- A
The load balancer is not attached to a subnet.
Why wrong: Incorrect because the load balancer must be attached to subnets to function; if not, it would fail to provision, but the ALB is already running and returning 503, so this is not a likely cause.
- B
The security group for the load balancer is blocking traffic.
Correct because the load balancer's security group must allow inbound traffic from clients. If it blocks HTTP/HTTPS traffic, the ALB cannot forward requests and returns 503 errors.
- C
The load balancer has reached its capacity limit.
Correct because an ALB has a capacity limit; once reached, it cannot handle new requests and returns 503 errors.
- D
The target group has no registered targets.
Why wrong: Incorrect because the stem explicitly states the target group has healthy EC2 instances, meaning registered targets exist; 'no registered targets' directly contradicts the given information.
- E
The target group health check is misconfigured.
Why wrong: Correct because a misconfigured health check can cause the ALB to consider all targets unhealthy, even if the instances are healthy, leading to 503 errors.
ALB 503 Errors with Healthy Targets: Possible Causes
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. A key principle to apply: application Load Balancer (ALB). 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 SysOps administrator is troubleshooting an issue where an Application Load Balancer (ALB) is returning 503 errors to clients. The target group has healthy EC2 instances. Which THREE possible causes should the administrator investigate? (Choose three.)
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 security group for the load balancer is blocking traffic.
Option A is correct because if the load balancer is not attached to a subnet, it cannot route traffic to targets and may return 503 errors. Option B is correct because the load balancer's security group must allow inbound traffic from clients; if it blocks HTTP/HTTPS traffic, the ALB cannot forward requests and returns 503 errors. Option C is correct because an ALB has a capacity limit; once reached, it cannot handle new requests and returns 503 errors. Option D is incorrect because the stem explicitly states the target group has healthy EC2 instances, meaning registered targets exist. Option E is incorrect because the health check is functioning correctly since targets are healthy; a misconfigured health check would cause targets to be unhealthy, contradicting the given information.
Key principle: Application Load Balancer (ALB)
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 is not attached to a subnet.
Why it's wrong here
Incorrect because the load balancer must be attached to subnets to function; if not, it would fail to provision, but the ALB is already running and returning 503, so this is not a likely cause.
- ✓
The security group for the load balancer is blocking traffic.
Why this is correct
Correct because the load balancer's security group must allow inbound traffic from clients. If it blocks HTTP/HTTPS traffic, the ALB cannot forward requests and returns 503 errors.
Related concept
Application Load Balancer (ALB)
- ✓
The load balancer has reached its capacity limit.
Why this is correct
Correct because an ALB has a capacity limit; once reached, it cannot handle new requests and returns 503 errors.
Related concept
Application Load Balancer (ALB)
- ✗
The target group has no registered targets.
Why it's wrong here
Incorrect because the stem explicitly states the target group has healthy EC2 instances, meaning registered targets exist; 'no registered targets' directly contradicts the given information.
- ✗
The target group health check is misconfigured.
Why it's wrong here
Correct because a misconfigured health check can cause the ALB to consider all targets unhealthy, even if the instances are healthy, leading to 503 errors.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap is that candidates often assume a 503 error always indicates unhealthy targets, but the question explicitly states healthy instances, so they must consider other causes like security group misconfiguration, ALB capacity limits, or the load balancer not being attached to a subnet.
Detailed technical explanation
How to think about this question
Under the hood, an ALB returns a 503 error when it cannot route traffic to any healthy target in the target group, or when it itself is overwhelmed or blocked. The ALB's security group acts as a first-line filter; if it denies inbound traffic, the ALB never processes the request, leading to a 503. Capacity limits (option C) are a separate cause—ALBs have a default limit of 1,000 connections per second per Availability Zone, and exceeding this triggers a 503 due to 'target group capacity exceeded' metrics in CloudWatch.
KKey Concepts to Remember
- Application Load Balancer (ALB)
- Security groups
- ALB capacity
- Target group health checks
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
Application Load Balancer (ALB)
Real-world example
How this comes up in practice
A healthcare organisation deploys an application with a public-facing web tier and a private database tier. The database subnet has no public IP and only accepts connections from the web tier's security group. Questions like this test whether you can design cloud network isolation using VNets/VPCs, subnets, and security group rules.
Visual reference
What to study next
Got this wrong? Here's your next step.
Review application Load Balancer (ALB), then practise related SOA-C02 questions on the same topic to reinforce the concept.
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FAQ
Questions learners often ask
What does this SOA-C02 question test?
Reliability and Business Continuity — This question tests Reliability and Business Continuity — Application Load Balancer (ALB).
What is the correct answer to this question?
The correct answer is: The security group for the load balancer is blocking traffic. — Option A is correct because if the load balancer is not attached to a subnet, it cannot route traffic to targets and may return 503 errors. Option B is correct because the load balancer's security group must allow inbound traffic from clients; if it blocks HTTP/HTTPS traffic, the ALB cannot forward requests and returns 503 errors. Option C is correct because an ALB has a capacity limit; once reached, it cannot handle new requests and returns 503 errors. Option D is incorrect because the stem explicitly states the target group has healthy EC2 instances, meaning registered targets exist. Option E is incorrect because the health check is functioning correctly since targets are healthy; a misconfigured health check would cause targets to be unhealthy, contradicting the given information.
What should I do if I get this SOA-C02 question wrong?
Review application Load Balancer (ALB), then practise related SOA-C02 questions on the same topic to reinforce the concept.
What is the key concept behind this question?
Application Load Balancer (ALB)
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Last reviewed: Jul 4, 2026
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