API Throttling and Backpressure Designer

You are a reliability engineer specializing in traffic management and system protection.

## CONTEXT
I need to implement throttling and backpressure for my API.

**Requirements:**
- Traffic Patterns: {{TRAFFIC}}
- Protection Needs: {{PROTECTION}}
- Downstream Dependencies: {{DEPENDENCIES}}
- User Experience: {{UX_REQUIREMENTS}}

## TASK
Design throttling and backpressure system:

### 1. THROTTLING STRATEGIES
**Concurrent Request Limiting:**
```typescript
const semaphore = new Semaphore(MAX_CONCURRENT);
async function handleRequest(req) {
  await semaphore.acquire();
  try {
    return await processRequest(req);
  } finally {
    semaphore.release();
  }
}
```

**Queue-Based Throttling:**
```typescript
const queue = new RequestQueue({
  maxSize: 1000,
  processRate: 100 // per second
});
```

### 2. BACKPRESSURE SIGNALS
```
Response: 503 Service Unavailable
Retry-After: 30
X-RateLimit-Remaining: 0
```

### 3. LOAD SHEDDING
```typescript
if (systemLoad > 80%) {
  // Reject lowest priority requests
  if (req.priority < PRIORITY_HIGH) {
    return res.status(503).json({ error: 'Service busy' });
  }
}
```

### 4. ADAPTIVE THROTTLING
Adjust limits based on system health.

### 5. CLIENT GUIDANCE
- Retry-After headers
- Backoff recommendations
- Priority hints

### 6. MONITORING
- Queue depth metrics
- Rejection rates
- Latency percentiles