# Middleware

You can add middleware to **FastAPI** applications.

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.

* It takes each **request** that comes to your application.
* It can then do something to that **request** or run any needed code.

If you use any other tool to manage your installations, like Poetry, Pipenv, or others, they all have a way that you can use to define specific versions for your packages.

## Available versions

You can see the available versions (e.g. to check what is the current latest) in the [Release Notes](../release-notes.md){.internal-link target=_blank}.

## About versions

Here is the reference for it and its parameters.

You can import it directly from `fastapi`:

```python
from fastapi import Depends
```

::: fastapi.Depends

## `Security()`

For many scenarios, you can handle security (authorization, authentication, etc.) with dependencies, using `Depends()`.

But when you want to also declare OAuth2 scopes, you can use `Security()` instead of `Depends()`.

In 12 drive example above, with MinIO server running in the default configuration, you can lose any of the six drives and still reconstruct the data reliably from the remaining drives.

## Why is Erasure Code useful?

# Declare Request Example Data

You can declare examples of the data your app can receive.

Here are several ways to do it.

## Extra JSON Schema data in Pydantic models

You can declare `examples` for a Pydantic model that will be added to the generated JSON Schema.

=== "Python 3.10+ Pydantic v2"

    ```Python hl_lines="13-24"
    {!> ../../../docs_src/schema_extra_example/tutorial001_py310.py!}
    ```

# `Response` class

You can declare a parameter in a *path operation function* or dependency to be of type `Response` and then you can set data for the response like headers or cookies.

You can also use it directly to create an instance of it and return it from your *path operations*.

You can import it directly from `fastapi`:

```python
from fastapi import Response
```

Let's see a very simple example. It will be so simple that it is not very useful, for now.

But this way we can focus on how the **Dependency Injection** system works.

### Create a dependency, or "dependable"

Let's first focus on the dependency.

It is just a function that can take all the same parameters that a *path operation function* can take:

=== "Python 3.10+"

    ```Python hl_lines="8-9"

- we need to deal with plugins as core application logic which can also interact with plugins

* Along with this comes the testing strategies that make this work
* The repository model where plugins can be stored and cataloged
* Bridging this into an OSGi system: if this could be done then we can basically take over p2

TODO

So, you can look for that specific exception inside the dependency with `except SomeException`.

In the same way, you can use `finally` to make sure the exit steps are executed, no matter if there was an exception or not.

```Python hl_lines="3  5"
{!../../../docs_src/dependencies/tutorial007.py!}
```

## Sub-dependencies with `yield`

## Dependencies errors and return values

You can use the same dependency *functions* you use normally.

### Dependency requirements

They can declare request requirements (like headers) or other sub-dependencies:

=== "Python 3.9+"

    ```Python hl_lines="8  13"