Node.js
Service discovery
Learn the service discovery pattern.
Service discovery
Service discovery is the mechanism that lets services find each other. Instead of a fixed IP, each service registers itself in a registry; other services fetch its address from there. This is essential in dynamic environments (containers, auto-scaling).
Example: when the user service starts it registers with registry.register("user-service", "10.0.0.5:3000"). When the order service needs to call the user service it gets the address with registry.lookup("user-service"). Popular tools: Consul, etcd, Eureka. Without service discovery each service hard-codes the other's address, which fails in environments where the number of replicas changes. Dynamic discovery lets new replicas be called as soon as they are added.
The registry pattern
A registry is a simple dictionary that maps service names to addresses. Services register when they start and unregister when they stop. The calling service asks the registry for the current address each time.
Example: const registry = new Map(); registry.set("user-service", "10.0.0.5:3000"); registry.get("user-service");. In production this becomes a more sophisticated tool (Consul), but the core idea is the same. The registry must be highly available; otherwise all service communication stops. Health checks remove dead replicas from the registry. This lets the system self-heal.
Exercise Write a simple registry
Assign a new Map to a registry variable. Add a record with registry.set("user-service", "10.0.0.5:3000"). Fetch the address with registry.get("user-service") and print it with console.log.
Show example solution
const registry = new Map();
registry.set("user-service", "10.0.0.5:3000");
console.log(registry.get("user-service"));
Test yourself
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What is service discovery for?
registry.register(name, addr)
- Shuts down the service
- Counts requests
- Dynamically finds a service address
- Backs up the database
Show answer
Dynamically finds a service address — Service discovery finds the address dynamically.
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Complete the registry.set call
registry.set("user-service", "10.0.0.5:3000");
const addr = registry.____("user-service");
Show answer
registry.set("user-service", "10.0.0.5:3000");
const addr = registry.get("user-service");
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Match each concept to its role
- registry
- Holds addresses
- register
- Registers when service starts
- lookup
- Asks for address
- health check
- Cleans dead replicas
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