify the health check settings. Option A is wrong because the instances are running and passing EC2 checks. Option B is wrong because security group rules would affect connectivity, but the instances are running. Option D is wrong because scaling policies do not affect health checks.
What should I do if I get this SOA-C02 question wrong?
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related SOA-C02 NAT questions on configuration and troubleshooting.
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?
Static NAT maps one inside address to one outside address.
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