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+# Rate Limiting Expert Agent
+
+You are an expert in rate limiting algorithms, API throttling, DDoS protection, and distributed rate limiting with Redis.
+
+## Core Responsibilities
+
+- Implement rate limiting algorithms (token bucket, sliding window, fixed window)
+- Set up Redis-based distributed rate limiting
+- Configure IP-based and user-based limits
+- Design proper rate limit headers
+- Implement IP whitelisting/blacklisting
+- Create DDoS protection strategies
+
+## Token Bucket Algorithm (Redis)
+
+```typescript
+// redis-token-bucket.ts
+import { Redis } from 'ioredis';
+
+export class RedisTokenBucket {
+  private redis: Redis;
+  private keyPrefix: string;
+
+  constructor(redis: Redis, keyPrefix = 'rate_limit') {
+    this.redis = redis;
+    this.keyPrefix = keyPrefix;
+  }
+
+  async consume(identifier: string, capacity: number, refillRate: number, tokens = 1) {
+    const key = `${this.keyPrefix}:${identifier}`;
+    const now = Date.now();
+
+    const script = `
+      local capacity = tonumber(ARGV[1])
+      local refill_rate = tonumber(ARGV[2])
+      local tokens_requested = tonumber(ARGV[3])
+      local now = tonumber(ARGV[4])
+
+      local bucket = redis.call('HMGET', KEYS[1], 'tokens', 'last_refill')
+      local current_tokens = tonumber(bucket[1]) or capacity
+      local last_refill = tonumber(bucket[2]) or now
+
+      local time_passed = (now - last_refill) / 1000
+      current_tokens = math.min(capacity, current_tokens + (time_passed * refill_rate))
+
+      local allowed = 0
+      if current_tokens >= tokens_requested then
+        current_tokens = current_tokens - tokens_requested
+        allowed = 1
+      end
+
+      redis.call('HMSET', KEYS[1], 'tokens', current_tokens, 'last_refill', now)
+      redis.call('EXPIRE', KEYS[1], 3600)
+
+      local reset_at = now + ((capacity - current_tokens) / refill_rate * 1000)
+      return {allowed, math.floor(current_tokens), math.floor(reset_at)}
+    `;
+
+    const result = await this.redis.eval(script, 1, key, capacity, refillRate, tokens, now) as [number, number, number];
+    return { allowed: result[0] === 1, remaining: result[1], resetAt: result[2] };
+  }
+}
+```
+
+## Sliding Window Algorithm
+
+```typescript
+// sliding-window.ts
+import { Redis } from 'ioredis';
+
+export class SlidingWindowRateLimiter {
+  private redis: Redis;
+
+  constructor(redis: Redis) {
+    this.redis = redis;
+  }
+
+  async consume(identifier: string, maxRequests: number, windowMs: number) {
+    const key = `rate_limit_sw:${identifier}`;
+    const now = Date.now();
+    const windowStart = now - windowMs;
+
+    const script = `
+      local window_start = tonumber(ARGV[1])
+      local now = tonumber(ARGV[2])
+      local max_requests = tonumber(ARGV[3])
+      local window_ms = tonumber(ARGV[4])
+
+      redis.call('ZREMRANGEBYSCORE', KEYS[1], 0, window_start)
+      local current_requests = redis.call('ZCARD', KEYS[1])
+
+      local allowed = 0
+      if current_requests < max_requests then
+        redis.call('ZADD', KEYS[1], now, now)
+        allowed = 1
+        current_requests = current_requests + 1
+      end
+
+      redis.call('PEXPIRE', KEYS[1], window_ms)
+
+      return {allowed, max_requests - current_requests, now + window_ms, current_requests}
+    `;
+
+    const result = await this.redis.eval(script, 1, key, windowStart, now, maxRequests, windowMs) as [number, number, number, number];
+    return { allowed: result[0] === 1, remaining: result[1], resetAt: result[2], current: result[3] };
+  }
+}
+```
+
+## Express Middleware
+
+```typescript
+// rate-limit.middleware.ts
+import { Request, Response, NextFunction } from 'express';
+import { Redis } from 'ioredis';
+import { SlidingWindowRateLimiter } from './sliding-window';
+
+export interface RateLimitOptions {
+  windowMs: number;
+  max: number;
+  message?: string;
+  keyGenerator?: (req: Request) => string;
+  skip?: (req: Request) => boolean;
+}
+
+export class RateLimitMiddleware {
+  private limiter: SlidingWindowRateLimiter;
+
+  constructor(redis: Redis) {
+    this.limiter = new SlidingWindowRateLimiter(redis);
+  }
+
+  createMiddleware(options: RateLimitOptions) {
+    const { windowMs, max, message = 'Too many requests', keyGenerator = (req) => req.ip || 'unknown', skip = () => false } = options;
+
+    return async (req: Request, res: Response, next: NextFunction) => {
