// Seek back to beginning.
	body.Seek(0, 0)

	// Add body
	req.Body = io.NopCloser(body)
	req.ContentLength = contentLength

	return req, nil
}

func assembleStreamingChunks(req *http.Request, body io.ReadSeeker, chunkSize int64,
	secretKey, signature string, currTime time.Time) (*http.Request, error,
) {
	regionStr := globalSite.Region
	var stream []byte
	var buffer []byte

}

func mainHandler(w http.ResponseWriter, r *http.Request) {
	body, err := io.ReadAll(r.Body)
	defer r.Body.Close()
	if err != nil {
		log.Printf("Error reading request body: %v", err)
		w.WriteHeader(http.StatusBadRequest)
		return
	}

	log.Printf(">>> %s %s\n", r.Method, r.URL.Path)
	var out bytes.Buffer
	json.Indent(&out, body, "", "    ")
	log.Printf("%s\n", out.String())

	w.WriteHeader(http.StatusOK)

To create the callback *path operation* use the same `APIRouter` you created above.

It should look just like a normal FastAPI *path operation*:

* It should probably have a declaration of the body it should receive, e.g. `body: InvoiceEvent`.
* And it could also have a declaration of the response it should return, e.g. `response_model=InvoiceEventReceived`.

```Python hl_lines="16-18  21-22  28-32"

from .datastructures import UploadFile as UploadFile
from .exceptions import HTTPException as HTTPException
from .exceptions import WebSocketException as WebSocketException
from .param_functions import Body as Body
from .param_functions import Cookie as Cookie
from .param_functions import Depends as Depends
from .param_functions import File as File
from .param_functions import Form as Form

![ReDoc](https://fastapi.tiangolo.com/img/index/index-02-redoc-simple.png)

## Example upgrade

Now modify the file `main.py` to receive a body from a `PUT` request.

Declare the body using standard Python types, thanks to Pydantic.

```Python hl_lines="4  9-12  25-27"
from typing import Union

from fastapi import FastAPI
from pydantic import BaseModel

    - tutorial/first-steps.md
    - tutorial/path-params.md
    - tutorial/query-params.md
    - tutorial/body.md
    - tutorial/query-params-str-validations.md
    - tutorial/path-params-numeric-validations.md
    - tutorial/body-multiple-params.md
    - tutorial/body-fields.md
    - tutorial/body-nested-models.md
    - tutorial/schema-extra-example.md
    - tutorial/extra-data-types.md
    - tutorial/cookie-params.md

## Beispiel Aktualisierung

Ändern Sie jetzt die Datei `main.py`, um den <abbr title="Body – Körper, Inhalt: Der eigentliche Inhalt einer Nachricht, nicht die Metadaten">Body</abbr> einer `PUT`-Anfrage zu empfangen.

Deklarieren Sie den Body mithilfe von Standard-Python-Typen, dank Pydantic.

```Python hl_lines="4  9-12  25-27"
from typing import Union

from fastapi import FastAPI

// it is used to ignore custom encoding charset in S3 XML body.
func nopCharsetConverter(label string, input io.Reader) (io.Reader, error) {
	return input, nil
}

// xmlDecoder provide decoded value in xml.
func xmlDecoder(body io.Reader, v interface{}, size int64) error {
	var lbody io.Reader
	if size > 0 {
		lbody = io.LimitReader(body, size)
	} else {
		lbody = body
	}
	d := xml.NewDecoder(lbody)

	if err != nil {
		writeErrorResponseJSON(ctx, w, toAdminAPIErr(ctx, err), r.URL)
		return
	}

	body, err := json.Marshal(gdesc)
	if err != nil {
		writeErrorResponseJSON(ctx, w, toAdminAPIErr(ctx, err), r.URL)
		return
	}

	writeSuccessResponseJSON(w, body)
}

// ListGroups - GET /minio/admin/v3/groups
func (a adminAPIHandlers) ListGroups(w http.ResponseWriter, r *http.Request) {

![ReDoc](https://fastapi.tiangolo.com/img/index/index-02-redoc-simple.png)

## Mejora al ejemplo

Ahora modifica el archivo `main.py` para recibir un <abbr title="cuerpo del mensaje HTTP">body</abbr> del `PUT` request.

Declara el body usando las declaraciones de tipo estándares de Python gracias a Pydantic.

```Python hl_lines="2  7-10  23-25"
from fastapi import FastAPI
from pydantic import BaseModel
from typing import Union