Service Discovery — Notes

Functional

• Register / deregister service instances.
• Health-checked endpoints.
• Lookup by service name (DNS or API).
• Watch for changes (push update on roster change).
• Tags / metadata for routing decisions.

Non-functional

• Sub-second propagation typical.
• Registry HA + survives partition.
• Lookups must be fast (cached client-side).

Capacity

• Tens of thousands of services × instances.
• High read fan-out from clients.

Models

• Client-side: clients query registry, load balance themselves.
• Server-side: gateway / LB consults registry; clients hit VIP.
• DNS-based: SRV records; simple but slow change propagation.
• xDS (Envoy): explicit subscribe + push.

Trade-offs

• CP vs AP registry: Consul (CP) vs Eureka (AP) — depends on whether stale routing is acceptable.
• Heartbeat-based TTLs vs active health checks: combine for safety.
• Sidecar mesh simplifies app code but adds operational layer.

Refs

• Consul, etcd, Eureka, ZooKeeper docs.
• "Service Discovery in a Microservices Architecture" Chris Richardson.
• Envoy xDS protocol spec.