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:
  1. Use shorter sticky session durations
  2. Implement client-side session storage
  3. Use distributed session stores
• Monitor the load distribution after disabling sticky sessions to ensure traffic is evenly distributed.