But in most cases, you will want to perform these steps only **once**.

So, you will want to have a **single process** to perform those **previous steps**, before starting the application.

## Use the `fastapi run` Command

In short, use `fastapi run` to serve your FastAPI application:

<div class="termy">

```console
$ <font color="#4E9A06">fastapi</font> run <u style="text-decoration-style:single">main.py</u>
<font color="#3465A4">INFO    </font> Using path <font color="#3465A4">main.py</font>

    ```

!!! info
    `Body` also has all the same extra validation and metadata parameters as `Query`,`Path` and others you will see later.

## Embed a single body parameter

Let's say you only have a single `item` body parameter from a Pydantic model `Item`.

By default, **FastAPI** will then expect its body directly.

# Bigger Applications - Multiple Files

If you are building an application or a web API, it's rarely the case that you can put everything on a single file.

**FastAPI** provides a convenience tool to structure your application while keeping all the flexibility.

!!! info
    If you come from Flask, this would be the equivalent of Flask's Blueprints.

## An example file structure

Let's say you have a file structure like this:

```
.
├── app

    * This SNI extension allows one single server (with a **single IP address**) to have **several HTTPS certificates** and serve **multiple HTTPS domains/applications**.
    * For this to work, a **single** component (program) running on the server, listening on the **public IP address**, must have **all the HTTPS certificates** in the server.

operations*

Later, when reading about how to structure bigger applications ([Bigger Applications - Multiple Files](../../tutorial/bigger-applications.md){.internal-link target=_blank}), possibly with multiple files, you will learn how to declare a single `dependencies` parameter for a group of *path operations*....

In the code, a `Bucket` represents the main entrypoint of communication with the database.

This utility function will:

* Connect to a **Couchbase** cluster (that might be a single machine).
    * Set defaults for timeouts.
* Authenticate in the cluster.
* Get a `Bucket` instance.
    * Set defaults for timeouts.
* Return it.

```Python hl_lines="12-21"

But here's the key point.

The security and dependency injection stuff is written once.

And you can make it as complex as you want. And still, have it written only once, in a single place. With all the flexibility.

But you can have thousands of endpoints (*path operations*) using the same security system.

{!../../../docs_src/first_steps/tutorial001.py!}
```

Copy that to a file `main.py`.

Run the live server:

<div class="termy">

```console
$ <font color="#4E9A06">fastapi</font> dev <u style="text-decoration-style:single">main.py</u>
<font color="#3465A4">INFO    </font> Using path <font color="#3465A4">main.py</font>

<a href="https://github.com/containous/traefik/releases" class="external-link" target="_blank">Download Traefik</a>, it's a single binary, you can extract the compressed file and run it directly from the terminal.

Then create a file `traefik.toml` with:

```TOML hl_lines="3"
[entryPoints]
  [entryPoints.http]
    address = ":9999"