How do you implement a job queue with priority and concurrency control?

Question

Accepted Answer

**Priority queue with concurrency** — process jobs by priority with limits.

```javascript
class PriorityQueue {
    constructor(concurrency = 3) {
        this.queue = [];
        this.running = 0;
        this.concurrency = concurrency;
    }

add(job, priority = 0) {
        return new Promise((resolve, reject) => {
            this.queue.push({ job, priority, resolve, reject });
            this.queue.sort((a, b) => b.priority - a.priority);
            this.process();
        });
    }

async process() {
        while (this.running < this.concurrency && this.queue.length > 0) {
            const item = this.queue.shift();
            this.running++;

try {
                const result = await item.job();
                item.resolve(result);
            } catch (err) {
                item.reject(err);
            } finally {
                this.running--;
                this.process();
            }
        }
    }
}

// Usage
const queue = new PriorityQueue(3);

queue.add(() => sendEmail(data), 10);  // high priority
queue.add(() => processImage(data), 5); // medium
queue.add(() => generateReport(data), 1); // low
```

**BullMQ alternative:**
```javascript
const queue = new Queue("jobs");
await queue.add("task", data, { priority: 10 });
```

Node.js: How do you implement a job queue with priority and concurrency control?