Token bucket rate limiter with Redis and PHP

Implement a token bucket rate limiter using Redis and Lua scripts in PHP

This guide shows you how to implement a distributed token bucket rate limiter using Redis and Lua scripts in PHP with the Predis client library.

Overview

Rate limiting is a critical technique for controlling the rate at which operations are performed. Common use cases include:

Limiting API requests per user or IP address
Preventing abuse and protecting against denial-of-service attacks
Ensuring fair resource allocation across multiple clients
Throttling background jobs or batch operations

The 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 PHP implementation using the Predis client library, taking advantage of PHP's associative arrays and Composer-based dependency management.

How it works

The token bucket algorithm works like a bucket that holds tokens:

Initialization: The bucket starts with a maximum capacity of tokens
Refill: Tokens are added to the bucket at a constant rate (for example, 1 token per second)
Consumption: Each request consumes one token from the bucket
Decision: If tokens are available, the request is allowed; otherwise, it's denied
Capacity limit: The bucket never exceeds its maximum capacity

This approach allows for burst traffic (using accumulated tokens) while enforcing an average rate limit over time.

Why use Redis?

Redis is ideal for distributed rate limiting because:

Atomic operations: Lua scripts execute atomically, preventing race conditions
Shared state: Multiple application servers can share the same rate limit counters
High performance: In-memory operations provide microsecond latency
Automatic expiration: Keys can be set to expire automatically (though not used in this implementation)

The Lua script

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.

Here's how the script works:

local key = KEYS[1]
local capacity = tonumber(ARGV[1])
local refill_rate = tonumber(ARGV[2])
local refill_interval = tonumber(ARGV[3])
local now = tonumber(ARGV[4])

-- Get current state or initialize
local bucket = redis.call('HMGET', key, 'tokens', 'last_refill')
local tokens = tonumber(bucket[1])
local last_refill = tonumber(bucket[2])

-- Initialize if this is the first request
if tokens == nil then
    tokens = capacity
    last_refill = now
end

-- Calculate token refill
local time_passed = now - last_refill
local refills = math.floor(time_passed / refill_interval)

if refills > 0 then
    tokens = math.min(capacity, tokens + (refills * refill_rate))
    last_refill = last_refill + (refills * refill_interval)
end

-- Try to consume a token
local allowed = 0
if tokens >= 1 then
    tokens = tokens - 1
    allowed = 1
end

-- Update state
redis.call('HMSET', key, 'tokens', tokens, 'last_refill', last_refill)

-- Return result: allowed (1 or 0) and remaining tokens
return {allowed, tokens}

Script breakdown

State retrieval: Uses HMGET to fetch the current token count and last refill time from a hash
Initialization: On first use, sets tokens to full capacity
Token refill calculation: Computes how many tokens should be added based on elapsed time
Capacity enforcement: Uses math.min() to ensure tokens never exceed capacity
Token consumption: Decrements the token count if available
State update: Uses HMSET to save the new state
Return value: Returns both the decision (allowed/denied) and remaining tokens

Why atomicity matters

Without atomic execution, race conditions could occur:

Double spending: Two requests could read the same token count and both succeed when only one should
Lost updates: Concurrent updates could overwrite each other's changes
Inconsistent state: Token count and refill time could become desynchronized

Using EVAL or EVALSHA ensures the entire operation executes atomically, making it safe for distributed systems.

Installation

Install the Predis client library using Composer:

composer require predis/predis

Using the PHP class

The TokenBucket class provides a simple interface for rate limiting (source):

<?php

require __DIR__ . '/vendor/autoload.php';

use Predis\Client;

// Create a Redis connection
$redis = new Client([
    'scheme' => 'tcp',
    'host'   => '127.0.0.1',
    'port'   => 6379,
]);

// Create a rate limiter: 10 requests per second
$limiter = new TokenBucket(
    redis: $redis,
    capacity: 10,        // Maximum burst size
    refillRate: 1,       // Add 1 token per interval
    refillInterval: 1.0  // Every 1 second
);

// Check if a request should be allowed
$result = $limiter->allow('user:123');

if ($result['allowed']) {
    echo "Request allowed. {$result['remaining']} tokens remaining.\n";
    // Process the request
} else {
    echo "Request denied. Rate limit exceeded.\n";
    // Return 429 Too Many Requests
}

The allow() method returns an associative array with two keys: allowed (boolean) and remaining (number of tokens left in the bucket).

