Rate Limiting

Controlling the rate of requests to a service to prevent overload, ensure fair usage, and protect against abuse

TL;DR

Rate limiting controls how many requests a client can make to a service within a time window. It protects backend resources from overload, ensures fair usage across clients, and defends against abuse like brute-force attacks. The two most common algorithms are token bucket (allows bursts) and sliding window (smooth enforcement).

Visual Overview

Rate Limiting Flow
┌────────────────────────────────────────────────────┐
│                                                    │
│  Client Request → Rate Limiter → Backend Service   │
│                       │                            │
│                       ↓                            │
│              Check: Within limit?                  │
│                    /     \                        │
│                  YES      NO                       │
│                   ↓        ↓                       │
│              ALLOWED    REJECTED                   │
│              (process)   (429 + Retry-After)       │
│                                                    │
└────────────────────────────────────────────────────┘

WHY RATE LIMIT?
┌────────────────────────────────────────────────────┐
│  1. Resource Protection                            │
│     └─ Prevent DB connection exhaustion            │
│                                                    │
│  2. Fair Usage                                     │
│     └─ One client can't starve others              │
│                                                    │
│  3. Cost Control                                   │
│     └─ Limit cloud/API spend                       │
│                                                    │
│  4. Security                                       │
│     └─ Stop brute-force, credential stuffing       │
└────────────────────────────────────────────────────┘

Core Explanation

What is Rate Limiting?

Real-World Analogy: Think of rate limiting like a nightclub bouncer. The club has a capacity of 100 people. The bouncer lets people in one at a time, but if the club is full, new arrivals must wait outside. Some VIPs (premium API users) might get a higher limit or skip the line.

Rate limiting enforces boundaries on how many requests a client can make:

  • Per user: Each authenticated user gets N requests/minute
  • Per IP: Unauthenticated requests limited by IP address
  • Per API key: Different tiers get different limits
  • Global: Protect the entire service from overload

How It Works

Every rate limiter needs to answer two questions:

  1. Identification: Who is making this request? (user ID, IP, API key)
  2. Counting: How many requests have they made recently?

The implementation varies by algorithm, but the flow is consistent:

Request arrives → Identify client → Check counter → Allow or Reject

Two Main Algorithms

Token Bucket vs Sliding Window
TOKEN BUCKET
┌────────────────────────────────────────────────────┐
│  Bucket: [●●●●●] Capacity: 5 tokens                │
│                                                    │
│  Request 1: [●●●●○] consume 1 → ALLOWED            │
│  Request 2: [●●●○○] consume 1 → ALLOWED            │
│  Request 3: [●●○○○] consume 1 → ALLOWED            │
│  Request 4: [●○○○○] consume 1 → ALLOWED            │
│  Request 5: [○○○○○] consume 1 → ALLOWED            │
│  Request 6: [○○○○○] no tokens → REJECTED           │
│                                                    │
│  After 2 seconds (refill 1/sec):                   │
│  Bucket: [●●○○○] → can handle 2 more requests      │
│                                                    │
│  ✓ Allows bursts up to capacity                    │
│  ✓ Steady-state rate = refill rate                 │
└────────────────────────────────────────────────────┘

SLIDING WINDOW COUNTER
┌────────────────────────────────────────────────────┐
│  Window: 60 seconds, Limit: 10 requests            │
│                                                    │
│  Previous window: 8 requests                       │
│  Current window: 3 requests                        │
│  Current position: 25% into window                 │
│                                                    │
│  Weighted count = 8 × 0.75 + 3 = 9                 │
│  9 < 10 → ALLOWED                                  │
│                                                    │
│  ✓ Prevents boundary burst problem                 │
│  ✓ Smooth rate enforcement                         │
└────────────────────────────────────────────────────┘

Token Bucket:

  • Tokens accumulate at a steady rate (refill)
  • Each request consumes one token
  • Burst allowed up to bucket capacity
  • Best for: APIs where legitimate traffic is bursty

Sliding Window Counter:

