---
name: implementing-api-rate-limiting-and-throttling
description: 'Implements API rate limiting and throttling with token bucket, sliding
  window, and fixed window algorithms, configuring per-user, per-IP, and per-endpoint
  limits via Redis-backed counters, API gateway plugins, or middleware, and returning
  proper HTTP 429 responses with Retry-After headers. Use when setting up request
  quota management or preventing brute force, credential stuffing, and resource exhaustion
  attacks against APIs.'
domain: cybersecurity
subdomain: api-security
tags:
- api-security
- rate-limiting
- throttling
- redis
- token-bucket
- abuse-prevention
version: 1.0.0
author: mahipal
license: Apache-2.0
nist_csf:
- PR.PS-01
- ID.RA-01
- PR.DS-10
- DE.CM-01
mitre_attack:
- T1190
- T1059.007
- T1552.001
- T1003
- T1110
---
# Implementing API Rate Limiting and Throttling

## When to Use

- Protecting authentication endpoints against brute force and credential stuffing attacks
- Preventing API abuse and resource exhaustion from automated scripts and bots
- Implementing fair usage quotas for different API consumer tiers (free, premium, enterprise)
- Defending against denial-of-service attacks at the application layer
- Meeting compliance requirements that mandate API abuse prevention controls

**Do not use** rate limiting as the sole defense against attacks. Combine with authentication, authorization, and WAF rules.

## Prerequisites

- Redis 6.0+ for distributed rate limit counters (or in-memory for single-instance deployments)
- API framework (Express.js, FastAPI, Spring Boot, or Django REST Framework)
- Monitoring system for rate limit metrics (Prometheus, CloudWatch, Datadog)
- Understanding of the API's normal traffic patterns and peak usage
- Load testing tool (k6, Gatling, or Locust) for validating rate limit behavior

## Workflow

### Step 1: Rate Limiting Strategy Design

Define rate limits per endpoint category and user tier:

```python
# Rate limit configuration
RATE_LIMITS = {
    # Authentication endpoints (most restrictive)
    "auth": {
        "login": {"requests": 5, "window_seconds": 60, "by": "ip"},
        "register": {"requests": 3, "window_seconds": 300, "by": "ip"},
        "forgot_password": {"requests": 3, "window_seconds": 3600, "by": "ip"},
        "verify_mfa": {"requests": 5, "window_seconds": 300, "by": "user"},
    },
    # Standard API endpoints
    "api": {
        "free": {"requests": 60, "window_seconds": 60, "by": "user"},
        "premium": {"requests": 300, "window_seconds": 60, "by": "user"},
        "enterprise": {"requests": 1000, "window_seconds": 60, "by": "user"},
    },
    # Resource-intensive endpoints
    "expensive": {
        "search": {"requests": 10, "window_seconds": 60, "by": "user"},
        "export": {"requests": 5, "window_seconds": 3600, "by": "user"},
        "bulk_import": {"requests": 2, "window_seconds": 3600, "by": "user"},
    },
    # Global limits
    "global": {
        "per_ip": {"requests": 1000, "window_seconds": 60, "by": "ip"},
        "per_user": {"requests": 5000, "window_seconds": 3600, "by": "user"},
    },
}
```

