go func(inbound chan []byte) {
			wg.Wait()
			defer xioutil.SafeClose(handlerIn)
			m.handleInbound(c, inbound, handlerIn)
		}(m.inbound)
	}
	// Fill outbound block.
	// Each token represents a message that can be sent to the client without blocking.
	// The client will refill the tokens as they confirm delivery of the messages.
	for i := 0; i < outboundCap; i++ {
		m.outBlock <- struct{}{}
	}

	// Handler goroutine.

		readerToBuf:   r2b,
		stashBuffer:   b,
	}
}

// Done will release any resources used by the parallelReader.
func (p *parallelReader) Done() {
	if p.stashBuffer != nil {
		globalBytePoolCap.Load().Put(p.stashBuffer)
		p.stashBuffer = nil
	}
}

// preferReaders can mark readers as preferred.
// These will be chosen before others.
func (p *parallelReader) preferReaders(prefer []bool) {

    * This will be done by sending a POST request (from *your API*) to some *external API* provided by that external developer (this is the "callback").

## The normal **FastAPI** app

Let's first see how the normal API app would look like before adding the callback.

It will have a *path operation* that will receive an `Invoice` body, and a query parameter `callback_url` that will contain the URL for the callback.

                Security scheme name.

                It will be included in the generated OpenAPI (e.g. visible at `/docs`).
                """
            ),
        ] = None,
        description: Annotated[
            Optional[str],
            Doc(
                """
                Security scheme description.

                It will be included in the generated OpenAPI (e.g. visible at `/docs`).

You can define logic (code) that should be executed before the application **starts up**. This means that this code will be executed **once**, **before** the application **starts receiving requests**.

The same way, you can define logic (code) that should be executed when the application is **shutting down**. In this case, this code will be executed **once**, **after** having handled possibly **many requests**.

package configurations

import common.Arch
import common.BuildToolBuildJvm
import common.Jvm
import common.KillProcessMode.KILL_ALL_GRADLE_PROCESSES
import common.KillProcessMode.KILL_LEAKED_PROCESSES_FROM_PREVIOUS_BUILDS
import common.KillProcessMode.KILL_PROCESSES_STARTED_BY_GRADLE
import common.Os
import common.VersionedSettingsBranch
import common.applyDefaultSettings
import common.buildToolGradleParameters

        assertEquals(
            listOf(
                "KILL_LEAKED_PROCESSES_FROM_PREVIOUS_BUILDS",
                "GRADLE_RUNNER",
                "KILL_ALL_GRADLE_PROCESSES",
                "CLEAN_UP_GIT_UNTRACKED_FILES_AND_DIRECTORIES",
                "GRADLE_RETRY_RUNNER",
                "KILL_PROCESSES_STARTED_BY_GRADLE",
                "CHECK_CLEAN_M2_ANDROID_USER_HOME"
            ),

}

func (b *bucketMetacache) debugf(format string, data ...interface{}) {
	if serverDebugLog {
		console.Debugf(format+"\n", data...)
	}
}

// findCache will attempt to find a matching cache for the provided options.
// If a cache with the same ID exists already it will be returned.
// If none can be found a new is created with the provided ID.
func (b *bucketMetacache) findCache(o listPathOptions) metacache {
	if b == nil {

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

## Check it

Now, every request under the path `/v1/` will be handled by the Flask application.

And the rest will be handled by **FastAPI**.

If you run it with Uvicorn and go to <a href="http://localhost:8000/v1/" class="external-link" target="_blank">http://localhost:8000/v1/</a> you will see the response from Flask:

```txt
Hello, World from Flask!
```

                Security scheme name.

                It will be included in the generated OpenAPI (e.g. visible at `/docs`).
                """
            ),
        ] = None,
        description: Annotated[
            Optional[str],
            Doc(
                """
                Security scheme description.

                It will be included in the generated OpenAPI (e.g. visible at `/docs`).