eventlet

Eventlet is a Python networking library designed to make it easier to write scalable, concurrent network applications. It uses green threads, or greenthreads, which are lightweight cooperative threads managed by a single Python process, enabling many connections to be handled with minimal per-connection overhead.

Eventlet achieves this through a cooperative event loop that multiplexes I/O across greenthreads. It relies on non-blocking I/O and can patch standard library modules via monkey patching so existing blocking calls yield control to the scheduler. This approach allows thousands of greenthreads to run concurrently within a single process, provided the code uses non-blocking I/O.

Eventlet provides a green socket API that mirrors Python's socket module, an asynchronous DNS resolver, and a WSGI server (eventlet.wsgi.server) for serving Python web applications. It also includes utilities such as a green pool of workers and helpers for networking operations. The core event loop can use backend multiplexers like libevent or other platform-specific selectors to monitor socket readiness.

Common usage patterns include applying eventlet's monkey_patch to replace blocking I/O, spawning greenthreads with eventlet.spawn, and using the eventlet.wsgi.server for WSGI applications or eventlet.listen for sockets. It is widely used in web services, microservices, and distributed systems that require scalable I/O-bound concurrency. Limitations include the need to adapt code to a cooperative model and the potential compatibility issues from monkey patching; CPU-bound tasks should be handled with separate processes or threading.