## An app with callbacks { #an-app-with-callbacks }

Let's see all this with an example.

Imagine you develop an app that allows creating invoices.

These invoices will have an `id`, `title` (optional), `customer`, and `total`.

The user of your API (an external developer) will create an invoice in your API with a POST request.

Then your API will (let's imagine):

But because of our changes in `GzipRequest.body`, the request body will be automatically decompressed when it is loaded by **FastAPI** when needed.

## Accessing the request body in an exception handler { #accessing-the-request-body-in-an-exception-handler }

/// tip

    }

    /**
     * Creates an {@link java.io.InputStream} for reading bytes from a file on
     * an SMB server represented by the {@link jcifs.smb.SmbFile} parameter. See
     * {@link jcifs.smb.SmbFile} for a detailed description and examples of
     * the smb URL syntax.
     *
     * @param file
     *            An <code>SmbFile</code> specifying the file to read from

### 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.

This provider might be charging you per request, and calling it might take some extra time than if you had a fixed mock user for tests.

{* ../../docs_src/dependencies/tutorial011_an_py39.py hl[9] *}

In this case, **FastAPI** won't ever touch or care about `__init__`, we will use it directly in our code.

## Create an instance { #create-an-instance }

We could create an instance of this class with:

{* ../../docs_src/dependencies/tutorial011_an_py39.py hl[18] *}

 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package okhttp3.sse

import okhttp3.Response

abstract class EventSourceListener {
  /**

So, in a URL like:

```
https://example.com/items/foo
```

...the path would be:

```
/items/foo
```

/// info

A "path" is also commonly called an "endpoint" or a "route".

///

While building an API, the "path" is the main way to separate "concerns" and "resources".

#### Operation { #operation }

"Operation" here refers to one of the HTTP "methods".

One of:

     * @return the generated key
     * @throws IOException if an I/O error occurs
     */
    protected abstract long makeKey(Request request) throws IOException;

    /**
     * Peek at the next key without removing it from the input stream
     *
     * @return the next key or null if none available
     * @throws IOException if an I/O error occurs
     */
    protected abstract Long peekKey() throws IOException;

/// info

Webhooks are available in OpenAPI 3.1.0 and above, supported by FastAPI `0.99.0` and above.

///

## An app with webhooks { #an-app-with-webhooks }

When you create a **FastAPI** application, there is a `webhooks` attribute that you can use to define *webhooks*, the same way you would define *path operations*, for example with `@app.webhooks.post()`.

    SQLModel will know that something declared as `str` will be a SQL column of type `TEXT` (or `VARCHAR`, depending on the database).

### Create an Engine { #create-an-engine }

A SQLModel `engine` (underneath it's actually a SQLAlchemy `engine`) is what **holds the connections** to the database.

You would have **one single `engine` object** for all your code to connect to the same database.