What stickiness means
Sticky sessions, also called session affinity, tell a load balancer to keep routing the same client to the same backend server. The balancer remembers the mapping, often by setting a cookie or hashing the client address, so every follow up request lands on the host that holds that user state.
Why anyone uses it
- A server may keep in memory session state like a shopping cart or login.
- Reusing the same host can warm local caches for that user.
The downsides
- If that server dies, its users lose their session state.
- Load can become uneven because some sticky users are heavier than others.
- Scaling and deploys are harder because you cannot freely move users.
The better default
The modern preference is stateless servers that store session data in a shared store such as a cache or database. Then any server can handle any request, the load balancer can route freely, and a dead host costs nobody their session.
Key idea
Sticky sessions pin a user to one server for local state, but stateless servers backed by a shared store scale and fail over more cleanly.