SAP-C02 Continuous Improvement for Existing Solutions Practice Question
A company is running a critical microservices application on Amazon ECS with Fargate launch type. The application uses an Application Load Balancer (ALB) to distribute traffic. Recently, the team noticed that the ALB's 5xx error rate has increased. The error is HTTP 503. The team suspects the target group is unhealthy. Which THREE steps should the team take to diagnose and resolve the issue?
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
✓
Verify the ECS service's desired count and compare with the number of healthy tasks in the target group.
Option A is correct because an HTTP 503 from an ALB typically means the target group has no healthy targets; comparing the ECS service's desired count against the number of healthy targets registered in the target group immediately reveals whether tasks are failing to register or failing health checks. Option C is correct because ECS service events and task status (via the ECS console, describe-services, or describe-tasks) show why tasks are stopping, restarting, or failing to reach a steady state, which is the root cause of unhealthy targets. Option E is correct because ALB access logs record the 503 responses and target health, while the target group health check settings (path, port, protocol, interval, timeout, healthy/unhealthy thresholds, and success codes) determine whether tasks are marked healthy; misconfigured health checks are a common cause of 503s. Option B is not appropriate because replacing the ALB with an NLB does not bypass the underlying problem and NLB health checks would still mark targets unhealthy, plus it changes the architecture unnecessarily. Option D is not relevant because sticky sessions (session affinity) affect request routing to already-healthy targets and do not cause or resolve 503 errors from an unhealthy target group.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Verify the ECS service's desired count and compare with the number of healthy tasks in the target group.
Why this is correct
Comparing the ECS service's desired count against healthy targets in the target group exposes capacity shortfalls: if tasks fail ALB health checks, they are deregistered, leaving too few healthy targets and triggering HTTP 503 responses. This directly addresses the suspected unhealthy target group constraint in the stem.
- ✗
Replace the ALB with a Network Load Balancer (NLB) to bypass health checks.
Why it's wrong here
An NLB operates at layer 4 and still performs health checks, so it neither bypasses them nor explains the 503s; the ALB is not the fault. NLBs are tempting for raw TCP/UDP throughput or static IPs, and would be correct if the workload needed ultra-low latency layer-4 routing rather than HTTP-aware load balancing.
- ✓
Check the ECS service events and task status to ensure tasks are running and passing health checks.
Why this is correct
ECS service events expose task-level failures such as image pull errors, port mismatches, or failed ELB health checks, while task status confirms whether targets registered with the target group are actually running. Since Fargate tasks have no host access, these control-plane signals are the primary way to identify why the ALB returns 503.
- ✗
Verify that sticky sessions (session affinity) are enabled on the target group.
Why it's wrong here
Sticky sessions affect which healthy target receives a request, not whether targets pass health checks, so enabling affinity cannot clear 503s caused by unhealthy targets. Session affinity suits stateful applications needing a user pinned to one backend, not diagnosing target-group health failures.
- ✓
Check the ALB access logs and health check settings for the target group.
Why this is correct
ALB access logs record target response codes and timing, while target group health check settings reveal why targets are marked unhealthy, such as wrong path or port. Together they pinpoint the cause of the HTTP 503 responses.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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