reject comments, commits, code, wiki edits, issues, and other contributions
that are not aligned to this Code of Conduct, or to ban temporarily or
permanently any contributor for other behaviors that they deem inappropriate,
threatening, offensive, or harmful. However, these actions shall respect the
licensing terms of the Project Developments that will always supersede such
Code of Conduct.

## Scope

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 { #use-case }

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

    /**
     * OS family that can be tested for. {@value}
     */
    private static final String FAMILY_OS2 = "os/2";

    /**
     * OS family that can be tested for. {@value}
     */
    private static final String FAMILY_NETWARE = "netware";

    /**
     * OS family that can be tested for. {@value}
     */
    private static final String FAMILY_DOS = "dos";

    /**
     * OS family that can be tested for. {@value}

- Some base services that are available to code hosted by the runtime. This varies by runtime.
- Additional constraints. For example, the CLI client runtimes limit the libraries that are available to the code in that runtime, for performance reasons. 

Other modules and platforms define the services that they contribute to the runtime.

// data and a flag, atEOF, that reports whether the [Reader] has no more data
// to give. The return values are the number of bytes to advance the input
// and the next token to return to the user, if any, plus an error, if any.
//
// Scanning stops if the function returns an error, in which case some of
// the input may be discarded. If that error is [ErrFinalToken], scanning

      "Licensor" shall mean the copyright owner or entity authorized by
      the copyright owner that is granting the License.

      "Legal Entity" shall mean the union of the acting entity and all
      other entities that control, are controlled by, or are under common
      control with that entity. For the purposes of this definition,
      "control" means (i) the power, direct or indirect, to cause the

Instead, you want to provide a different dependency that will be used only during tests (possibly only some specific tests), and will provide a value that can be used where the value of the original dependency was used.

### Use cases: external service { #use-cases-external-service }

An example could be that you have an external authentication provider that you need to call.

You send it a token and it returns an authenticated user.

      "Licensor" shall mean the copyright owner or entity authorized by
      the copyright owner that is granting the License.

      "Legal Entity" shall mean the union of the acting entity and all
      other entities that control, are controlled by, or are under common
      control with that entity. For the purposes of this definition,
      "control" means (i) the power, direct or indirect, to cause the

### FastAPI Data Filtering { #fastapi-data-filtering }

Now, for FastAPI, it will see the return type and make sure that what you return includes **only** the fields that are declared in the type.

The **server** program (for example **Uvicorn** via **FastAPI CLI**) is capable of interpreting these headers, and then passing that information to your application.