### Step 2: Sliding Window Rate Limiter (Redis)

```python
import redis
import time
import hashlib
from functools import wraps
from flask import Flask, request, jsonify, g

app = Flask(__name__)
redis_client = redis.Redis(host='localhost', port=6379, db=0, decode_responses=True)

class SlidingWindowRateLimiter:
    """Sliding window rate limiter using Redis sorted sets."""

    def __init__(self, redis_conn):
        self.redis = redis_conn

    def is_allowed(self, key, max_requests, window_seconds):
        """Check if request is allowed and record it."""
        now = time.time()
        window_start = now - window_seconds
        pipe = self.redis.pipeline()

        # Remove expired entries
        pipe.zremrangebyscore(key, 0, window_start)
        # Count requests in current window
        pipe.zcard(key)
        # Add current request
        pipe.zadd(key, {f"{now}:{hashlib.md5(str(now).encode()).hexdigest()[:8]}": now})
        # Set TTL on the key
        pipe.expire(key, window_seconds + 1)

        results = pipe.execute()
        current_count = results[1]

        if current_count >= max_requests:
            # Calculate retry-after
            oldest = self.redis.zrange(key, 0, 0, withscores=True)
            if oldest:
                retry_after = int(oldest[0][1] + window_seconds - now) + 1
            else:
                retry_after = window_seconds
            return False, current_count, max_requests, retry_after

        return True, current_count + 1, max_requests, 0

rate_limiter = SlidingWindowRateLimiter(redis_client)

def rate_limit(max_requests, window_seconds, key_func=None):
    """Decorator for rate limiting API endpoints."""
    def decorator(f):
        @wraps(f)
        def wrapped(*args, **kwargs):
            # Determine the rate limit key
            if key_func:
                identifier = key_func()
            elif hasattr(g, 'user_id'):
                identifier = f"user:{g.user_id}"
            else:
                identifier = f"ip:{request.remote_addr}"

            key = f"ratelimit:{request.endpoint}:{identifier}"
            allowed, current, limit, retry_after = rate_limiter.is_allowed(
                key, max_requests, window_seconds)

            # Always set rate limit headers
            headers = {
                "X-RateLimit-Limit": str(limit),
                "X-RateLimit-Remaining": str(max(0, limit - current)),
                "X-RateLimit-Reset": str(int(time.time()) + window_seconds),
            }

            if not allowed:
                headers["Retry-After"] = str(retry_after)
                response = jsonify({
                    "error": "rate_limit_exceeded",
                    "message": "Too many requests. Please try again later.",
                    "retry_after": retry_after
                })
                response.status_code = 429
                for h, v in headers.items():
                    response.headers[h] = v
                return response

            response = f(*args, **kwargs)
            for h, v in headers.items():
                response.headers[h] = v
            return response
        return wrapped
    return decorator

# Apply rate limiting to endpoints
@app.route('/api/v1/auth/login', methods=['POST'])
@rate_limit(max_requests=5, window_seconds=60,
            key_func=lambda: f"ip:{request.remote_addr}")
def login():
    # Login logic
    return jsonify({"message": "Login successful"})

@app.route('/api/v1/users/me', methods=['GET'])
@rate_limit(max_requests=60, window_seconds=60)
def get_profile():
    # Profile logic
    return jsonify({"user": "data"})

@app.route('/api/v1/search', methods=['GET'])
@rate_limit(max_requests=10, window_seconds=60)
def search():
    # Search logic
    return jsonify({"results": []})
```

### Step 3: Token Bucket Rate Limiter

```python
import redis
import time

class TokenBucketRateLimiter:
    """Token bucket rate limiter allowing burst traffic within limits."""

    def __init__(self, redis_conn):
        self.redis = redis_conn

    def is_allowed(self, key, max_tokens, refill_rate, refill_interval=1):
        """
        Token bucket algorithm:
        - max_tokens: Maximum burst capacity
        - refill_rate: Tokens added per refill_interval
        - refill_interval: Seconds between refills
        """
        now = time.time()
        bucket_key = f"tb:{key}"

        # Lua script for atomic token bucket operation
        lua_script = """
        local key = KEYS[1]
        local max_tokens = tonumber(ARGV[1])
        local refill_rate = tonumber(ARGV[2])
        local refill_interval = tonumber(ARGV[3])
        local now = tonumber(ARGV[4])

        local bucket = redis.call('hmget', key, 'tokens', 'last_refill')
        local tokens = tonumber(bucket[1])
        local last_refill = tonumber(bucket[2])

        if tokens == nil then
            tokens = max_tokens
            last_refill = now
        end

        -- Refill tokens
        local elapsed = now - last_refill
        local refills = math.floor(elapsed / refill_interval)
        if refills > 0 then
            tokens = math.min(max_tokens, tokens + (refills * refill_rate))
            last_refill = last_refill + (refills * refill_interval)
        end

        local allowed = 0
        if tokens >= 1 then
            tokens = tokens - 1
            allowed = 1
        end

        redis.call('hmset', key, 'tokens', tokens, 'last_refill', last_refill)
        redis.call('expire', key, math.ceil(max_tokens / refill_rate * refill_interval) + 10)

        return {allowed, tokens, max_tokens}
        """

        result = self.redis.eval(lua_script, 1, bucket_key,
                                  max_tokens, refill_rate, refill_interval, now)
        allowed = bool(result[0])
        remaining = int(result[1])
        limit = int(result[2])

        return allowed, remaining, limit
```

