FastAPI Uvicorn

What is Uvicorn?

Uvicorn is a lightning-fast ASGI server implementation for Python that supports asynchronous web frameworks like FastAPI and Starlette. It is designed to handle asynchronous workloads and efficiently serve HTTP requests for Python applications. Uvicorn is widely used for running FastAPI applications and is based on the ASGI (Asynchronous Server Gateway Interface) specification, which allows for handling async/await code natively.

How do you install Uvicorn?

Uvicorn can be installed using the pip package manager. You can either install it as a standalone package or include it when installing a framework like FastAPI.

Example of installing Uvicorn:

pip install uvicorn

If you're using FastAPI, you can install both FastAPI and Uvicorn together:

pip install fastapi uvicorn

How do you run a FastAPI application using Uvicorn?

To run a FastAPI application with Uvicorn, you need to specify the Python file that contains the FastAPI app, and Uvicorn will serve it. You can specify the app with the syntax module_name:app_name, and Uvicorn will start the ASGI server to handle requests.

Example of running a FastAPI app using Uvicorn:

uvicorn main:app --reload

In this example, Uvicorn is instructed to run the FastAPI app defined in the main.py file. The --reload flag is used to enable automatic reloading when changes are made to the code, making it ideal for development environments.

What is the purpose of the --reload flag in Uvicorn?

The --reload flag is used in Uvicorn to enable automatic reloading of the application during development. When this flag is set, Uvicorn watches for file changes in the application and reloads the server whenever changes are detected. This eliminates the need to manually restart the server after making code updates.

Example of running Uvicorn with the --reload flag:

uvicorn main:app --reload

This command will automatically reload the app when changes are made to the main.py file, speeding up the development workflow.

How do you run Uvicorn in production mode?

In production, you should run Uvicorn without the --reload flag and with multiple workers to handle concurrent requests. You can specify the number of workers using the --workers option, which allows Uvicorn to handle more requests simultaneously.

Example of running Uvicorn in production mode:

uvicorn main:app --host 0.0.0.0 --port 8000 --workers 4

In this example, Uvicorn is started with 4 workers, allowing the app to handle multiple requests in parallel. It also listens on 0.0.0.0, making it accessible from any IP address.

How do you run Uvicorn behind a reverse proxy like Nginx?

When deploying a FastAPI app using Uvicorn in production, it is common to run Uvicorn behind a reverse proxy like Nginx. Nginx acts as a front-end server that forwards client requests to Uvicorn and handles things like SSL termination, load balancing, and serving static files.

Example of an Nginx configuration to proxy requests to Uvicorn:

server {
    listen 80;
    server_name your_domain.com;

    location / {
        proxy_pass http://127.0.0.1:8000;
        proxy_set_header Host $host;
        proxy_set_header X-Real-IP $remote_addr;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
        proxy_set_header X-Forwarded-Proto $scheme;
    }
}

In this example, Nginx is configured to forward requests to the Uvicorn server running on http://127.0.0.1:8000. Nginx also sets appropriate headers to pass client information to Uvicorn.

What is the --host option in Uvicorn?

The --host option in Uvicorn specifies the IP address or hostname that Uvicorn will bind to. By default, Uvicorn binds to 127.0.0.1, making the application accessible only from the local machine. To make the app accessible from other machines or to expose it to the internet, you can bind Uvicorn to 0.0.0.0, which allows connections from any IP address.

Example of using the --host option:

uvicorn main:app --host 0.0.0.0

In this example, Uvicorn listens on 0.0.0.0, making the FastAPI app accessible from other devices in the network or from the internet.

What is the --port option in Uvicorn?

The --port option in Uvicorn specifies the port number on which Uvicorn will serve the application. The default port is 8000, but you can specify any available port depending on your deployment needs.

Example of using the --port option:

uvicorn main:app --port 8080

In this example, Uvicorn runs the FastAPI app on port 8080 instead of the default port 8000.

What is the --workers option in Uvicorn?

The --workers option in Uvicorn specifies the number of worker processes that will handle requests. Increasing the number of workers allows Uvicorn to handle multiple requests concurrently, making the application more scalable in production environments. The number of workers typically depends on the number of CPU cores available on the server.

Example of using the --workers option:

uvicorn main:app --workers 4

In this example, Uvicorn runs 4 worker processes to handle concurrent requests, improving the application's ability to scale.

How do you serve static files with Uvicorn?

Although Uvicorn is designed primarily to serve dynamic content, you can serve static files (like CSS, JavaScript, or images) directly using a web framework like FastAPI. FastAPI provides built-in support for serving static files using StaticFiles from Starlette.

Example of serving static files with FastAPI and Uvicorn:

from fastapi import FastAPI
from fastapi.staticfiles import StaticFiles

app = FastAPI()

app.mount("/static", StaticFiles(directory="static"), name="static")

In this example, the StaticFiles class is used to serve files from the static directory. You can access these static files at /static/ in the browser, and Uvicorn will serve them.

How do you enable SSL/TLS in Uvicorn?

You can enable SSL/TLS in Uvicorn by specifying an SSL certificate and key using the ssl_context argument. This allows Uvicorn to serve the application over HTTPS, providing encrypted communication between the server and clients.

Example of running Uvicorn with SSL/TLS:

uvicorn main:app --ssl-keyfile=key.pem --ssl-certfile=cert.pem

In this example, Uvicorn uses the provided SSL certificate cert.pem and private key key.pem to enable HTTPS.

How do you configure logging in Uvicorn?

Uvicorn provides built-in support for logging, and you can customize the logging level using the --log-level option. The available logging levels are critical, error, warning, info, debug, and trace.

Example of configuring logging in Uvicorn:

uvicorn main:app --log-level debug

In this example, Uvicorn is started with the debug logging level, which provides detailed information about the application's runtime behavior.

How do you test Uvicorn applications?

You can test Uvicorn applications using standard testing frameworks like pytest along with testing tools like TestClient from FastAPI, which allows you to simulate HTTP requests to your Uvicorn-powered app during tests.

Example of testing a Uvicorn application:

from fastapi.testclient import TestClient
from main import app

client = TestClient(app)

def test_read_root():
    response = client.get("/")
    assert response.status_code == 200
    assert response.json() == {"Hello": "World"}

In this example, the TestClient class is used to send a GET request to the root endpoint /, and the response is checked to ensure the app behaves as expected.