SAP-C02 Design for New Solutions Practice Question
A company runs a web application on Amazon EC2 instances behind an Application Load Balancer (ALB). They want to implement a blue/green deployment strategy with minimal impact on users. Which approach should they use?
⚠ Common exam trap
Many exam-takers confuse blue/green deployment with rolling updates or assuming that a new ALB and DNS change are required, when in fact ALB listener rule modifications provide immediate traffic switching without DNS propagation delays.
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
✓
Create a new target group with the green instances. Modify the ALB listener rule to forward traffic to the new target group.
Blue/green deployment with an ALB is achieved by creating a new target group for the green instances and modifying the ALB listener rule to forward traffic to the new target group. This allows instant traffic shifting with zero downtime, as the ALB can route traffic between target groups without changing the DNS or creating a new ALB. The blue (old) target group remains available for instant rollback if needed.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Create a new target group with the green instances. Modify the ALB listener rule to forward traffic to the new target group.
Why this is correct
Registering green instances in a new target group and repointing the ALB listener rule shifts traffic at the load balancer, giving instant cutover and rollback to blue. This achieves blue/green with minimal user impact, unlike DNS-based approaches with caching delays.
- ✗
Stop all instances, launch new instances with the new version, and update the target group.
Why it's wrong here
Stopping every instance removes all healthy targets from the load balancer, causing an outage rather than a blue/green cutover. It is tempting because it guarantees no mixed versions run, and would suit a maintenance window where downtime is acceptable, not minimal user impact.
- ✗
Update the existing target group's instances to the new version, one at a time.
Why it's wrong here
Replacing instances within one target group is a rolling update, so blue and green versions serve traffic simultaneously and rollback is slow. It is tempting because it avoids duplicate infrastructure, and would be correct when gradual in-place deployment with no instant rollback requirement is acceptable.
- ✗
Create a new ALB and update DNS to point to the new ALB.
Why it's wrong here
Swapping DNS to a new ALB leaves existing connections and cached records pointing at the old load balancer, causing uneven traffic and propagation delays. It is tempting because DNS weighting is a known pattern, but ALB listener rules with weighted target groups shift traffic without DNS changes.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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