### Step 4: Tiered Rate Limiting with User Plans

```python
from enum import Enum

class UserTier(Enum):
    FREE = "free"
    PREMIUM = "premium"
    ENTERPRISE = "enterprise"

TIER_LIMITS = {
    UserTier.FREE: {
        "default": (60, 60),          # 60 req/min
        "search": (10, 60),           # 10 req/min
        "export": (5, 3600),          # 5 req/hour
        "daily_total": (1000, 86400), # 1000 req/day
    },
    UserTier.PREMIUM: {
        "default": (300, 60),
        "search": (50, 60),
        "export": (20, 3600),
        "daily_total": (10000, 86400),
    },
    UserTier.ENTERPRISE: {
        "default": (1000, 60),
        "search": (200, 60),
        "export": (100, 3600),
        "daily_total": (100000, 86400),
    },
}

def get_rate_limit_for_request(user_tier, endpoint_category="default"):
    """Get rate limit configuration based on user tier and endpoint."""
    tier_config = TIER_LIMITS.get(user_tier, TIER_LIMITS[UserTier.FREE])
    limit_config = tier_config.get(endpoint_category, tier_config["default"])
    return limit_config  # (max_requests, window_seconds)

class TieredRateLimitMiddleware:
    """Middleware that applies rate limits based on user subscription tier."""

    def __init__(self, app, redis_conn):
        self.app = app
        self.limiter = SlidingWindowRateLimiter(redis_conn)

    def __call__(self, environ, start_response):
        # Extract user info from request
        user_id = environ.get("HTTP_X_USER_ID")
        user_tier = UserTier(environ.get("HTTP_X_USER_TIER", "free"))
        endpoint = environ.get("PATH_INFO", "/")

        # Determine endpoint category
        category = "default"
        if "/search" in endpoint:
            category = "search"
        elif "/export" in endpoint:
            category = "export"

        max_requests, window = get_rate_limit_for_request(user_tier, category)
        key = f"tiered:{user_id or environ.get('REMOTE_ADDR')}:{category}"

        allowed, current, limit, retry_after = self.limiter.is_allowed(
            key, max_requests, window)

        if not allowed:
            status = "429 Too Many Requests"
            headers = [
                ("Content-Type", "application/json"),
                ("Retry-After", str(retry_after)),
                ("X-RateLimit-Limit", str(limit)),
                ("X-RateLimit-Remaining", "0"),
            ]
            start_response(status, headers)
            body = f'{{"error":"rate_limit_exceeded","retry_after":{retry_after},"tier":"{user_tier.value}"}}'
            return [body.encode()]

        return self.app(environ, start_response)
```

### Step 5: Distributed Rate Limiting for Microservices

```python
# Centralized rate limiting service using Redis Cluster
import redis
from redis.cluster import RedisCluster

class DistributedRateLimiter:
    """Rate limiter for microservice architectures using Redis Cluster."""

    def __init__(self):
        self.redis = RedisCluster(
            startup_nodes=[
                {"host": "redis-node-1", "port": 6379},
                {"host": "redis-node-2", "port": 6379},
                {"host": "redis-node-3", "port": 6379},
            ],
            decode_responses=True
        )

    def check_and_increment(self, service_name, user_id, endpoint,
                             max_requests, window_seconds):
        """Atomic check-and-increment using Redis Lua script."""
        key = f"rl:{{{service_name}}}:{user_id}:{endpoint}"

        # Lua script ensures atomicity across the check and increment
        lua_script = """
        local key = KEYS[1]
        local max_requests = tonumber(ARGV[1])
        local window = tonumber(ARGV[2])
        local now = tonumber(ARGV[3])
        local window_start = now - window

        -- Remove old entries
        redis.call('zremrangebyscore', key, '-inf', window_start)

        -- Count current entries
        local count = redis.call('zcard', key)

        if count >= max_requests then
            -- Get oldest entry for retry-after calculation
            local oldest = redis.call('zrange', key, 0, 0, 'WITHSCORES')
            local retry_after = 0
            if #oldest > 0 then
                retry_after = math.ceil(tonumber(oldest[2]) + window - now)
            end
            return {0, count, retry_after}
        end

        -- Add new entry
        redis.call('zadd', key, now, now .. ':' .. math.random(100000))
        redis.call('expire', key, window + 1)

        return {1, count + 1, 0}
        """

        result = self.redis.eval(lua_script, 1, key,
                                  max_requests, window_seconds, time.time())
        return {
            "allowed": bool(result[0]),
            "current": int(result[1]),
            "retry_after": int(result[2]),
        }
```

