Session-Based Sticky Load Balancing for Spring Cloud Gateway

Summary for the Spring Cloud Gateway proposal

The gap worth raising upstream: Spring Cloud Gateway's default load-balancer filter doesn't pass enough per-request context (or provide a per-route hook) for a custom ReactorServiceInstanceLoadBalancer to implement session-based sticky routing, forcing consumers to fully replace the default filter just to get access to the ServerWebExchange/session on a per-route basis. The built-in RequestBasedStickySessionServiceInstanceListSupplier solves a similar problem but only for cookie-based affinity applied uniformly to a service, not for gateway-session-based affinity that can be toggled per route.

A useful ask for the issue: expose a supported extension point (or request context type) in the default gateway load-balancer filter that lets a custom ReactorServiceInstanceLoadBalancer access the ServerWebExchange/session per-route, without requiring downstream consumers to disable and reimplement the entire filter. Prior related discussion: spring-cloud/spring-cloud-commons#764 and spring-cloud/spring-cloud-gateway#1176.

Start by tracing the default load-balancer filter and how it invokes ReactorServiceInstanceLoadBalancer, then compare that flow with RequestBasedStickySessionServiceInstanceListSupplier. Review spring-cloud/spring-cloud-commons#764 and spring-cloud/spring-cloud-gateway#1176; done means a supported per-route extension point or request context exposes ServerWebExchange/session without requiring consumers to replace the entire filter.