Service Discovery
The mechanism by which a caller finds a currently healthy network address for a service whose instances are ephemeral.
Static configuration works until instances are created and destroyed by an autoscaler, which is the normal state of a container platform. Discovery replaces it with a registry that instances register into and that health checks prune.
Two shapes. Client-side: the caller queries the registry and picks an instance itself, which allows sophisticated load-balancing but puts registry logic into every application. Server-side: the caller talks to a stable virtual address and a load balancer or platform resolves it, which is what Kubernetes Services and cloud load balancers do, and what most systems should use.
Whichever shape, the property that actually matters is how fast an unhealthy instance leaves rotation. That interval is a direct term in your availability calculation.