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 { #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/"

/// 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}.

					e := f.updateCache.find(parent.Key())
					if e == nil || e.Compacted {
						foundAny = true
						break
					}
					next := f.updateCache.searchParent(parent)
					if next == nil {
						foundAny = true
						break
					}
					parent = *next
				}
				if !foundAny {
					// Add non-compacted empty entry.
					f.updateCache.replaceHashed(h, &thisHash, dataUsageEntry{})
				}
			}

			return
		}
	}
	b.err = io.ErrNoProgress
}

func (b *Reader) readErr() error {
	err := b.err
	b.err = nil
	return err
}

// Peek returns the next n bytes without advancing the reader. The bytes stop
// being valid at the next read call. If necessary, Peek will read more bytes
// into the buffer in order to make n bytes available. If Peek returns fewer

    assertThrows(
        ConcurrentModificationException.class,
        () -> {
          Iterator<Entry<K, V>> iterator = getMap().entrySet().iterator();
          put(e3());
          iterator.next();
        });
  }

  @MapFeature.Require({FAILS_FAST_ON_CONCURRENT_MODIFICATION, SUPPORTS_PUT})
  @CollectionSize.Require(absent = ZERO)
  public void testPutAbsentConcurrentWithKeySetIteration() {
    assertThrows(

        }

        return type.cast(object);
    }

    /**
     * Gets the next element from an enumeration and casts it to the specified type
     * @param <T> the target type
     * @param type the target class type
     * @param enumeration the enumeration to read from
     * @return next element from enumeration cast to type
     * @throws PACDecodingException if types are incompatible
     */

  private ClusterException(Collection<? extends Throwable> exceptions) {
    super(
        exceptions.size() + " exceptions were thrown. The first exception is listed as a cause.",
        exceptions.iterator().next());
    ArrayList<? extends Throwable> temp = new ArrayList<>(exceptions);
    this.exceptions = Collections.unmodifiableCollection(temp);
  }

  /** See {@link #create(Collection)}. */

    for (Class<? super T> itf : allInterfaceClasses) {
      Iterator<Class<?>> iterator = interfaceClasses.iterator();
      boolean addToSet = true;
      while (iterator.hasNext()) {
        Class<?> current = iterator.next();
        if (current == itf || itf.isAssignableFrom(current)) {
          // Skip any super interface of the ones that are already included.
          addToSet = false;
          break;
        }
      }

    Iterator<T> iteratorA = iterableA.iterator();
    Iterator<T> iteratorB = iterableB.iterator();

    while (iteratorA.hasNext() && iteratorB.hasNext()) {
      if (!elementEquivalence.equivalent(iteratorA.next(), iteratorB.next())) {
        return false;
      }
    }

    return !iteratorA.hasNext() && !iteratorB.hasNext();
  }

  @Override
  protected int doHash(Iterable<T> iterable) {
    int hash = 78721;

                final DfsReferral start = dr;
                do {
                    final String domain = dr.server.toLowerCase();
                    entry.map.put(domain, new HashMap());
                    dr = dr.next;
                } while (dr != start);

                _domains = entry;
                return _domains.map;
            }
        } catch (final IOException ioe) {
            if (LogStream.level >= 3) {