Sie sehen eine einfache Seite wie:

<img src="/img/tutorial/websockets/image01.png">

Sie können Nachrichten in das Eingabefeld tippen und absenden:

<img src="/img/tutorial/websockets/image02.png">

Und Ihre **FastAPI**-Anwendung mit WebSockets antwortet:

<img src="/img/tutorial/websockets/image03.png">

{!../../../docs_src/events/tutorial002.py!}
```

Hier schreibt die `shutdown`-Eventhandler-Funktion eine Textzeile `"Application shutdown"` in eine Datei `log.txt`.

!!! info
    In der Funktion `open()` bedeutet `mode="a"` „append“ („anhängen“), sodass die Zeile nach dem, was sich in dieser Datei befindet, hinzugefügt wird, ohne den vorherigen Inhalt zu überschreiben.

!!! tip "Tipp"

Including external libraries also based on Pydantic, as <abbr title="Object-Relational Mapper">ORM</abbr>s, <abbr title="Object-Document Mapper">ODM</abbr>s for databases.

This also means that in many cases you can pass the same object you get from a request **directly to the database**, as everything is validated automatically.

✅ 🔢 🗃 ⚓️ 🔛 Pydantic, <abbr title="Object-Relational Mapper">🐜</abbr>Ⓜ, <abbr title="Object-Document Mapper">🏭</abbr>Ⓜ 💽.

👉 ⛓ 👈 📚 💼 👆 💪 🚶‍♀️ 🎏 🎚 👆 🤚 ⚪️➡️ 📨 **🔗 💽**, 🌐 ✔ 🔁.

🎏 ✔ 🎏 🌌 🤭, 📚 💼 👆 💪 🚶‍♀️ 🎚 👆 🤚 ⚪️➡️ 💽 **🔗 👩‍💻**.

# OpenAPI Callbacks

You could create an API with a *path operation* that could trigger a request to an *external API* created by someone else (probably the same developer that would be *using* your API).

# Exceptions - `HTTPException` and `WebSocketException`

These are the exceptions that you can raise to show errors to the client.

When you raise an exception, as would happen with normal Python, the rest of the execution is aborted. This way you can raise these exceptions from anywhere in the code to abort a request and show the error to the client.

You can use:

* `HTTPException`
* `WebSocketException`

!!! tip
    The `shutdown` would happen when you are **stopping** the application.

    Maybe you need to start a new version, or you just got tired of running it. 🤷

### Lifespan function

So, the section:

```Python
    uvicorn.run(app, host="0.0.0.0", port=8000)
```

will run.

---

This won't happen if you import that module (file).

So, if you have another file `importer.py` with:

```Python
from myapp import app

# Some more code
```

# Background Tasks

You can define background tasks to be run *after* returning a response.

This is useful for operations that need to happen after a request, but that the client doesn't really have to be waiting for the operation to complete before receiving the response.

This includes, for example:

* Email notifications sent after performing an action:

<img src="/img/tutorial/body/image02.png">

## Editor support

In your editor, inside your function you will get type hints and completion everywhere (this wouldn't happen if you received a `dict` instead of a Pydantic model):

<img src="/img/tutorial/body/image03.png">

You also get error checks for incorrect type operations:

<img src="/img/tutorial/body/image04.png">