## The normal **FastAPI** app

Let's first see how the normal API app would look like before adding the callback.

It will have a *path operation* that will receive an `Invoice` body, and a query parameter `callback_url` that will contain the URL for the callback.

This part is pretty normal, most of the code is probably already familiar to you:

```Python hl_lines="9-13  36-53"
{!../../../docs_src/openapi_callbacks/tutorial001.py!}
```

!!! tip

// PolicyRule holds information that describes a policy rule, but does not contain information
// about who the rule applies to or which namespace the rule applies to.
message PolicyRule {
  // Verbs is a list of Verbs that apply to ALL the ResourceKinds contained in this rule. '*' represents all verbs.
  repeated string verbs = 1;

  // The list ordering isn't significant, may contain duplicates, and possibly be incomplete.
  repeated ResourceRule resourceRules = 1;

  // NonResourceRules is the list of actions the subject is allowed to perform on non-resources.
  // The list ordering isn't significant, may contain duplicates, and possibly be incomplete.
  repeated NonResourceRule nonResourceRules = 2;

| `contact` | `dict` | The contact information for the exposed API. It can contain several fields. <details><summary><code>contact</code> fields</summary><table><thead><tr><th>Parameter</th><th>Type</th><th>Description</th></tr></thead><tbody><tr><td><code>name</code></td><td><code>str</code></td><td>The identifying name of the contact person/organization.</td></tr><tr><td><code>url</code></td><td><code>str</code></td><td>The...

      if (first) {
        firstIsRandalcat = codePoint in randalcatSet
      } else if (firstIsRandalcat) {
        // 'If a string contains any RandALCat character, the string MUST NOT contain any LCat
        // character.'
        if (codePoint in lcatSet) return null

        // 'If a string contains any RandALCat character, a RandALCat character MUST be the last
        // character of the string.'

#   and scripts on Linux create a persistent container called "tf"
#   which mounts your TensorFlow directory into the container.
#
#   Important: because the container is persistent, you cannot change TFCI
#   variables in between script executions. To forcibly remove the
#   container and start fresh, run "docker rm -f tf". Removing the container
#   destroys some temporary bazel data and causes longer builds.
#

        append(request.url)
      } else {
        append(requestPath(request.url))
      }
      append(" HTTP/1.1")
    }

  /**
   * Returns true if the request line should contain the full URL with host and port (like "GET
   * http://android.com/foo HTTP/1.1") or only the path (like "GET /foo HTTP/1.1").
   */
  private fun includeAuthorityInRequestLine(
    request: Request,

                    analyze(inlineFunction)
                }
            }
            analyzedElements.addAll(nextInlineFunctions)
        }
    }

    /**
     * Returns the list of files that contain all reached inline functions.
     */
    fun allFiles(): List<KtFile> = analyzedElements.mapTo(mutableSetOf()) { it.containingKtFile }.toList()

    /**

  // If the signer cannot issue the certificate, a condition of type "Failed" is added and this field remains empty.
  //
  // Validation requirements:
  //  1. certificate must contain one or more PEM blocks.
  //  2. All PEM blocks must have the "CERTIFICATE" label, contain no headers, and the encoded data
  //   must be a BER-encoded ASN.1 Certificate structure as described in section 4 of RFC5280.

// PolicyRule holds information that describes a policy rule, but does not contain information
// about who the rule applies to or which namespace the rule applies to.
message PolicyRule {
  // Verbs is a list of Verbs that apply to ALL the ResourceKinds contained in this rule. '*' represents all verbs.
  repeated string verbs = 1;