AWS Sticky Sessions (Session Affinity) in Load Balancers

Sticky Sessions, also known as Session Affinity, allow an Elastic Load Balancer (ELB) to consistently route a client’s requests to the same backend instance. This ensures session continuity, preventing users from losing session data when making multiple requests.

How Sticky Sessions Work

• Normally, an Application Load Balancer (ALB) distributes traffic evenly across multiple backend instances.
• With sticky sessions enabled, a cookie is used to associate a client with a specific instance.
• Future requests from the same client will always be routed to the same backend instance until the cookie expires.

Use Case for Sticky Sessions

✅ Maintaining session data (e.g., user logins, shopping carts).

✅ Ensuring stateful applications function correctly without relying on external session storage.

⚠️ Potential downside: Can lead to uneven load distribution if some users generate significantly more traffic than others.

Supported Load Balancers

• Application Load Balancer (ALB)
• Network Load Balancer (NLB)
• Classic Load Balancer (CLB)

Types of Cookies in Sticky Sessions

1️⃣ Application-Based Cookie

• Generated by the application itself (not by the load balancer).