The only new thing is the `callbacks=invoices_callback_router.routes` as an argument to the *path operation decorator*. We'll see what that is next.

## Documenting the callback

The actual callback code will depend heavily on your own API app.

And it will probably vary a lot from one app to the next.

It could be just one or two lines of code, like:

```Python
callback_url = "https://example.com/api/v1/invoices/events/"

        }

        return false
      }

      override fun next(): String {
        if (!hasNext()) throw NoSuchElementException()
        val result = nextUrl!!
        nextUrl = null
        canRemove = true
        return result
      }

      override fun remove() {
        check(canRemove) { "remove() before next()" }
        delegate.remove()
      }
    }
  }

I'll show you some concrete examples in the next chapters.

---

Then the next concepts to consider are all about the program running your actual API (e.g. Uvicorn).

## Program and Process

    set("c", "c1", "c2")
    val iterator = cache.snapshots()
    assertThat(iterator.hasNext()).isTrue()
    iterator.next().use {
      assertThat(it.key()).isEqualTo("a")
      it.assertValue(0, "a1")
      it.assertValue(1, "a2")
    }
    assertThat(iterator.hasNext()).isTrue()
    iterator.next().use {
      assertThat(it.key()).isEqualTo("b")
      it.assertValue(0, "b1")
      it.assertValue(1, "b2")
    }

This would be typical after a user initiates a force refresh by a pull to refresh type action.

```java
    val urlIterator = cache.urls()
    while (urlIterator.hasNext()) {
      if (urlIterator.next().startsWith("https://www.google.com/")) {
        urlIterator.remove()
      }
    }
```

### Troubleshooting

1. Valid cacheable responses are not being cached

 *    we must send a `CONNECT` request, and handle authorization challenges from the proxy.
 *  * Optional [TLS handshake][connectTls].
 *
 * Each step may fail. If a retry is possible, a new instance is created with the next plan, which
 * will be configured differently.
 */
class ConnectPlan(
  private val taskRunner: TaskRunner,
  private val connectionPool: RealConnectionPool,
  private val readTimeoutMillis: Int,

## Create a middleware

To create a middleware you use the decorator `@app.middleware("http")` on top of a function.

The middleware function receives:

* The `request`.
* A function `call_next` that will receive the `request` as a parameter.
    * This function will pass the `request` to the corresponding *path operation*.
    * Then it returns the `response` generated by the corresponding *path operation*.

## ساخت یک میان افزار

برای ایجاد یک میان‌افزار، از دکوریتور `@app.middleware("http")` در بالای یک تابع استفاده می‌شود.

تابع میان افزار دریافت می کند:
* `درخواست`
* تابع `call_next` که `درخواست` را به عنوان پارامتر دریافت می کند
    * این تابع `درخواست` را به *path operation* مربوطه ارسال می کند.
    * سپس `پاسخ` تولید شده توسط *path operation* مربوطه را برمی‌گرداند.

!!! info
    If you are using containers, for example with Docker or Kubernetes, I'll tell you more about that in the next chapter: [FastAPI in Containers - Docker](docker.md){.internal-link target=_blank}.

## Supported Versions

Security updates are applied to the latest feature release (1.x) of the compiler, standard library, and build plugins.

A fix will be shipped with the next incremental (1.x.y) or bug fix release (1.x.yz).

All fixes are also applied to the master branch to be included in all upcoming releases.

## Reporting a Vulnerability