## Key Concepts

| Term | Definition |
|------|------------|
| **Sliding Window** | Rate limiting algorithm that tracks requests in a rolling time window, providing smoother rate enforcement than fixed windows |
| **Token Bucket** | Algorithm where tokens are added at a fixed rate and consumed per request, allowing controlled bursts up to the bucket capacity |
| **Fixed Window** | Simplest rate limiting where requests are counted per fixed time window (e.g., per minute), susceptible to burst at window boundaries |
| **429 Too Many Requests** | HTTP status code indicating the client has exceeded the rate limit, accompanied by Retry-After header |
| **Retry-After Header** | HTTP response header telling the client how many seconds to wait before retrying, essential for well-behaved API clients |
| **Distributed Rate Limiting** | Rate limiting across multiple server instances using shared state (Redis, Memcached) to maintain accurate global counters |

## Tools & Systems

- **Redis**: In-memory data store used for distributed rate limit counters with atomic operations via Lua scripts
- **Kong Rate Limiting Plugin**: API gateway plugin supporting fixed-window and sliding-window rate limiting with Redis backend
- **express-rate-limit**: Express.js middleware for simple rate limiting with Redis, Memcached, or in-memory stores
- **Flask-Limiter**: Flask extension for rate limiting with support for multiple backends and configurable limits per endpoint
- **Envoy Rate Limit Service**: Centralized rate limiting service for Envoy-based service mesh architectures

## Common Scenarios

### Scenario: Implementing Rate Limiting for a Public API

**Context**: A company launches a public API with free, premium, and enterprise tiers. The API must protect against abuse while providing fair access to paying customers. The API runs on 6 instances behind an AWS ALB.

**Approach**:
1. Deploy Redis Cluster (3 nodes) for distributed rate limit state
2. Implement sliding window rate limiter using Redis sorted sets with Lua scripts for atomicity
3. Configure per-tier limits: Free (60 req/min), Premium (300 req/min), Enterprise (1000 req/min)
4. Add stricter limits on authentication endpoints (5 req/min per IP) regardless of tier
5. Implement resource-intensive endpoint limits (search: 10 req/min free, export: 5 req/hour)
6. Set rate limit response headers on every response (X-RateLimit-Limit, X-RateLimit-Remaining, X-RateLimit-Reset)
7. Return 429 with Retry-After header and JSON error body when limits are exceeded
8. Set up Prometheus metrics for rate limit hits and CloudWatch alarms for unusual patterns

**Pitfalls**:
- Using in-memory rate limiting without shared state across instances, allowing limit bypass by hitting different servers
- Not implementing rate limiting on authentication endpoints separately from general API limits
- Using fixed windows that allow burst at window boundaries (2x the limit in a short period)
- Not including rate limit headers on successful responses, giving clients no visibility into their quota
- Trusting X-Forwarded-For for IP identification without validating it against the load balancer

## Output Format

```
## Rate Limiting Implementation Report

**API**: Public API v2
**Algorithm**: Sliding Window (Redis Sorted Sets)
**Backend**: Redis Cluster (3 nodes)
**Deployment**: 6 API instances behind AWS ALB

### Rate Limit Configuration

| Tier | Default | Search | Export | Auth (per IP) |
|------|---------|--------|--------|---------------|
| Free | 60/min | 10/min | 5/hour | 5/min |
| Premium | 300/min | 50/min | 20/hour | 5/min |
| Enterprise | 1000/min | 200/min | 100/hour | 10/min |

### Validation Results (k6 load test)

- Free tier: Rate limited at 61st request (correct)
- Premium tier: Rate limited at 301st request (correct)
- Cross-instance: Rate limiting consistent across all 6 instances
- Redis failover: Rate limiting degrades gracefully (allows traffic) when Redis is unreachable
- Retry-After header: Accurate within 1 second of actual reset time
- Response overhead: < 2ms added latency per request for rate limit check
```