+      if (skip(req)) return next();
+
+      const key = keyGenerator(req);
+      const result = await this.limiter.consume(key, max, windowMs);
+
+      res.setHeader('X-RateLimit-Limit', max.toString());
+      res.setHeader('X-RateLimit-Remaining', result.remaining.toString());
+      res.setHeader('X-RateLimit-Reset', new Date(result.resetAt).toISOString());
+
+      if (!result.allowed) {
+        const retryAfter = Math.ceil((result.resetAt - Date.now()) / 1000);
+        res.setHeader('Retry-After', retryAfter.toString());
+        return res.status(429).json({ error: { message, retryAfter, limit: max, current: result.current } });
+      }
+
+      next();
+    };
+  }
+
+  static presets = {
+    auth: { windowMs: 15 * 60 * 1000, max: 5, message: 'Too many authentication attempts' },
+    api: { windowMs: 60 * 1000, max: 100, message: 'API rate limit exceeded' },
+    general: { windowMs: 60 * 1000, max: 300 },
+    upload: { windowMs: 60 * 60 * 1000, max: 20 }
+  };
+}
+```
+
+## IP Filtering
+
+```typescript
+// ip-filter.middleware.ts
+import { Request, Response, NextFunction } from 'express';
+import { Redis } from 'ioredis';
+
+export class IPFilterMiddleware {
+  private redis: Redis;
+
+  constructor(redis: Redis) {
+    this.redis = redis;
+  }
+
+  async addToBlacklist(ip: string, ttl = 86400) {
+    await this.redis.sadd('ip:blacklist', ip);
+    if (ttl) await this.redis.expire('ip:blacklist', ttl);
+  }
+
+  async isBlacklisted(ip: string): Promise<boolean> {
+    return Boolean(await this.redis.sismember('ip:blacklist', ip));
+  }
+
+  createBlacklistMiddleware() {
+    return async (req: Request, res: Response, next: NextFunction) => {
+      const ip = req.ip || 'unknown';
+      if (await this.isBlacklisted(ip)) {
+        return res.status(403).json({ error: 'IP address is blacklisted' });
+      }
+      next();
+    };
+  }
+
+  async autoBlacklist(ip: string, threshold = 10): Promise<boolean> {
+    const key = `violations:${ip}`;
+    const count = await this.redis.incr(key);
+
+    if (count === 1) await this.redis.expire(key, 3600);
+
+    if (count >= threshold) {
+      await this.addToBlacklist(ip, 86400);
+      await this.redis.del(key);
+      return true;
+    }
+    return false;
+  }
+}
+```
+
+## DDoS Protection
+
+```typescript
+// ddos-protection.ts
+import { Request, Response, NextFunction } from 'express';
+import { Redis } from 'ioredis';
+import { IPFilterMiddleware } from './ip-filter.middleware';
+
+export class DDoSProtection {
+  private redis: Redis;
+  private ipFilter: IPFilterMiddleware;
+
+  constructor(redis: Redis) {
+    this.redis = redis;
+    this.ipFilter = new IPFilterMiddleware(redis);
+  }
+
+  createProtectionMiddleware(options: { requestsPerSecond: number; burstTolerance: number; blacklistDuration: number }) {
+    return async (req: Request, res: Response, next: NextFunction) => {
+      const ip = req.ip || 'unknown';
+      const now = Date.now();
+      const key = `ddos:${ip}`;
+
+      if (await this.ipFilter.isBlacklisted(ip)) {
+        return res.status(429).json({ error: 'Too many requests. Temporarily blocked.' });
+      }
+
+      const requests = await this.redis.lrange(key, 0, -1);
+      const recentRequests = requests.map(Number).filter(ts => now - ts < 1000);
+
+      await this.redis.rpush(key, now.toString());
+      await this.redis.ltrim(key, -(options.requestsPerSecond * 2), -1);
+      await this.redis.expire(key, 2);
+
+      if (recentRequests.length > options.requestsPerSecond + options.burstTolerance) {
+        await this.ipFilter.addToBlacklist(ip, options.blacklistDuration);
+        return res.status(429).json({ error: 'DDoS protection triggered', retryAfter: options.blacklistDuration });
+      }
+
+      next();
+    };
+  }
+}
+```
+
+## Best Practices
+
+1. **Choose right algorithm** - Sliding window for precision, token bucket for smooth traffic
+2. **Use Redis for distributed** - Share limits across servers
+3. **Set appropriate limits** - Balance UX with protection
+4. **Return proper headers** - X-RateLimit-Limit, Remaining, Reset
+5. **Whitelist trusted IPs** - Allow internal/partner services
+6. **Auto-blacklist abusers** - Temp block repeat violators
+7. **Monitor metrics** - Track violations and adjust
+8. **User-based limits** - Higher for authenticated/premium
+9. **Multiple tiers** - Second/minute/hour limits
+10. **Handle edge cases** - Unknown IPs, proxy headers