### Bigger Errors - Crashes

Nevertheless, there might be cases where we write some code that **crashes the entire application** making Uvicorn and Python crash. 💥

And still, you would probably not want the application to stay dead because there was an error in one place, you probably want it to **continue running** at least for the *path operations* that are not broken.

That means that, clients using your API don't have to check if the value exists or not, they can **assume the field will always be there**, but just that in some cases it will have the default value of `None`.

The way to describe this in OpenAPI, is to mark that field as **required**, because it will always be there.

Because of that, the JSON Schema for a model can be different depending on if it's used for **input or output**:

# 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**.

## Webhooks steps

their faces.  There was a sound of many footsteps, and Alice
looked round, eager to see the Queen.

  First came ten soldiers carrying clubs; these were all shaped
like the three gardeners, oblong and flat, with their hands and
feet at the corners:  next the ten courtiers; these were
ornamented all over with diamonds, and walked two and two, as the
soldiers did.  After these came the royal children; there were

   * needed.
   *
   * A single spinlock ("busy") is used for initializing and
   * resizing the table, as well as populating slots with new Cells.
   * There is no need for a blocking lock; when the lock is not
   * available, threads try other slots (or the base).  During these
   * retries, there is increased contention and reduced locality,
   * which is still better than alternatives.
   *
   * Per-thread hash codes are initialized to random values.

   * needed.
   *
   * A single spinlock ("busy") is used for initializing and
   * resizing the table, as well as populating slots with new Cells.
   * There is no need for a blocking lock; when the lock is not
   * available, threads try other slots (or the base).  During these
   * retries, there is increased contention and reduced locality,
   * which is still better than alternatives.
   *
   * Per-thread hash codes are initialized to random values.

	// ErrDuplicatedKey occurs when there is a unique key constraint violation
	ErrDuplicatedKey = errors.New("duplicated key not allowed")
	// ErrForeignKeyViolated occurs when there is a foreign key constraint violation
	ErrForeignKeyViolated = errors.New("violates foreign key constraint")
	// ErrCheckConstraintViolated occurs when there is a check constraint violation

    * HTTP Digest, etc.
* `oauth2`: all the OAuth2 ways to handle security (called "flows").
    * Several of these flows are appropriate for building an OAuth 2.0 authentication provider (like Google, Facebook, Twitter, GitHub, etc):
        * `implicit`
        * `clientCredentials`
        * `authorizationCode`
    * But there is one specific "flow" that can be perfectly used for handling authentication in the same application directly:

This is similar to the 200 HTTP status codes (from 200 to 299). Those "200" status codes mean that somehow there was a "success" in the request.

The status codes in the 400 range mean that there was an error from the client.

Remember all those **"404 Not Found"** errors (and jokes)?

## Use `HTTPException`

   * to reference the FinalizableReferenceQueue class then its ClassLoader cannot be
   * garbage-collected, even if there are no more instances of FinalizableReferenceQueue itself.
   * The code in FinalizableReferenceQueue goes to considerable trouble to ensure that there are
   * no such references and the tests here check that that trouble has not been in vain.
   *