{
  "id": "ruby",
  "title": "Token bucket rate limiter with Redis and Ruby",
  "url": "https://redis.io/docs/latest/develop/use-cases/rate-limiter/ruby/",
  "summary": "Implement a token bucket rate limiter using Redis and Lua scripts in Ruby",
  "tags": [
    "docs",
    "develop",
    "stack",
    "oss",
    "rs",
    "rc"
  ],
  "last_updated": "2026-04-16T13:29:55-07:00",
  "children": [],
  "page_type": "content",
  "content_hash": "f9366ec4cad185c607da05b8c9712a91d74e6461b8069a88d71b11845bf1cdf4",
  "sections": [
    {
      "id": "overview",
      "title": "Overview",
      "role": "overview",
      "text": "This guide shows you how to implement a distributed token bucket rate limiter using Redis and Lua scripts in Ruby with the [`redis-rb`](https://redis.io/docs/latest/develop/clients/ruby) client library."
    },
    {
      "id": "overview",
      "title": "Overview",
      "role": "overview",
      "text": "Rate limiting is a critical technique for controlling the rate at which operations are performed. Common use cases include:\n\n* Limiting API requests per user or IP address\n* Preventing abuse and protecting against denial-of-service attacks\n* Ensuring fair resource allocation across multiple clients\n* Throttling background jobs or batch operations\n\nThe **token bucket algorithm** is a popular rate limiting approach that allows bursts of traffic while maintaining an average rate limit over time. This guide covers the Ruby implementation using the [`redis-rb`](https://redis.io/docs/latest/develop/clients/ruby) gem."
    },
    {
      "id": "how-it-works",
      "title": "How it works",
      "role": "content",
      "text": "The token bucket algorithm works like a bucket that holds tokens:\n\n1. **Initialization**: The bucket starts with a maximum capacity of tokens\n2. **Refill**: Tokens are added to the bucket at a constant rate (for example, 1 token per second)\n3. **Consumption**: Each request consumes one token from the bucket\n4. **Decision**: If tokens are available, the request is allowed; otherwise, it's denied\n5. **Capacity limit**: The bucket never exceeds its maximum capacity\n\nThis approach allows for burst traffic (using accumulated tokens) while enforcing an average rate limit over time."
    },
    {
      "id": "why-use-redis",
      "title": "Why use Redis?",
      "role": "content",
      "text": "Redis is ideal for distributed rate limiting because:\n\n* **Atomic operations**: Lua scripts execute atomically, preventing race conditions\n* **Shared state**: Multiple application servers can share the same rate limit counters\n* **High performance**: In-memory operations provide microsecond latency\n* **Automatic expiration**: Keys can be set to expire automatically (though not used in this implementation)"
    },
    {
      "id": "the-lua-script",
      "title": "The Lua script",
      "role": "content",
      "text": "The core of this implementation is a Lua script that runs atomically on the Redis server. This ensures that checking and updating the token bucket happens in a single operation, preventing race conditions in distributed environments.\n\nHere's how the script works:\n\n[code example]"
    },
    {
      "id": "script-breakdown",
      "title": "Script breakdown",
      "role": "content",
      "text": "1. **State retrieval**: Uses [`HMGET`](https://redis.io/docs/latest/commands/hmget) to fetch the current token count and last refill time from a hash\n2. **Initialization**: On first use, sets tokens to full capacity\n3. **Token refill calculation**: Computes how many tokens should be added based on elapsed time\n4. **Capacity enforcement**: Uses `math.min()` to ensure tokens never exceed capacity\n5. **Token consumption**: Decrements the token count if available\n6. **State update**: Uses [`HMSET`](https://redis.io/docs/latest/commands/hmset) to save the new state\n7. **Return value**: Returns both the decision (allowed/denied) and remaining tokens"
    },
    {
      "id": "why-atomicity-matters",
      "title": "Why atomicity matters",
      "role": "content",
      "text": "Without atomic execution, race conditions could occur:\n\n* **Double spending**: Two requests could read the same token count and both succeed when only one should\n* **Lost updates**: Concurrent updates could overwrite each other's changes\n* **Inconsistent state**: Token count and refill time could become desynchronized\n\nUsing [`EVAL`](https://redis.io/docs/latest/commands/eval) or [`EVALSHA`](https://redis.io/docs/latest/commands/evalsha) ensures the entire operation executes atomically, making it safe for distributed systems."
    },
    {
      "id": "installation",
      "title": "Installation",
      "role": "setup",
      "text": "Install the `redis` gem:\n\n[code example]\n\nOr add it to your `Gemfile`:\n\n[code example]\n\nThen run:\n\n[code example]"
    },
    {
      "id": "using-the-ruby-module",
      "title": "Using the Ruby module",
      "role": "content",
      "text": "The `TokenBucket` class provides a simple interface for rate limiting\n([source](token_bucket.rb)):\n\n[code example]\n\nRuby's keyword arguments make the constructor parameters self-documenting, and the `allow` method returns a Hash with `:allowed` and `:remaining` keys."
