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Server Workers - Uvicorn with Workers

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Let's check back those deployment concepts from before:

  • Security - HTTPS
  • Running on startup
  • Restarts
  • Replication (the number of processes running)
  • Memory
  • Previous steps before starting

Up to this point, with all the tutorials in the docs, you have probably been running a server program, for example, using the fastapi command, that runs Uvicorn, running a single process.

When deploying applications you will probably want to have some replication of processes to take advantage of multiple cores and to be able to handle more requests.

As you saw in the previous chapter about Deployment Concepts, there are multiple strategies you can use.

Here I'll show you how to use Uvicorn with worker processes using the fastapi command or the uvicorn command directly.

Info

If you are using containers, for example with Docker or Kubernetes, I'll tell you more about that in the next chapter: FastAPI in Containers - Docker.

In particular, when running on Kubernetes you will probably not want to use workers and instead run a single Uvicorn process per container, but I'll tell you about it later in that chapter.

Multiple Workers

You can start multiple workers with the --workers command line option:

If you use the fastapi command:

fast →fastapi run --workers 4 main.py
   FastAPI   Starting production server 🚀

             Searching for package file structure from directories with
             __init__.py files
             Importing from /home/user/code/awesomeapp

    module   🐍 main.py

      code   Importing the FastAPI app object from the module with the
             following code:

             from main import app

       app   Using import string: main:app

    server   Server started at http://0.0.0.0:8000
    server   Documentation at http://0.0.0.0:8000/docs

             Logs:

      INFO   Uvicorn running on http://0.0.0.0:8000 (Press CTRL+C to
             quit)
      INFO   Started parent process [27365]
      INFO   Started server process [27368]
      INFO   Started server process [27369]
      INFO   Started server process [27370]
      INFO   Started server process [27367]
      INFO   Waiting for application startup.
      INFO   Waiting for application startup.
      INFO   Waiting for application startup.
      INFO   Waiting for application startup.
      INFO   Application startup complete.
      INFO   Application startup complete.
      INFO   Application startup complete.
      INFO   Application startup complete.

restart ↻

If you prefer to use the uvicorn command directly:

fast →uvicorn main:app --host 0.0.0.0 --port 8080 --workers 4INFO:     Uvicorn running on http://0.0.0.0:8080 (Press CTRL+C to quit)
INFO:     Started parent process [27365]
INFO:     Started server process [27368]
INFO:     Waiting for application startup.
INFO:     Application startup complete.
INFO:     Started server process [27369]
INFO:     Waiting for application startup.
INFO:     Application startup complete.
INFO:     Started server process [27370]
INFO:     Waiting for application startup.
INFO:     Application startup complete.
INFO:     Started server process [27367]
INFO:     Waiting for application startup.
INFO:     Application startup complete.

restart ↻

The only new option here is --workers telling Uvicorn to start 4 worker processes.

You can also see that it shows the PID of each process, 27365 for the parent process (this is the process manager) and one for each worker process: 27368, 27369, 27370, and 27367.

Deployment Concepts

Here you saw how to use multiple workers to parallelize the execution of the application, take advantage of multiple cores in the CPU, and be able to serve more requests.

From the list of deployment concepts from above, using workers would mainly help with the replication part, and a little bit with the restarts, but you still need to take care of the others:

  • Security - HTTPS
  • Running on startup
  • Restarts
  • Replication (the number of processes running)
  • Memory
  • Previous steps before starting

Containers and Docker

In the next chapter about FastAPI in Containers - Docker I'll explain some strategies you could use to handle the other deployment concepts.

I'll show you how to build your own image from scratch to run a single Uvicorn process. It is a simple process and is probably what you would want to do when using a distributed container management system like Kubernetes.

Recap

You can use multiple worker processes with the --workers CLI option with the fastapi or uvicorn commands to take advantage of multi-core CPUs, to run multiple processes in parallel.

You could use these tools and ideas if you are setting up your own deployment system while taking care of the other deployment concepts yourself.

Check out the next chapter to learn about FastAPI with containers (e.g. Docker and Kubernetes). You will see that those tools have simple ways to solve the other deployment concepts as well. ✨

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