Configuration parameters

capacity: Maximum number of tokens in the bucket (controls burst size)
refillRate: Number of tokens added per refill interval
refillInterval: Time in seconds between refills

For example:

capacity: 10, refillRate: 1, refillInterval: 1.0 allows 10 requests per second with bursts up to 10
capacity: 100, refillRate: 10, refillInterval: 1.0 allows 10 requests per second with bursts up to 100
capacity: 60, refillRate: 1, refillInterval: 60.0 allows 1 request per minute with bursts up to 60

Rate limit keys

The key parameter identifies what you're rate limiting. Common patterns:

Per user: user:{userId} - Limit each user independently
Per IP address: ip:{ipAddress} - Limit by client IP
Per API endpoint: api:{endpoint}:{userId} - Different limits per endpoint
Global: global:api - Single limit shared across all requests

Script caching with EVALSHA

The PHP implementation uses 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 class automatically falls back to EVAL and reloads the script.

// The class handles script caching automatically.
// First call loads the script, subsequent calls use EVALSHA.
$result1 = $limiter->allow('user:123'); // Uses EVAL + caches
$result2 = $limiter->allow('user:123'); // Uses EVALSHA (faster)

Running the demo

Get the source files

The demo consists of two PHP files. Download them from the php source folder on GitHub, or grab them with curl:

mkdir rate-limiter-demo && cd rate-limiter-demo
BASE=https://raw.githubusercontent.com/redis/docs/main/content/develop/use-cases/rate-limiter/php
curl -O $BASE/TokenBucket.php
curl -O $BASE/demo_server.php

Start the demo server

A demonstration web server is included to show the rate limiter in action (source):

# Install dependencies
composer require predis/predis

# Run the demo server
php -S localhost:8080 demo_server.php

The demo provides an interactive web interface where you can:

Submit requests and see them allowed or denied in real-time
View the current token count
Adjust rate limit parameters dynamically
Test different rate limiting scenarios

The demo assumes Redis is running on localhost:6379 but you can specify a different host and port using the REDIS_HOST and REDIS_PORT environment variables:

REDIS_HOST=myhost REDIS_PORT=6380 php -S localhost:8080 demo_server.php

Visit http://localhost:8080 in your browser to try it out.

Response headers

It's common to include rate limit information in HTTP response headers:

$capacity = 10;
$refillInterval = 1.0;

$limiter = new TokenBucket(
    redis: $redis,
    capacity: $capacity,
    refillRate: 1,
    refillInterval: $refillInterval,
);

$result = $limiter->allow("user:{$userId}");

// Add standard rate limit headers
header('X-RateLimit-Limit: ' . $capacity);
header('X-RateLimit-Remaining: ' . (int) $result['remaining']);
header('X-RateLimit-Reset: ' . (int) (time() + $refillInterval));

if (!$result['allowed']) {
    header('Retry-After: ' . (int) $refillInterval);
    http_response_code(429);
    echo json_encode(['error' => 'Too Many Requests']);
    exit;
}

Customization

Using as PSR-15 middleware

You can wrap the rate limiter as PSR-15 middleware for use with frameworks like Slim or Laravel:

use Psr\Http\Message\ResponseInterface;
use Psr\Http\Message\ServerRequestInterface;
use Psr\Http\Server\MiddlewareInterface;
use Psr\Http\Server\RequestHandlerInterface;

class RateLimitMiddleware implements MiddlewareInterface
{
    public function __construct(
        private TokenBucket $limiter,
        private \Closure $keyFn,
    ) {}

    public function process(
        ServerRequestInterface $request,
        RequestHandlerInterface $handler,
    ): ResponseInterface {
        $key = ($this->keyFn)($request);
        $result = $this->limiter->allow($key);

        if (!$result['allowed']) {
            $response = new \Slim\Psr7\Response(429);
            $response->getBody()->write(
                json_encode(['error' => 'Rate limit exceeded'])
            );
            return $response->withHeader('Content-Type', 'application/json');
        }