    },
    {
      "id": "configuration-parameters",
      "title": "Configuration parameters",
      "role": "configuration",
      "text": "* **capacity**: Maximum number of tokens in the bucket (controls burst size)\n* **refill_rate**: Number of tokens added per refill interval\n* **refill_interval**: Time in seconds between refills\n\nFor example:\n* `capacity: 10, refill_rate: 1, refill_interval: 1.0` allows 10 requests per second with bursts up to 10\n* `capacity: 100, refill_rate: 10, refill_interval: 1.0` allows 10 requests per second with bursts up to 100\n* `capacity: 60, refill_rate: 1, refill_interval: 60.0` allows 1 request per minute with bursts up to 60"
    },
    {
      "id": "rate-limit-keys",
      "title": "Rate limit keys",
      "role": "content",
      "text": "The `key` parameter identifies what you're rate limiting. Common patterns:\n\n* **Per user**: `user:{user_id}` - Limit each user independently\n* **Per IP address**: `ip:{ip_address}` - Limit by client IP\n* **Per API endpoint**: `api:{endpoint}:{user_id}` - Different limits per endpoint\n* **Global**: `global:api` - Single limit shared across all requests"
    },
    {
      "id": "script-caching-with-evalsha",
      "title": "Script caching with EVALSHA",
      "role": "content",
      "text": "The Ruby implementation uses [`EVALSHA`](https://redis.io/docs/latest/commands/evalsha) for optimal performance. On first use, the Lua script is loaded into Redis with `SCRIPT LOAD`, and subsequent calls use the cached SHA1 hash. If the script is evicted from the cache, the module automatically falls back to [`EVAL`](https://redis.io/docs/latest/commands/eval) and reloads the script.\n\n[code example]"
    },
    {
      "id": "running-the-demo",
      "title": "Running the demo",
      "role": "content",
      "text": "A demonstration web server is included to show the rate limiter in action\n([source](demo_server.rb)):\n\n[code example]\n\nThe demo provides an interactive web interface where you can:\n\n* Submit requests and see them allowed or denied in real-time\n* View the current token count\n* Adjust rate limit parameters dynamically\n* Test different rate limiting scenarios\n\nThe demo assumes Redis is running on `localhost:6379` but you can specify a different host and port using the `--redis-host HOST` and `--redis-port PORT` command-line arguments. Visit `http://localhost:8080` in your browser to try it out."
    },
    {
      "id": "response-headers",
      "title": "Response headers",
      "role": "returns",
      "text": "It's common to include rate limit information in HTTP response headers:\n\n[code example]"
    },
    {
      "id": "customization",
      "title": "Customization",
      "role": "content",
      "text": ""
    },
    {
      "id": "using-with-rack-middleware",
      "title": "Using with Rack middleware",
      "role": "content",
      "text": "You can wrap the rate limiter as Rack middleware for easy integration with any Rack-based framework (Rails, Sinatra, Hanami):\n\n[code example]"
    },
    {
      "id": "error-handling",
      "title": "Error handling",
      "role": "errors",
      "text": "The `allow` method may raise an error if the Redis connection is lost. Wrap calls in a `begin/rescue` block for production use:\n\n[code example]"
    },
    {
      "id": "learn-more",
      "title": "Learn more",
      "role": "related",
      "text": "* [EVAL command](https://redis.io/docs/latest/commands/eval) - Execute Lua scripts\n* [EVALSHA command](https://redis.io/docs/latest/commands/evalsha) - Execute cached Lua scripts\n* [Lua scripting](https://redis.io/docs/latest/develop/programmability/eval-intro) - Introduction to Redis Lua scripting\n* [HMGET command](https://redis.io/docs/latest/commands/hmget) - Get multiple hash fields\n* [HMSET command](https://redis.io/docs/latest/commands/hmset) - Set multiple hash fields\n* [Ruby client](https://redis.io/docs/latest/develop/clients/ruby) - Redis Ruby client documentation"
    }
  ],
  "examples": [
    {
      "id": "the-lua-script-ex0",
      "language": "lua",
