Apache Traffic Server is a high-performance HTTP/HTTPS forward and reverse proxy server with caching capabilities, used in production CDNs and large-scale web infrastructures. It handles HTTP/1.1, HTTP/2, HTTP/3 (over QUIC), and WebSocket connections, supporting TLS termination, URL rewriting, access control, and extensive logging. The architecture is event-driven and multi-threaded (thread-per-core design) with a plugin system that allows dynamic loading of C, Lua, and (deprecated) C++ plugins. Core subsystems include the event system (iocore/eventsystem), network I/O (iocore/net), cache (iocore/cache), HTTP state machine (proxy/http), DNS resolution (iocore/dns), logging (proxy/logging), and management RPC (mgmt/rpc). Configuration is primarily YAML-based, with legacy flat-file formats still supported. Key entry points are the main server binary (traffic_server), the management CLI (traffic_ctl), and log analysis tools (traffic_logcat, traffic_top). The primary data flow begins with client connection acceptance, proceeds through the HTTP state machine (HttpSM) which performs cache lookup, origin server connection (on miss), response processing, and logging. Plugins can hook into multiple transaction phases to modify headers, rewrite URLs, enforce access control, or transform content. External dependencies include OpenSSL (or BoringSSL/quictls), PCRE2, Lua, libxml2, libcurl, and compression libraries (zlib, brotli, zstd). Several third-party libraries are vendored in-tree (yamlcpp, Catch2, highway, swoc), which reduces external build complexity but introduces maintenance overhead.