        $response = $handler->handle($request);
        return $response->withHeader(
            'X-RateLimit-Remaining',
            (string) (int) $result['remaining']
        );
    }
}

// Apply per-IP rate limiting
$middleware = new RateLimitMiddleware(
    $limiter,
    fn(ServerRequestInterface $req) => 'ip:' . $req->getServerParams()['REMOTE_ADDR']
);

Error handling

The allow() method may throw a Predis\Connection\ConnectionException if the Redis connection is lost. Wrap calls in try/catch blocks for production use:

try {
    $result = $limiter->allow('user:123');
    // Handle result
} catch (\Predis\Connection\ConnectionException $e) {
    error_log('Rate limiter error: ' . $e->getMessage());
    // Fail open or closed depending on your policy
}

Alternative rate limiting algorithms

The token bucket algorithm above handles most use cases but Redis supports other rate limiter patterns that might fit your requirements better. The table below lists four other algorithms alongside token bucket and summarizes their features:

Algorithm	Memory	Accuracy	Burst behavior	Best for
Token bucket	1 key (hash)	Exact	Controlled bursts	APIs with bursty traffic
Fixed window counter	1 key (string)	Approximate	2x burst at boundaries	Simple API limits
Sliding window log	O(n) entries	Exact	No bursts	High-value APIs, audit trails
Sliding window counter	2 keys (string)	Near-exact	Smoothed boundaries	General-purpose APIs
Leaky bucket (policing)	1 key (hash)	Exact	No bursts	Strict no-burst enforcement

The sections below give example implementations of these other algorithms. The three time-based algorithms call redis.call('TIME') inside the Lua script to derive the current timestamp from the Redis server clock. This eliminates clock drift when the limiter runs across multiple application servers. The fixed window counter reads no clock: the key's TTL defines the window.

Fixed window counter

Counts requests within discrete, non-overlapping time intervals. Simplest algorithm — one key per window, one EVAL round trip.

<?php

use Predis\Client;

const FIXED_WINDOW_SCRIPT = <<<'LUA'
local key    = KEYS[1]
local limit  = tonumber(ARGV[1])
local window = tonumber(ARGV[2])

local count = redis.call('INCR', key)
if count == 1 then
    redis.call('EXPIRE', key, window)
end

local ttl = redis.call('PTTL', key)

if count > limit then
    return {0, ttl}
end
return {1, ttl}
LUA;

/**
 * @return array{allowed: bool, retry_after_ms: int}
 */
function fixedWindowAllow(Client $redis, string $key, int $limit, int $windowSeconds): array
{
    [$allowed, $ttl] = $redis->eval(FIXED_WINDOW_SCRIPT, 1, $key, $limit, $windowSeconds);

    return [
        'allowed' => $allowed === 1,
        'retry_after_ms' => $allowed === 1 ? 0 : (int) $ttl,
    ];
}

Trade-off: A client can make 2x requests by sending limit requests at the end of one window and limit requests at the start of the next.

Sliding window log

Records the exact timestamp of every request in a sorted set. Provides a true rolling window with no boundary bursts.

<?php

use Predis\Client;

const SLIDING_WINDOW_LOG_SCRIPT = <<<'LUA'
local key    = KEYS[1]
local limit  = tonumber(ARGV[1])
local window = tonumber(ARGV[2])
local member = ARGV[3]

local t      = redis.call('TIME')
local now    = tonumber(t[1]) + tonumber(t[2]) / 1e6
local cutoff = now - window

redis.call('ZREMRANGEBYSCORE', key, '-inf', cutoff)

local count = redis.call('ZCARD', key)

if count < limit then
    redis.call('ZADD', key, now, member)
    redis.call('EXPIRE', key, window * 2)
    return {1, 0}
end

local oldest = redis.call('ZRANGE', key, 0, 0, 'WITHSCORES')
local retry_after_ms = 0
if oldest[2] then
    retry_after_ms = math.floor((tonumber(oldest[2]) + window - now) * 1000)
end

return {0, retry_after_ms}
LUA;

function slidingWindowLogAllow(Client $redis, string $key, int $limit, int $windowSeconds): array
{
    $member = bin2hex(random_bytes(16));

    [$allowed, $retryAfterMs] = $redis->eval(
        SLIDING_WINDOW_LOG_SCRIPT, 1, $key, $limit, $windowSeconds, $member
    );

    return ['allowed' => $allowed === 1, 'retry_after_ms' => (int) $retryAfterMs];
}

Trade-off: Memory grows O(n) with request volume. Not ideal for high-volume, high-cardinality rate limiting.

