How do you implement a rate-limited notification system?

Question

Accepted Answer

**Rate-limited notifications** — send emails/SMS without exceeding limits.

```javascript
const BullMQ = require("bullmq");

class NotificationService {
    constructor() {
        this.emailQueue = new BullMQ.Queue("emails", {
            limiter: {
                max: 100,        // max jobs
                duration: 60000, // per minute
            },
        });

this.smsQueue = new BullMQ.Queue("sms", {
            limiter: {
                max: 10,
                duration: 60000,
            },
        });
    }

async sendEmail(to, subject, body) {
        return this.emailQueue.add("send", {
            to, subject, body,
        }, {
            attempts: 3,
            backoff: { type: "exponential", delay: 5000 },
        });
    }

async sendSMS(to, message) {
        return this.smsQueue.add("send", { to, message });
    }
}

// Consumer
const emailWorker = new BullMQ.Worker("emails", async (job) => {
    const { to, subject, body } = job.data;
    await transporter.sendMail({ from: "noreply@app.com", to, subject, html: body });
});

// Per-user rate limiting
const userLimits = new Map();

async function canSend(userId, type, maxPerHour) {
    const key = `${userId}:${type}`;
    const count = userLimits.get(key) || 0;
    if (count >= maxPerHour) return false;
    userLimits.set(key, count + 1);
    return true;
}

app.post("/notify", async (req, res) => {
    if (!await canSend(req.user.id, "email", 10)) {
        return res.status(429).json({ error: "Too many notifications" });
    }
    await notificationService.sendEmail(req.user.email, "Update", "New content");
    res.json({ sent: true });
});
```

Node.js: How do you implement a rate-limited notification system?