  • Counts requests in a sliding time window
  • Weights previous window to prevent boundary bursts
  • Smooth enforcement, no burst allowance
  • Best for: Strict rate enforcement, billing limits

Real Systems Using Rate Limiting

SystemAlgorithm StyleNotesUse Case
GitHub APIToken bucket styleTiered by authentication; check current docs for limitsDeveloper API access
StripeSliding windowDifferent limits for live vs test modePayment processing
Twitter/X APITiered windowsVaries significantly by endpoint and tierSocial media API
AWS API GatewayToken bucketFully configurable per stageAPI management
CloudflareLeaky bucketRule-based configurationEdge rate limiting

Note: Specific limits change frequently. Always verify current limits in official documentation.

Case Study: API Gateway Rate Limiting

Multi-Tier Rate Limiting
MULTI-TIER API LIMITS (Illustrative Example)
┌────────────────────────────────────────────────────┐
│  Tier        │ Rate Limit    │ Burst   │ Pricing   │
│──────────────┼───────────────┼─────────┼────────── │
│  Free        │ Low           │ Small   │ Free      │
│  Developer   │ Medium        │ Medium  │ Basic     │
│  Pro         │ High          │ Large   │ Premium   │
│  Enterprise  │ Custom        │ Custom  │ Custom    │
└────────────────────────────────────────────────────┘
│  (Exact limits vary by provider—verify current docs) │

REQUEST FLOW WITH TIERS
┌────────────────────────────────────────────────────┐
│  Request + API Key                                 │
│         ↓                                          │
│  Lookup tier from API key                          │
│         ↓                                          │
│  Check rate limit for tier                         │
│         ↓                                          │
│  ┌─────────────────────────────────────┐           │
│  │ Within limit? ──YES──► Process      │           │
│  │       │                             │           │
│  │      NO                             │           │
│  │       ↓                             │           │
│  │ Return 429 + Retry-After header     │           │
│  │ + X-RateLimit-Remaining: 0          │           │
│  │ + X-RateLimit-Reset: 1640000000     │           │
│  └─────────────────────────────────────┘           │
└────────────────────────────────────────────────────┘

When to Use Rate Limiting

✓ Perfect Use Cases

Rate Limiting Use Cases
PUBLIC APIS
Scenario: REST API exposed to external developers
Requirement: Prevent abuse, ensure fair access
Configuration: 1000 requests/hour per API key
Trade-off: Legitimate high-volume users need higher tier

MULTI-TENANT SAAS
Scenario: Shared infrastructure, many customers
Requirement: One customer can't impact others
Configuration: Per-tenant limits based on plan
Trade-off: Complexity in limit management

LOGIN ENDPOINTS
Scenario: Authentication endpoints
Requirement: Prevent credential stuffing attacks
Configuration: 5 attempts/minute per IP
Trade-off: Legitimate users with typos may be blocked

WEBHOOK RECEIVERS
Scenario: Receiving webhooks from external services
Requirement: Prevent flood from misbehaving senders
Configuration: 100 requests/second per source
Trade-off: May drop legitimate spikes

✕ When NOT to Use (or Use Carefully)

When Rate Limiting May Not Fit
INTERNAL SERVICE-TO-SERVICE
Problem: Adds latency to trusted internal calls
Alternative: Circuit breakers, back-pressure
When OK: Still useful for resource protection

REAL-TIME CRITICAL PATHS
Problem: 429s disrupt user experience
Alternative: Graceful degradation, queuing
When OK: As last resort protection

BATCH PROCESSING
Problem: Legitimate jobs need high throughput
Alternative: Separate batch endpoints with higher limits
When OK: With burst capacity for batch jobs

Interview Application

Common Interview Question

Q: “You’re designing an API for a public service. How would you implement rate limiting? What algorithm would you choose?”

