Middleware is essential to any web application development framework, and Python's FastAPI is no exception. In FastAPI, middleware can be used to add additional functionality to the request and response cycle, such as authentication, rate limiting, or logging.
This blog will explore how to create and use middleware in FastAPI. We will cover the following topics:
- What is middleware?
- Creating middleware in FastAPI
- Adding middleware to FastAPI
- Testing the middleware
So, let's get started!
1. What is middleware?
Middleware is a software layer between the client and server in a web application. It intercepts the requests and responses and can add, modify or delete the information as it passes through. In other words, middleware is a set of functions executed before or after the web application handles the request.
Middleware is useful for adding additional functionality to an application, such as authentication, logging, rate limiting, or modifying requests and responses. It can also perform pre-processing or post-processing of the request/response data.
2. Creating middleware in FastAPI
Creating middleware in FastAPI is straightforward. Middleware is implemented as a function that takes two arguments: the request and the call_next function. The request object contains information about the incoming request, while the call_next function is used to call the next middleware function or the request handler.
Here's an example of a simple middleware function that logs the request method and URL,
from fastapi import FastAPI
app = FastAPI()
@app.middleware("http")
async def log_request(request, call_next):
print(f"Received request: {request.method} {request.url}")
response = await call_next(request)
return response
In this example, we define a middleware function log_request that takes two arguments request and call_next. The request object contains information about the incoming request and the call_next function is used to call the next middleware function or the request handler.
The log_request function logs the incoming request method and URL and then calls the next middleware function or the request handler using call_next(request).
3. Adding middleware to FastAPI
To add middleware to FastAPI, we need to use the app.middleware decorator. The decorator takes one argument, the type of middleware, which can be "http" for HTTP middleware or "websocket" for WebSocket middleware.
Here's an example of adding the log_request middleware function to our FastAPI application,
from fastapi import FastAPI
app = FastAPI()
@app.middleware("http")
async def log_request(request, call_next):
print(f"Received request: {request.method} {request.url}")
response = await call_next(request)
return response
In this example, we add the log_request middleware function to our FastAPI application using the app.middleware decorator. We then define a simple route using the @app.get decorator.
When we run the application and make a request to the root endpoint (http://localhost:8000/), we should see the log message in the console,
Received request: GET http://localhost:8000/