Gunicorn

Gunicorn, short for Green Unicorn, is a Python WSGI HTTP server designed for UNIX-like operating systems. It serves Python web applications by implementing the WSGI specification and acts as an interface between a web server and Python frameworks or apps. It is framework-agnostic and commonly used with Django, Flask, Pyramid, and other WSGI-compatible applications.

Gunicorn uses a pre-fork worker model with a master process that manages multiple worker processes. This design provides process isolation and can improve reliability and throughput on multi-core systems. The default worker class is synchronous, but Gunicorn can run asynchronous workers through libraries such as gevent or eventlet, and it also offers a threaded worker class (gthread). In addition, Gunicorn can run ASGI applications by using the UvicornWorker class, enabling support for asynchronous frameworks like FastAPI when paired with uvicorn.

Usage and configuration are straightforward. A typical start command is gunicorn module:app, where module:app refers to the Python module and application callable. Common options include --bind to specify the host and port or a UNIX socket, -w to set the number of workers, --reload for automatic reloading during development, and --preload to load the application before forking. Gunicorn is commonly used behind a reverse proxy such as Nginx, with configuration handling timeouts, logging, and access controls.

Limitations and deployment notes: Gunicorn is designed for UNIX-like environments and does not have formal Windows support; many deployments on Windows use Windows Subsystem for Linux or alternative servers. It competes with other servers such as uWSGI, mod_wsgi, Waitress, and Daphne, each with different feature sets and compatibility considerations.

License and status: Gunicorn is an open-source project released under a permissive BSD-style license and is maintained by a community of contributors.