!!! note
    Some response codes (see the next section) indicate that the response does not have a body.

    FastAPI knows this, and will produce OpenAPI docs that state there is no response body.

## About HTTP status codes

!!! note
    If you already know what HTTP status codes are, skip to the next section.

In HTTP, you send a numeric status code of 3 digits as part of the response.

TYPE java.lang.Runnable

FIELD com.google.common.base.Converter.reverse
FIELD com.google.common.collect.AbstractBiMap.EntrySet.iterator.$.entry com.google.common.collect.AbstractBiMap.EntrySet.iterator.$.next.$
FIELD com.google.common.collect.AbstractMapBasedMultimap.map
FIELD com.google.common.collect.AbstractMultimap.asMap com.google.common.collect.AbstractMapBasedMultimap.NavigableAsMap

    assertThat(actualUrls).containsExactly(*expectedUrls)
  }

  fun finishJob(url: String) {
    val i = calls.iterator()
    while (i.hasNext()) {
      val call = i.next()
      if (call.request.url.toString() == url) {
        i.remove()
        dispatcherTest.dispatcher.finishedAccessor(call)
        return
      }
    }
    throw AssertionError("No such job: $url")
  }

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/"

import java.io.IOException
import java.net.ConnectException
import java.net.HttpURLConnection
import java.net.ProtocolException
import java.net.Proxy
import java.net.Socket
import java.net.UnknownServiceException
import java.security.cert.X509Certificate
import java.util.concurrent.TimeUnit
import javax.net.ssl.SSLPeerUnverifiedException
import javax.net.ssl.SSLSocket
import kotlin.concurrent.withLock

        }

        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")
    }

  `Never mind!' said the King, with an air of great relief.
`Call the next witness.'  And he added in an undertone to the
Queen, `Really, my dear, YOU must cross-examine the next witness.
It quite makes my forehead ache!'

  Alice watched the White Rabbit as he fumbled over the list,
feeling very curious to see what the next witness would be like,
`--for they haven't got much evidence YET,' she said to herself.

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