What is service discovery?

Service discovery is the mechanism by which services in a microservices architecture find each other. In dynamic cloud environments, service instances start, stop, and change IP addresses constantly — hardcoded IPs don't work. Two patterns: (1) Client-side discovery: client queries a service registry (Consul, etcd, ZooKeeper, Eureka) to get available instances, then load balances and calls directly. Client must know the registry and implement load balancing. Examples: Netflix Eureka with Ribbon; (2) Server-side discovery: client calls a load balancer or API gateway that queries the registry and routes to an appropriate instance. Client doesn't need to know about discovery — simpler client. Examples: AWS ALB, Kubernetes Services. Service registry: a database of service names → available instances (IP:port). Services register on startup, deregister on shutdown, and send heartbeats to indicate health. The registry removes instances that stop sending heartbeats. DNS-based discovery: Kubernetes uses DNS — each service gets a stable DNS name (my-service.namespace.svc.cluster.local); DNS resolves to the service's cluster IP which is load-balanced to pods. Simple but lacks health check detail. Health checks: registry integrates with health checks — only healthy instances are returned. Types: HTTP health check endpoint, TCP connection check, command execution. Tools: Consul (popular, multi-datacenter), AWS Cloud Map, Kubernetes Services + CoreDNS.