sessionaffinity

SessionAffinity is a technique used by load balancers and service proxies to route all requests from a given client to a single backend server for the duration of a session. The goal is to preserve session state and avoid repeated initialization or authentication on each request.

Common approaches to implement session affinity include: Client IP affinity, which uses the client's IP address to select and consistently route requests to the same backend; Cookie-based affinity, in which the load balancer issues a persistent cookie that binds the client to a specific backend; Session token affinity, where the application-provided token determines the backend; and hash-based affinity, which applies a deterministic hash of client identity to choose a backend while aiming to balance load.

Benefits include improved user experience and reduced server-side session retrieval costs for apps that store state on the backend. Drawbacks include skewed load distribution, reduced resilience to instance failures, and operational complexity during scaling or rolling upgrades. Client IP can be unreliable when clients are behind NAT or proxies; cookie-based methods can be affected by client settings or privacy constraints; and backend migrations can disrupt sessions unless designed for migration or session transfer.

Platform patterns vary. In Kubernetes, a Service can specify sessionAffinity: ClientIP with an optional timeoutSeconds to control affinity duration. Cloud load balancers offer stickiness through duration-based cookies or load balancer generated cookies. NGINX and HAProxy support sticky sessions via cookies or tokens, configurable per backend or route.

See also: load balancing, session persistence, sticky sessions, and stateful service design.