## Una aplicación con callbacks

Veamos todo esto con un ejemplo.

Imagina que desarrollas una aplicación que permite crear facturas.

Estas facturas tendrán un `id`, `title` (opcional), `customer`, y `total`.

El usuario de tu API (un desarrollador externo) creará una factura en tu API con un request POST.

Luego tu API (imaginemos):

* Enviará la factura a algún cliente del desarrollador externo.

src_obj4_etag=$(echo "${stat_out4}" | jq '.etag')
src_obj4_size=$(echo "${stat_out4}" | jq '.size')
src_obj4_md5=$(echo "${stat_out4}" | jq '.metadata."X-Amz-Server-Side-Encryption-Customer-Key-Md5"')

# Stat the objects from replicated site
echo "Stat minio2/test-bucket/encrypted"
./mc stat --no-list minio2/test-bucket/encrypted --insecure --json

et()?"moment.utc":"moment.parseZone",n="Z"),e="["+s+'("]',a=0<=this.year()&&this.year()<=9999?"YYYY":"YYYYYY",t=n+'[")]',this.format(e+a+"-MM-DD[T]HH:mm:ss.SSS"+t)},"undefined"!=typeof Symbol&&null!=Symbol.for&&(yt[Symbol.for("nodejs.util.inspect.custom")]=function(){return"Moment<"+this.format()+">"}),yt.toJSON=function(){return this.isValid()?this.toISOString():null},yt.toString=function(){return this.clone().locale("en").format("ddd MMM DD YYYY HH:mm:ss [GMT]ZZ")},yt.unix=function(){return Ma...

- `X-Amz-Server-Side-Encryption-Customer-Key`: Base64 encoded new key.
- `X-Amz-Copy-Source-Server-Side-Encryption-Customer-Key`: Base64 encoded current key.

Such a special COPY request is also known as S3 SSE-C key rotation.

### Server-Side Encryption with a KMS

//     setting will be set to this value. This setting will be only applied to newly created jobs (manual changes with Active Full
//     processing possible from the customer side).
//     Optional value, default 1024, allowed values 256,512,1024,4096,8192, value defined in KB size.
//

				continue
			}
			if object.IsDeleteMarker {
				log.Println("SKIPPED: DELETE marker object:", objFullPath(object))
				continue
			}
			if _, ok := object.UserMetadata["X-Amz-Server-Side-Encryption-Customer-Algorithm"]; ok {
				log.Println("SKIPPED: Objects encrypted with SSE-C do not have md5sum as ETag:", objFullPath(object))
				continue
			}

		Description:    "Requests specifying Server Side Encryption with Customer provided keys must provide an appropriate secret key.",
		HTTPStatusCode: http.StatusBadRequest,
	},
	ErrMissingSSECustomerKeyMD5: {
		Code:           "InvalidArgument",
		Description:    "Requests specifying Server Side Encryption with Customer provided keys must provide the client calculated MD5 of the secret key.",

Nós criamos uma função assíncrona chamada `lifespan()` com `yield` como este:

{* ../../docs_src/events/tutorial003.py hl[16,19] *}

Aqui nós estamos simulando a *inicialização* custosa do carregamento do modelo colocando a (falsa) função de modelo no dicionário com modelos de _machine learning_ antes do `yield`. Este código será executado **antes** da aplicação **começar a receber requisições**, durante a *inicialização*.

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// the handler should ensure to remove these keys before sending them to the object layer.
// Currently these keys are:
//   - X-Amz-Server-Side-Encryption-Customer-Key
//   - X-Amz-Copy-Source-Server-Side-Encryption-Customer-Key
func (api objectAPIHandlers) NewMultipartUploadHandler(w http.ResponseWriter, r *http.Request) {
	ctx := newContext(r, w, "NewMultipartUpload")