HAProxy

HAProxy, short for High Availability Proxy, is open-source software that provides high-availability load balancing and reverse proxy capabilities for TCP and HTTP applications. It is designed to handle large numbers of concurrent connections with low latency and is widely used to improve the reliability and scalability of web services. HAProxy runs on Linux and other Unix-like systems and is written in C. The project originated in 2001, created by Willy Tarreau to meet the needs of high-traffic sites; it has since become one of the most widely deployed load balancers in production. It is available in a Community Edition and a commercial Enterprise edition offered by HAProxy Technologies.

In operation, HAProxy accepts client connections on one or more front-end ports, dispatches requests to back-end servers according to configurable load-balancing algorithms such as round robin, least connections, or source IP hash, and performs health checks to detect failed back-end nodes. It supports both layer 4 (TCP) and layer 7 (HTTP) load balancing, SSL/TLS termination, and advanced routing based on ACLs, headers, cookies, and URL patterns. Features include session persistence, rate limiting, dynamic reconfiguration, HTTP compression, and monitoring with a built-in statistics page and a runtime API.

HAProxy is favored for its performance, predictability, and extensive configuration options, making it a common choice for high-traffic websites, APIs, and microservices architectures. It integrates with orchestration platforms and monitoring stacks and has a large ecosystem of accompanying tools and extensions.