HAProxy

Free, high-performance reverse proxy offering load balancing and proxying for TCP and HTTP-based applications. Suited for high-traffic sites with event-driven architecture, multi-threading, and low latency. Handles millions of requests per second with sub-millisecond response times, and scales efficiently on modern hardware including multi-core systems.

HAProxy is the de-facto standard open source load balancer, powers a significant portion of the world's most visited sites, and is shipped with most mainstream Linux distributions and many cloud platforms. Notable adopters include Airbnb, Alibaba, Adobe, DISQUS, and GitHub. Compared to alternatives like Nginx and Envoy, it stands out for its long history of extreme reliability: users often report the process running uninterrupted for years, with rigorous development practices including LTS branches maintained for five years.

Key capabilities:

• TCP, HTTP, HTTP/2, HTTP/3, QUIC, and gRPC support with multiple load balancing algorithms
• ACL-based content switching and Lua scripting for routing and custom logic
• Built-in health checks, seamless reloads, and runtime server addition or removal
• Stats interface and Prometheus integration for observability
• ACME for certificate management and support for multiple SSL libraries including OpenSSL, WolfSSL, and AWS-LC

Engineers deploy it at the edge for SSL termination, in front of web servers for horizontal scaling, or between microservices for internal traffic routing. It fits well in container and Kubernetes setups, with official Docker images and the ability to handle dynamic backends via DNS or service discovery. The dataplane API enables configuration updates without restarts, and integration with monitoring stacks supports capacity planning and incident response.