gunicorn vs uvicorn notes

TLTR: gunicorn for WSGI app (like Flask), uvicorn for ASGI app (like FastAPI)


Gunicorn is an application server supports the WSGI standard. This means that Gunicorn can serve applications written in frameworks such as Flask or Django (more so for versions released before 2021). The way it works is that it creates and maintains their operability a configurable number of application instances (aka workers) that serve requests from clients. Gunicorn itself is not compatible with FastAPI because FastAPI uses the fresh ASGI standard.

Uvicorn is an app server supports the ASGI protocol. However, it's capabilities as a process (workers) manager leave much to be desired.

But Uvicorn has a Gunicorn-compatible worker class. Using that combination, Gunicorn would act as a process manager, listening on the port and the IP. And it would transmit the communication to the worker processes running the Uvicorn class. And then the Gunicorn-compatible Uvicorn worker class would be in charge of converting the data sent by Gunicorn to the ASGI standard for FastAPI to use it.

If you have a cluster of machines with Kubernetes, Docker Swarm or another similar complex system to manage distributed containers on multiple machines, then you will probably want to handle replication at the cluster level instead of using a process manager (like Gunicorn with workers) in each container. One of those distributed container management systems like Kubernetes normally has some integrated way of handling replication of containers while still supporting load balancing for the incoming requests. All at the cluster level. In those cases, you would probably want to build a Docker image from scratch, installing your dependencies, and running a single Uvicorn process instead of running something like Gunicorn with Uvicorn workers.