Celestial

A lightweight, high-performance, asynchronous HTTP server engine written from scratch in low-level C.

Celestial bypasses heavyweight application frameworks to interface directly with the operating system kernel. Utilizing OS-level event demultiplexing (kqueue on macOS/BSD) and non-blocking I/O via the Reactor Pattern, Celestial achieves massive concurrency with minimal memory overhead and zero CPU idle waste.

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Key Features

• Reactor Pattern Architecture: Built on non-blocking file descriptors (O_NONBLOCK) dispatched through a central event loop.
• Zero-Copy Kernel Notification: Uses OS-native demultiplexers (kqueue / kevent) to process socket events with 0% CPU overhead while idle.
• Bare-Metal Footprint: Tiny memory baseline (~1 MB) compared to heavy runtimes like Node.js or JVM.
• Modular C Design: Clean interface separation across header (.h) and implementation (.c) files.
• Zero Dependencies: Pure C standard library and POSIX/BSD system calls—no external packages or third-party engines.

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Architecture

Instead of allocating a thread per connection (which scales poorly), Celestial uses a Single-Threaded Non-Blocking Reactor Loop—the exact same architectural pattern powering Nginx, Redis, and Node.js (libuv):

+-------------------------------------------------------------------+
|                        CELESTIAL REACTOR                          |
|                                                                   |
|   1. Non-Blocking Sockets   --->  fcntl(fd, O_NONBLOCK)          |
|   2. Event Demultiplexer    --->  kqueue() / kevent()             |
|   3. Event Loop             --->  while (1)                       |
|   4. Handlers               --->  handle_accept() / handle_read() |
+-------------------------------------------------------------------+
                                  |
                                  v
                   +-----------------------------+
                   |          OS KERNEL          |
                   |   Monitors active sockets   |
                   +-----------------------------+