How do you implement a custom service discovery client?

Question

Accepted Answer

**Service discovery** — find and connect to services dynamically.

```javascript
const Consul = require("consul");

class ServiceDiscovery {
    constructor() {
        this.consul = new Consul();
        this.cache = new Map();
    }

async register(name, address, port) {
        await this.consul.agent.service.register({
            name,
            address,
            port,
            check: {
                http: `http://${address}:${port}/health`,
                interval: "10s",
            },
        });
    }

async discover(name) {
        const cached = this.cache.get(name);
        if (cached && Date.now() - cached.timestamp < 5000) {
            return cached.instances;
        }

const instances = await this.consul.health.service(name, {
            passing: true,
        });

const healthy = instances
            .filter(i => i.Service.Meta.healthy !== "false")
            .map(i => ({
                address: i.Service.Address,
                port: i.Service.Port,
            }));

this.cache.set(name, {
            instances: healthy,
            timestamp: Date.now(),
        });

return healthy;
    }

async getUrl(serviceName, path) {
        const instances = await this.discover(serviceName);
        if (!instances.length) throw new Error("No instances");
        const instance = instances[Math.floor(Math.random() * instances.length)];
        return `http://${instance.address}:${instance.port}${path}`;
    }
}

// Usage
const discovery = new ServiceDiscovery();
await discovery.register("user-service", "localhost", 3001);

const url = await discovery.getUrl("user-service", "/users");
const response = await fetch(url);
```

Node.js: How do you implement a custom service discovery client?