mirror of https://github.com/tiangolo/fastapi.git
61 lines
2.6 KiB
Markdown
61 lines
2.6 KiB
Markdown
You can add middleware to **FastAPI** applications.
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A "middleware" is a function that works with every **request** before it is processed by any specific *path operation*. And also with every **response** before returning it.
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* It takes each **request** that comes to your application.
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* It can then do something to that **request** or run any needed code.
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* Then it passes the **request** to be processed by the rest of the application (by some *path operation*).
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* It then takes the **response** generated by the application (by some *path operation*).
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* It can do something to that **response** or run any needed code.
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* Then it returns the **response**.
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## Create a middleware
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To create a middleware you use the decorator `@app.middleware("http")` on top of a function.
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The middleware function receives:
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* The `request`.
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* A function `call_next` that will receive the `request` as a parameter.
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* This function will pass the `request` to the corresponding *path operation*.
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* Then it returns the `response` generated by the corresponding *path operation*.
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* You can then modify further the `response` before returning it.
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```Python hl_lines="9 10 12 15"
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{!./src/middleware/tutorial001.py!}
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```
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!!! tip
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This technique is used in the tutorial about <a href="https://fastapi.tiangolo.com/tutorial/sql-databases/" target="_blank">SQL (Relational) Databases</a>.
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!!! tip
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Have in mind that custom proprietary headers can be added <a href="https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers" target="_blank">using the 'X-' prefix</a>.
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But if you have custom headers that you want a client in a browser to be able to see, you need to add them to your <a href="https://fastapi.tiangolo.com/tutorial/cors/" target="_blank">CORS configurations</a>, using the parameter `expose_headers` documented in <a href="https://www.starlette.io/middleware/#corsmiddleware" target="_blank">Starlette's CORS docs</a>.
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### Before and after the `response`
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You can add code to be run with the `request`, before any *path operation* receives it.
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And also after the `response` is generated, before returning it.
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For example, you could add a custom header `X-Process-Time` containing the time in seconds that it took to process the request and generate a response:
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```Python hl_lines="11 13 14"
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{!./src/middleware/tutorial001.py!}
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```
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## Starlette's Middleware
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You can also add any other <a href="https://www.starlette.io/middleware/" target="_blank">Starlette Middleware</a>.
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These are classes instead of plain functions.
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Including:
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* `CORSMiddleware` (described in the next section).
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* `GZipMiddleware`.
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* `SentryMiddleware`.
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* ...and others.
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