SAP-C02 Continuous Improvement for Existing Solutions Practice Question
A company is running a stateful web application on EC2 instances in an Auto Scaling group. Users report that their sessions are lost when instances are terminated during scale-in. What should a solutions architect do to preserve session state?
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
✓
Store session state in Amazon ElastiCache.
ElastiCache provides a centralized, fast session store that persists across instance terminations, ensuring session state is retained even when instances are scaled in. Option A is wrong because using lifecycle hooks to save session data to Amazon S3 is unreliable due to the high latency and the risk that the termination process may not complete the save in time. Option B is wrong because sticky sessions (session affinity) on the Application Load Balancer can cause uneven load distribution and still result in session loss if all instances in a target group are replaced or if the specific instance with the session is terminated. Option D is wrong because increasing the Auto Scaling group's cooldown period only delays the next scaling activity but does not preserve session state when instances are eventually terminated.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Use lifecycle hooks to save session data to Amazon S3 before instance termination.
Why it's wrong here
Lifecycle hooks fire on termination and cannot persist sessions for requests already in flight, so users still lose state during scale-in. They suit graceful draining, deregistration or cleanup tasks, not continuous session storage that must survive instance replacement.
- ✗
Enable sticky sessions (session affinity) on the Application Load Balancer.
Why it's wrong here
Session affinity pins a client to one instance, so terminating that instance still destroys the session; it cannot preserve state across scale-in. Sticky sessions suit short-lived state or caching on stable fleets, not sessions that must survive instance termination.
- ✓
Store session state in Amazon ElastiCache.
Why this is correct
ElastiCache externalises session data from instance memory into a shared Redis or Memcached tier, so any instance in the Auto Scaling group can serve subsequent requests. Scale-in termination no longer discards sessions, satisfying the requirement to preserve state.
- ✗
Increase the Auto Scaling group's cooldown period to prevent rapid scaling.
Why it's wrong here
Cooldown merely delays subsequent scaling activities; it does not stop the terminating instance from discarding its in-memory sessions. Cooldown periods suit smoothing flapping metrics and avoiding thrashing, not preserving state that must outlive any individual instance.
Quick reference
AWS S3 Storage Class Comparison
| Storage Class | Min Duration | Retrieval | Use Case |
|---|---|---|---|
| S3 Standard | None | Immediate | Frequently accessed data |
| S3 Standard-IA | 30 days | Immediate | Infrequent access, rapid retrieval |
| S3 One Zone-IA | 30 days | Immediate | Non-critical infrequent data |
| S3 Intelligent-Tiering | None | Immediate–hours | Unknown or changing access patterns |
| S3 Glacier Instant | 90 days | Milliseconds | Archive with instant retrieval |
| S3 Glacier Flexible | 90 days | Minutes–hours | Archive, flexible retrieval |
| S3 Glacier Deep Archive | 180 days | Hours | Long-term compliance archive |
Go deeper
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Written by Johnson Ajibi, MSc IT Security
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