      "code": "local key = KEYS[1]\nlocal capacity = tonumber(ARGV[1])\nlocal refill_rate = tonumber(ARGV[2])\nlocal refill_interval = tonumber(ARGV[3])\nlocal now = tonumber(ARGV[4])\n\n-- Get current state or initialize\nlocal bucket = redis.call('HMGET', key, 'tokens', 'last_refill')\nlocal tokens = tonumber(bucket[1])\nlocal last_refill = tonumber(bucket[2])\n\n-- Initialize if this is the first request\nif tokens == nil then\n    tokens = capacity\n    last_refill = now\nend\n\n-- Calculate token refill\nlocal time_passed = now - last_refill\nlocal refills = math.floor(time_passed / refill_interval)\n\nif refills > 0 then\n    tokens = math.min(capacity, tokens + (refills * refill_rate))\n    last_refill = last_refill + (refills * refill_interval)\nend\n\n-- Try to consume a token\nlocal allowed = 0\nif tokens >= 1 then\n    tokens = tokens - 1\n    allowed = 1\nend\n\n-- Update state\nredis.call('HMSET', key, 'tokens', tokens, 'last_refill', last_refill)\n\n-- Return result: allowed (1 or 0) and remaining tokens\nreturn {allowed, tokens}",
      "section_id": "the-lua-script"
    },
    {
      "id": "installation-ex0",
      "language": "bash",
      "code": "gem install redis",
      "section_id": "installation"
    },
    {
      "id": "installation-ex1",
      "language": "ruby",
      "code": "gem 'redis', '~> 5.0'",
      "section_id": "installation"
    },
    {
      "id": "installation-ex2",
      "language": "bash",
      "code": "bundle install",
      "section_id": "installation"
    },
    {
      "id": "using-the-ruby-module-ex0",
      "language": "ruby",
      "code": "require 'redis'\nrequire_relative 'token_bucket'\n\n# Create a Redis connection\nredis = Redis.new(host: 'localhost', port: 6379)\n\n# Create a rate limiter: 10 requests per second\nlimiter = TokenBucket.new(\n  redis: redis,\n  capacity: 10,          # Maximum burst size\n  refill_rate: 1,        # Add 1 token per interval\n  refill_interval: 1.0   # Every 1 second\n)\n\n# Check if a request should be allowed\nresult = limiter.allow('user:123')\n\nif result[:allowed]\n  puts \"Request allowed. #{result[:remaining]} tokens remaining.\"\n  # Process the request\nelse\n  puts 'Request denied. Rate limit exceeded.'\n  # Return 429 Too Many Requests\nend",
      "section_id": "using-the-ruby-module"
    },
    {
      "id": "script-caching-with-evalsha-ex0",
      "language": "ruby",
      "code": "# The module handles script caching automatically.\n# First call loads the script, subsequent calls use EVALSHA.\nresult1 = limiter.allow('user:123') # Uses EVAL + caches\nresult2 = limiter.allow('user:123') # Uses EVALSHA (faster)",
      "section_id": "script-caching-with-evalsha"
    },
    {
      "id": "running-the-demo-ex0",
      "language": "bash",
      "code": "# Install dependencies\ngem install redis webrick\n\n# Run the demo server\nruby demo_server.rb",
      "section_id": "running-the-demo"
    },
    {
      "id": "response-headers-ex0",
      "language": "ruby",
      "code": "result = limiter.allow(\"user:#{user_id}\")\n\n# Add standard rate limit headers\nresponse['X-RateLimit-Limit'] = limiter.capacity.to_s\nresponse['X-RateLimit-Remaining'] = result[:remaining].to_i.to_s\nresponse['X-RateLimit-Reset'] = (Time.now.to_i + limiter.refill_interval).to_s\n\nunless result[:allowed]\n  response.status = 429 # Too Many Requests\n  response['Retry-After'] = limiter.refill_interval.ceil.to_s\nend",
      "section_id": "response-headers"
    },
    {
      "id": "using-with-rack-middleware-ex0",
      "language": "ruby",
      "code": "class RateLimitMiddleware\n  def initialize(app, limiter:, key_proc:)\n    @app = app\n    @limiter = limiter\n    @key_proc = key_proc\n  end\n\n  def call(env)\n    key = @key_proc.call(env)\n    result = @limiter.allow(key)\n\n    if result[:allowed]\n      status, headers, body = @app.call(env)\n      headers['X-RateLimit-Remaining'] = result[:remaining].to_i.to_s\n      [status, headers, body]\n    else\n      [429, { 'Content-Type' => 'application/json', 'Retry-After' => @limiter.refill_interval.ceil.to_s },\n       ['{\"error\":\"Rate limit exceeded\"}']]\n    end\n  end\nend\n\n# Apply per-IP rate limiting\nuse RateLimitMiddleware, limiter: limiter, key_proc: ->(env) { \"ip:#{env['REMOTE_ADDR']}\" }",
      "section_id": "using-with-rack-middleware"
    },
    {
      "id": "error-handling-ex0",
      "language": "ruby",
      "code": "begin\n  result = limiter.allow('user:123')\n  # Handle result\nrescue Redis::BaseError => e\n  puts \"Rate limiter error: #{e.message}\"\n  # Fail open or closed depending on your policy\nend",
      "section_id": "error-handling"
    }
  ]
}