npm install bullmq

// queue.ts — add jobs from your API handler
import { Queue } from 'bullmq';

export const emailQueue = new Queue('emails', {
  connection: { host: 'localhost', port: 6379 },
  defaultJobOptions: {
    attempts: 3,
    backoff: { type: 'exponential', delay: 2000 },
  },
});

// In your Express/Fastify route:
app.post('/api/signup', async (req, res) => {
  const { email, name } = req.body;

  // Create user in DB synchronously
  const user = await db.users.create({ email, name });

  // Enqueue welcome email — don't block the HTTP response
  await emailQueue.add('welcome-email', { userId: user.id, email, name });

  res.status(201).json({ id: user.id });
});

// worker.ts — run in a separate process
import { Worker } from 'bullmq';
import { sendWelcomeEmail } from './mailer';

const worker = new Worker(
  'emails',
  async (job) => {
    if (job.name === 'welcome-email') {
      await sendWelcomeEmail(job.data.email, job.data.name);
      console.log(`Welcome email sent to ${job.data.email}`);
    }
  },
  {
    connection: { host: 'localhost', port: 6379 },
    concurrency: 5, // process up to 5 jobs simultaneously
  }
);

worker.on('failed', (job, err) => {
  console.error(`Job ${job?.id} failed:`, err.message);
});

// scheduler.ts — recurring jobs (like cron)
import { QueueScheduler } from 'bullmq';

const scheduler = new QueueScheduler('emails', {
  connection: { host: 'localhost', port: 6379 },
});

await emailQueue.add(
  'weekly-digest',
  { type: 'digest' },
  { repeat: { pattern: '0 9 * * MON' } } // every Monday at 9am
);