Sticky Sessions: Pinning a User to a Server

Sticky sessions (session affinity) pin a user's requests to a single server in a scaled-out application. This load balancing strategy is essential for stateful services like Socket.IO, where a connection and its state live on one machine. The load balancer uses the client's IP or a cookie to ensure all subsequent requests from that user land on the correct server. The footgun is forgetting to enable it, causing users to lose their sessions as they're bounced between servers, leading to broken connections.