Sliding window counter

Blends two fixed-window counters using a weighted average to approximate a true sliding window. Near-exact accuracy with the same low memory footprint as a fixed window. The two keys use hash tags so they map to the same slot in Redis Cluster.

<?php

use Predis\Client;

const SLIDING_WINDOW_COUNTER_SCRIPT = <<<'LUA'
local base   = KEYS[1]
local limit  = tonumber(ARGV[1])
local window = tonumber(ARGV[2])

local t   = redis.call('TIME')
local now = tonumber(t[1]) + tonumber(t[2]) / 1e6

local window_num = math.floor(now / window)
local elapsed     = (now % window) / window

local curr_key = base .. ':' .. window_num
local prev_key = base .. ':' .. (window_num - 1)

local prev = tonumber(redis.call('GET', prev_key) or 0)
local curr = tonumber(redis.call('GET', curr_key) or 0)

local estimate = prev * (1 - elapsed) + curr

if estimate >= limit then
    return {0, 0}
end

local new_count = redis.call('INCR', curr_key)
if new_count == 1 then
    redis.call('EXPIRE', curr_key, window * 2)
end

return {1, 0}
LUA;

function slidingWindowCounterAllow(Client $redis, string $key, int $limit, int $windowSeconds): array
{
    [$allowed] = $redis->eval(
        SLIDING_WINDOW_COUNTER_SCRIPT, 1, '{' . $key . '}', $limit, $windowSeconds
    );

    return ['allowed' => $allowed === 1];
}

Trade-off: The weighted estimate may let slightly more or fewer requests through than the exact limit. Negligible for most apps.

Leaky bucket (policing)

A virtual bucket fills with incoming requests and drains at a fixed rate. If the bucket is full, requests are rejected immediately. This is the policing variant — requests are allowed or denied instantly with no delay.

<?php

use Predis\Client;

const LEAKY_BUCKET_SCRIPT = <<<'LUA'
local key       = KEYS[1]
local capacity  = tonumber(ARGV[1])
local leak_rate = tonumber(ARGV[2])

local t   = redis.call('TIME')
local now = tonumber(t[1]) + tonumber(t[2]) / 1e6

local data      = redis.call('HGETALL', key)
local level     = 0
local last_leak = now

if #data > 0 then
    for i = 1, #data, 2 do
        if data[i] == 'level' then
            level = tonumber(data[i+1])
        elseif data[i] == 'last_leak' then
            last_leak = tonumber(data[i+1])
        end
    end
end

local elapsed = now - last_leak
level = math.max(0, level - elapsed * leak_rate)

if level + 1 > capacity then
    return {0, math.floor((level + 1 - capacity) / leak_rate * 1000)}
end

level = level + 1
local ttl = math.ceil(capacity / leak_rate) + 1

redis.call('HSET', key, 'level', level, 'last_leak', now)
redis.call('EXPIRE', key, ttl)

return {1, 0}
LUA;

function leakyBucketAllow(Client $redis, string $key, int $capacity, float $leakRate): array
{
    [$allowed, $retryAfterMs] = $redis->eval(
        LEAKY_BUCKET_SCRIPT, 1, $key, $capacity, $leakRate
    );

    return ['allowed' => $allowed === 1, 'retry_after_ms' => (int) $retryAfterMs];
}

Trade-off: Overflow traffic is rejected immediately. Clients must handle 429 Too Many Requests and retry with backoff.

Learn more

EVAL command - Execute Lua scripts
EVALSHA command - Execute cached Lua scripts
Lua scripting - Introduction to Redis Lua scripting
HMGET command - Get multiple hash fields
HMSET command - Set multiple hash fields
PHP client - Redis PHP client documentation

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