API Gateway for C++ Services: From Idea to Production

API Gateway for C++ Services: When Performance Matters Most

Imagine: you have a dozen microservices in C++, each a true performance beast. Clients send thousands of requests per second, and the system starts slowing down because of... the gateway. Sound familiar? Most developers drag Nginx with Lua or heavyweight Kong into the C++ ecosystem, when they could write their own API Gateway in C++ — fast, lightweight, and predictable.

In this article, I'll show you how to build your own API gateway in C++ from scratch. We'll break down the architecture, write working code in pure C++, and compare it with popular solutions. You'll learn when a custom API Gateway is a justified choice and when it's better to use an off-the-shelf solution.

Why Write an API Gateway in C++?

Let's be honest: most projects don't need a custom gateway. Nginx, Envoy, or Kong solve 90% of tasks. But there are scenarios where C++ is irreplaceable:

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• Latency under 1 ms — financial systems, game servers, real-time trading
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• Millions of RPS — high-load APIs, CDNs, IoT platforms
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• Strict memory requirements — embedded systems, edge devices
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• Tight integration with C++ code — calling native libraries with zero overhead
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Moreover, an API Gateway C++ gives you full control over every byte. You decide how to handle headers, how to cache, how to balance load. No magic — only predictable performance.

Architecture of an API Gateway in C++

A typical API gateway in C++ consists of several layers:

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1. Transport layer — receiving and sending HTTP requests (boost::beast, libcurl, custom socket)
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2. Router — matching URLs to backend services
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3. Middleware — authentication, rate limiting, logging, CORS
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4. Load Balancer — round-robin, least-connections, weighted
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5. Cache — in-memory response cache (LRU, LFU)
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Here's a schematic view:

Client → [API Gateway C++] → Middleware → Router → Balancer → Backend A\n                               ↓\n                           [Cache]

Each layer is a separate module. You can assemble them like building blocks. Want to add JWT validation? Write a middleware. Need a new balancing algorithm? Add a class.

Writing an API Gateway in C++: A Practical Example

Let's use modern C++20 and the boost::beast library for HTTP. Our gateway will:

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• Accept requests on port 8080
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• Route based on URL prefix
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• Validate API keys
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• Limit to 100 requests per minute per IP
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• Balance between two backends
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Step 1: Basic Structure

#include <boost/beast/core.hpp>\n#include <boost/beast/http.hpp>\n#include <boost/beast/version.hpp>\n#include <boost/asio/ip/tcp.hpp>\n#include <unordered_map>\n#include <memory>\n#include <string>\n\nnamespace beast = boost::beast;\nnamespace http = beast::http;\nnamespace net = boost::asio;\nusing tcp = net::ip::tcp;\n\n// Structure for storing backend information\nstruct Backend {\n    std::string host;\n    std::string port;\n    int weight;\n};\n\n// Router class\nclass Router {\n    std::unordered_map<std::string, std::vector<Backend>> routes_;\n    std::unordered_map<std::string, size_t> counters_; // round-robin\n\npublic:\n    void add_route(const std::string& prefix, std::vector<Backend> backends) {\n        routes_[prefix] = std::move(backends);\n        counters_[prefix] = 0;\n    }\n\n    Backend get_backend(const std::string& path) {\n