You could create an API with a *path operation* that could trigger a request to an *external API* created by someone else (probably the same developer that would be *using* your API).

The process that happens when your API app calls the *external API* is named a "callback". Because the software that the external developer wrote sends a request to your API and then your API *calls back*, sending a request to an *external API* (that was probably created by the same developer).

    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 }

# OpenAPI Webhooks { #openapi-webhooks }

There are cases where you want to tell your API **users** that your app could call *their* app (sending a request) with some data, normally to **notify** of some type of **event**.

This means that instead of the normal process of your users sending requests to your API, it's **your API** (or your app) that could **send requests to their system** (to their API, their app).

This is normally called a **webhook**.

Let's imagine that we want to have a dependency that checks if the query parameter `q` contains some fixed content.

But we want to be able to parameterize that fixed content.

## A "callable" instance { #a-callable-instance }

In Python there's a way to make an instance of a class a "callable".

Not the class itself (which is already a callable), but an instance of that class.

The correct place is:

* In the key `content`, that has as value another JSON object (`dict`) that contains:
    * A key with the media type, e.g. `application/json`, that contains as value another JSON object, that contains:
        * A key `schema`, that has as the value the JSON Schema from the model, here's the correct place.

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

    /**
     * Deletes old documents that have the same URL but different document IDs
     * as the documents in the provided list. This prevents duplicate documents
     * from accumulating in the index.
     *
     * @param searchEngineClient the search engine client to use for deletion
     * @param docList the list of new documents to check against
     * @return the number of old documents that were deleted
     */

# Middleware { #middleware }

You can add middleware to **FastAPI** applications.

A "middleware" is a function that works with every **request** before it is processed by any specific *path operation*. And also with every **response** before returning it.

* It takes each **request** that comes to your application.
* It can then do something to that **request** or run any needed code.

To declare types that have type parameters (internal types), like `list`, `dict`, `tuple`,
pass the internal type(s) as "type parameters" using square brackets: `[` and `]`

```Python
my_list: list[str]
```

That's all standard Python syntax for type declarations.

Use that same standard syntax for model attributes with internal types.

## Wildcards { #wildcards }

It's also possible to declare the list as `"*"` (a "wildcard") to say that all are allowed.

But that will only allow certain types of communication, excluding everything that involves credentials: Cookies, Authorization headers like those used with Bearer Tokens, etc.