This can be very useful for setting up **resources** that you need to use for the whole app, and that are **shared** among requests, and/or that you need to **clean up** afterwards. For example, a database connection pool, or loading a shared machine learning model.

## Use Case

Let's start with an example **use case** and then see how to solve it with this.

Let's imagine that you have some **machine learning models** that you want to use to handle requests. 🤖

  - type: textarea
    attributes:
      label: 1. What are you trying to do?
    validations:
      required: true

  - type: textarea
    attributes:
      label: 2. What's the best code you can write to accomplish that without the new feature?
    validations:
      required: true

  - type: textarea
    attributes:
      label: 3. What would that same code look like if we added your feature?
    validations:

```txt
fastapi==0.45.0
```

that would mean that you would use exactly the version `0.45.0`.

Or you could also pin it with:

```txt
fastapi>=0.45.0,<0.46.0
```

that would mean that you would use the versions `0.45.0` or above, but less than `0.46.0`, for example, a version `0.45.2` would still be accepted.

<div class="termy">

```console
$ pip install "fastapi[all]"

---> 100%
```

</div>

...that also includes `uvicorn`, that you can use as the server that runs your code.

!!! note
    You can also install it part by part.

    This is what you would probably do once you want to deploy your application to production:

    ```
    pip install fastapi
    ```

```

The `@app.get("/")` tells **FastAPI** that the function right below is in charge of handling requests that go to:

* the path `/`
* using a <abbr title="an HTTP GET method"><code>get</code> operation</abbr>

!!! info "`@decorator` Info"
    That `@something` syntax in Python is called a "decorator".

    You put it on top of a function. Like a pretty decorative hat (I guess that's where the term came from).

  HttpUrl baseUrl = server.url("/v1/chat/");

  // Exercise your application code, which should make those HTTP requests.
  // Responses are returned in the same order that they are enqueued.
  Chat chat = new Chat(baseUrl);

  chat.loadMore();
  assertEquals("hello, world!", chat.messages());

  chat.loadMore();
  chat.loadMore();
  assertEquals(""
      + "hello, world!\n"
      + "sup, bra?\n"

   - [artifactId] tells maven what the artifact name is for this particular
       project. Each project should have a distinct artifactId.

   - [version] tells maven what release of this artifact we're trying to produce.
       The fact that a project has a distinct pom.xml should indicate a separate
       release cycle that is also distinct to that project, so a concrete version
       declaration is required.

#### JVM platform

This is a platform that builds on the core and software platforms to add support for developing software that runs on the JVM.
This includes software that is implemented using Java, Kotlin or some other JVM language.

This platform provides specific support for Java, Groovy and Scala, and includes the foojay toolchain plugin. 

#### Extensibility platform

## OAuth2

OAuth2 is a specification that defines several ways to handle authentication and authorization.

It is quite an extensive specification and covers several complex use cases.

It includes ways to authenticate using a "third party".

That's what all the systems with "login with Facebook, Google, Twitter, GitHub" use underneath.

### OAuth 1

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).

The process that happens when your API app calls the *external API* is named a "callback". Because the software that the external developer wrote sends a request to your API and then your API *calls back*, sending a request to an *external API* (that was probably created by the same developer).