Rate Limiting in C++: Libraries and Tools

Rate Limiting in C++: A Complete Guide to Request Limiting

Rate limiting is a critically important mechanism for protecting servers, APIs, and distributed systems from overloads, DDoS attacks, and random traffic spikes. In C++, implementing rate limiting requires efficient algorithms and the right choice of libraries. In this article, we will break down the main approaches, popular libraries, and provide ready-made code examples.

What is rate limiting and why is it needed?

Rate limiting sets the maximum number of requests a client or system can perform within a specific time period. The main goals are:

Protection from abuse and server overload
Ensuring fair distribution of resources among users
Preventing DDoS attacks and brute-force attacks
Controlling API quota consumption

Basic rate limiting algorithms

In C++, three classic algorithms are most commonly used:

1. Token Bucket

The most popular algorithm. It represents a "bucket" of tokens that are replenished at a constant rate. Each request takes one token. If there are no tokens, the request is rejected.

#include <chrono>#include <mutex>

class TokenBucket {private:    double tokens;    double maxTokens;    double refillRate; // tokens per second    std::chrono::steady_clock::time_point lastRefill;    std::mutex mtx;

public:    TokenBucket(double maxTokens, double refillRate)         : tokens(maxTokens), maxTokens(maxTokens), refillRate(refillRate) {        lastRefill = std::chrono::steady_clock::now();    }

    bool tryConsume() {        std::lock_guard<std::mutex> lock(mtx);        refill();        if (tokens >= 1.0) {            tokens -= 1.0;            return true;        }        return false;    }

private:    void refill() {        auto now = std::chrono::steady_clock::now();        auto elapsed = std::chrono::duration_cast<std::chrono::milliseconds>(now - lastRefill).count() / 1000.0;        tokens = std::min(maxTokens, tokens + elapsed * refillRate);        lastRefill = now;    }};

2. Leaky Bucket

Requests enter a queue (bucket) which is processed at a fixed rate. If the bucket overflows, requests are discarded.

#include <queue>#include <chrono>

class LeakyBucket {private:    std::queue<int> bucket;    size_t capacity;    double leakRate; // requests per second    std::chrono::steady_clock::time_point lastLeak;    std::mutex mtx;

public:    LeakyBucket(size_t capacity, double leakRate)         : capacity(capacity), leakRate(leakRate) {        lastLeak = std::chrono::steady_clock::now();    }

    bool addRequest(int request) {        std::lock_guard<std::mutex> lock(mtx);        leak();        if (bucket.size() < capacity) {            bucket.push(request);            return true;        }        return false;    }

private:    void leak() {        auto now = std::chrono::steady_clock::now();        auto elapsed = std::chrono::duration_cast<std::chrono::milliseconds>(now - lastLeak).count() / 1000.0;        size_t toRemove = static_cast<size_t>(elapsed * leakRate);        while (toRemove > 0 && !bucket.empty()) {            bucket.pop();            toRemove--;        }        lastLeak = now;    }};