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Sticky Sessions (Session Affinity)

Ensures the same client is always routed to the same backend instance

Supported by CLB, ALB, and NLB

For CLB and ALB: implemented with cookies (configurable expiration time)

Use case: Maintain user session data without re-authentication or losing state

Trade-off: May cause uneven traffic distribution between instances

Cookie Types

Application-based Cookies

Custom Cookie

Generated by target application

Can include any required custom attributes

Name must be set per target group

Cannot use reserved names: AWSALB, AWSALBAPP, AWSALBTG

Application Cookie

Generated by load balancer

Name: AWSALBAPP

Duration-based Cookies

Generated by load balancer

Names:

ALB: AWSALB
CLB: AWSELB

Example Flow

Client connects → Load balancer assigns target → Cookie is set

On subsequent requests, LB routes the client to the same target until cookie expires