AWS Certified Solutions Architect - Associate
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A company is building a new furniture inventory application. The company has deployed the application on a fleet of Amazon EC2 instances across multiple Availability Zones. The EC2 instances run behind an Application Load Balancer (ALB) in their VPC.
A solutions architect has observed that incoming traffic seems to favor one EC2 instance, resulting in latency for some requests.
What should the solutions architect do to resolve this issue?
Other
Community
UAnonymous
A
Disable session affinity (sticky sessions) on the ALB
B
Replace the ALB with a Network Load Balancer
C
Increase the number of EC2 instances in each Availability Zone
D
Adjust the frequency of the health checks on the ALB's target group
Explanation:
Explanation
When traffic seems to favor one EC2 instance behind an Application Load Balancer (ALB), this is typically caused by session affinity (sticky sessions) being enabled. Here's why:
How Sticky Sessions Work
- Session affinity ensures that requests from a particular client are always routed to the same target instance.
- This is useful for applications that store session data locally on the instance.
- However, it can lead to uneven traffic distribution if some clients generate more requests than others.
The Problem
- The question states that traffic "favors one EC2 instance" - this is a classic symptom of sticky sessions.
- When sticky sessions are enabled, all requests from a particular client go to the same instance, which can create load imbalances.
- This results in latency for some requests because the overloaded instance cannot handle all traffic efficiently.
Why Other Options Are Incorrect
B. Replace the ALB with a Network Load Balancer
- Network Load Balancers (NLBs) operate at Layer 4 and don't support session affinity at the application layer.
- However, replacing the ALB is unnecessary and disruptive when the issue can be resolved by simply disabling sticky sessions.
C. Increase the number of EC2 instances in each Availability Zone
- Adding more instances doesn't solve the root cause of uneven distribution.
- The traffic would still favor certain instances due to sticky sessions.
- This would be an inefficient use of resources.
D. Adjust the frequency of the health checks on the ALB's target group
- Health check frequency affects how quickly the ALB detects unhealthy instances.
- It doesn't address traffic distribution patterns or session affinity issues.
Best Practice
- Disable sticky sessions unless your application specifically requires them.
- For applications that need session persistence, consider using Amazon ElastiCache or Amazon DynamoDB to store session data externally.
- This allows for true load balancing across all instances while maintaining session state.
Additional Considerations
- If the application requires session persistence, you could:
- Use shorter sticky session durations
- Implement client-side session storage
- Use distributed session stores
- Monitor the load distribution after disabling sticky sessions to ensure traffic is evenly distributed.
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