Varnish HTTP Cache

Varnish Cache is an open-source HTTP accelerator and reverse proxy created by Poul-Henning Kamp in 2006, providing high-performance caching of HTTP responses with throughput exceeding one hundred thousand requests per second on modern hardware, written in C for maximum performance, designed to sit in front of web servers and cache their responses for dramatically reduced response times and server load, adopted by some of the most trafficked websites including Wikipedia, The Guardian, Reddit, and The New York Times for their content delivery. The caching architecture stores HTTP responses in a shared memory cache, with the cache organized into hash entries indexed by URL, host header, and custom keys, enabling sub-millisecond cache lookups without disk I/O, while the event-driven architecture uses a worker thread per connection model with non-blocking I/O, enabling the cache to handle tens of thousands of concurrent connections with minimal overhead. The configuration language is a domain-specific language for defining caching behavior, with syntax for receiving requests, looking up cache entries, fetching from backend servers, delivering responses, and handling errors, enabling complex caching rules including cache invalidation, content purging, cookie handling, device detection, and geo-based routing. The grace mode for serving stale content during backend failures. The backend health checking. The ESI for edge-side includes. The real-time log streaming. The statistics through the stat command. Designed by Poul-Henning Kamp. Designed for web performance.