Strong Answer:

“I’d implement rate limiting at the API gateway level with a token bucket algorithm. Here’s my approach:

Why Token Bucket:

  1. Allows legitimate bursts: Users often make multiple quick requests (page load, app startup)
  2. Simple state: Just two values per client (tokens, last_refill_time)
  3. Configurable: Capacity controls burst size, refill rate controls steady-state

Implementation:

  • Store state in Redis for distributed rate limiting
  • Key: ratelimit:{user_id} with token count and timestamp
  • Use Redis MULTI/EXEC for atomic check-and-decrement

Configuration:

  • Capacity: 100 tokens (max burst)
  • Refill: 10 tokens/second (100 req/sec steady-state)
  • Different limits per API tier

Response Headers:

  • X-RateLimit-Limit: Maximum requests allowed
  • X-RateLimit-Remaining: Requests left in window
  • X-RateLimit-Reset: Unix timestamp when limit resets
  • Retry-After: Seconds to wait (on 429)

Edge Cases:

  • Clock skew: Use Redis server time, not client time
  • Distributed: Single Redis cluster for consistency
  • Failover: Fail open (allow) if Redis unavailable—better to risk abuse than block all users”

Follow-up: How would you handle distributed rate limiting across multiple regions?

“For multi-region, I’d use local rate limiting with global synchronization:

  1. Each region has local Redis for low-latency checks
  2. Async sync between regions (eventual consistency)
  3. Accept that users might get slightly more than limit globally
  4. Alternative: Single global Redis with latency cost

The trade-off is accuracy vs latency. For most APIs, slightly exceeding limits across regions is acceptable.”

Code Example

Token Bucket Rate Limiter (Python + Redis)

import time
import redis

class TokenBucketRateLimiter:
    """
    Distributed token bucket rate limiter using Redis.

    Allows bursts up to capacity while enforcing average rate.
    """

    def __init__(self, redis_client: redis.Redis, capacity: int, refill_rate: float):
        """
        Args:
            redis_client: Redis connection
            capacity: Maximum tokens (burst size)
            refill_rate: Tokens added per second
        """
        self.redis = redis_client
        self.capacity = capacity
        self.refill_rate = refill_rate

    def is_allowed(self, key: str) -> tuple[bool, dict]:
    # ... omitted: keep concept snippets short

    # Simulate requests
    for i in range(25):
        allowed, info = limiter.is_allowed(user_id)

        if allowed:
            print(f"Request {i+1}: ALLOWED (remaining: {info['remaining']})")
        else:
            print(f"Request {i+1}: REJECTED (retry at: {info['reset_at']})")
            # In real code: return 429 with Retry-After header

Express.js Middleware Example

const rateLimit = require('express-rate-limit');
const RedisStore = require('rate-limit-redis');
const Redis = require('ioredis');

const redis = new Redis({
  host: process.env.REDIS_HOST,
  port: 6379,
});

// Create rate limiter middleware
const apiLimiter = rateLimit({
  store: new RedisStore({
    sendCommand: (...args) => redis.call(...args),
  }),

  // 100 requests per 15 minutes
  windowMs: 15 * 60 * 1000,
  max: 100,

  // Return rate limit info in headers
  standardHeaders: true,
  legacyHeaders: false,
  // ... omitted: keep concept snippets short
app.use('/api/', apiLimiter);

// Stricter limit for auth endpoints
const authLimiter = rateLimit({
  windowMs: 60 * 1000,  // 1 minute
  max: 5,               // 5 attempts per minute
  message: 'Too many login attempts, please try again later',
});

app.use('/api/auth/login', authLimiter);

Related Content

See It In Action:

Related Concepts:

Quick Self-Check

  • Can explain rate limiting in 60 seconds?
  • Understand difference between token bucket and sliding window?
  • Know what HTTP 429 means and what headers to return?
  • Can implement distributed rate limiting with Redis?
  • Understand why sliding window prevents boundary burst?
  • Know when to use rate limiting vs circuit breaker?

Production signal

Why this concept matters

Interview 80% of API design interviews
Production All production APIs
Performance Resource protection
Scale Fair usage enforcement