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The branch needs to be available on the remote that the BWC makes of the
repository you run the tests from. Using the remote is a handy trick to make
sure that a branch is available and is up to date in the case of multiple runs.

Example:

Say you need to make a change to `master` and have a BWC layer in `5.x`. You
will need to:

  sufficient copyright rights in its Contribution, if any, to grant
  the copyright license set forth in this Agreement.

  e) Notwithstanding the terms of any Secondary License, no
  Contributor makes additional grants to any Recipient (other than
  those set forth in this Agreement) as a result of such Recipient's
  receipt of the Program under the terms of a Secondary License
  (if permitted under the terms of Section 3).

          latch2.countDown()
        }
      }
    val request2Listener: EventListener =
      object : EventListener() {
        override fun connectStart(
          call: Call,
          inetSocketAddress: InetSocketAddress,
          proxy: Proxy,
        ) {
          // Let request1 proceed to make a connection.
          latch1.countDown()

    * Tell me how you use FastAPI (I love to hear that).
    * Hear when I make announcements or release new tools.
    * You can also [follow @fastapi on X (Twitter)](https://x.com/fastapi) (a separate account).
* [Follow me on **LinkedIn**](https://www.linkedin.com/in/tiangolo/).
    * Hear when I make announcements or release new tools (although I use X (Twitter) more often 🤷‍♂).

## OpenAPI { #openapi }

OpenAPI (previously known as Swagger) is the open specification for building APIs (now part of the Linux Foundation).

**FastAPI** is based on **OpenAPI**.

That's what makes it possible to have multiple automatic interactive documentation interfaces, code generation, etc.

OpenAPI has a way to define multiple security "schemes".

## Restarts { #restarts }

Similar to making sure your application is run on startup, you probably also want to make sure it is **restarted** after failures.

### We Make Mistakes { #we-make-mistakes }

We, as humans, make **mistakes**, all the time. Software almost *always* has **bugs** hidden in different places. 🐛

> and you can rely on it after doing initial sanity check and targeted local testing.

### Copyright and License

When updating/modifying a file, please do not make changes to the copyright header.

When creating a new file, please make sure to add a header as defined below.

#### Required Files for Copyright Headers:

- Source code files (e.g., `.java`, `.kt`, `.groovy`).

/// note

Having a default value of any type, including `None`, makes the parameter optional (not required).

///

## Required parameters { #required-parameters }

When we don't need to declare more validations or metadata, we can make the `q` query parameter required just by not declaring a default value, like:

```Python
q: str
```

instead of:

You will see examples of this in the next chapters, about relational and NoSQL databases, security, etc.

## **FastAPI** compatibility { #fastapi-compatibility }

The simplicity of the dependency injection system makes **FastAPI** compatible with:

* all the relational databases
* NoSQL databases
* external packages
* external APIs
* authentication and authorization systems
* API usage monitoring systems