HAProxy Load Balancer

HAProxy is an open-source TCP and HTTP load balancer and reverse proxy created by Willy Tarreau in 2001, providing high-availability load balancing, SSL termination, connection pooling, health checking, rate limiting, and traffic routing with throughput exceeding two million requests per second on modern hardware, written in C for maximum performance, designed as the industry standard for load balancing with deployments in some of the most demanding environments including GitHub, Reddit, Stack Overflow, Tumblr, and Airbnb for distributing traffic across their backend servers. The load balancing algorithms include round-robin for even distribution, static round-robin for deterministic assignment, leastconn for distributing based on connection count, source for sticky sessions based on client IP hash, uri for caching based on URI hash, and first for filling servers sequentially, with each algorithm configurable per backend and supporting server weights for uneven distribution, slow start for gradually ramping up recovered servers, and backup servers for failover scenarios. The health checking system continuously monitors backend server health through TCP connection checks, HTTP request checks with expected status codes and response patterns, SSL hello checks, and custom script-based checks, with the health checking supporting configurable check intervals, fall thresholds for marking servers as down after consecutive failures, and rise thresholds for marking recovered servers as up after consecutive successes, while the inter-fallback feature enables automatic failover between different server groups. The SSL termination with SNI-based routing. The connection pooling for backend reuse. The rate limiting through stick tables. The access control lists for traffic filtering. The HTTP compression. The statistics interface. The dynamic configuration through the runtime API. Designed by Willy Tarreau. Designed for network engineers.