///

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/"

	// the actual benchmark for PutObject starts here. Reset the benchmark timer.
	b.ResetTimer()

	if err := req.ParseForm(); err != nil {
		b.Fatal(err)
	}

	b.RunParallel(func(pb *testing.PB) {
		for pb.Next() {
			req.Form.Get("uploadId")
		}
	})

	// Benchmark ends here. Stop timer.
	b.StopTimer()
}

// BenchmarkURLQuery - benchmark URL memory allocations
func BenchmarkURLQuery(b *testing.B) {

      if (!hasNext()) {
        throw new NoSuchElementException();
      }

      // requireNonNull is safe because of the hasNext check.
      BiEntry<K, V> entry = requireNonNull(next);
      next = entry.nextInKeyInsertionOrder;
      toRemove = entry;
      remaining--;
      return output(entry);
    }

    @Override
    public void remove() {
      if (modCount != expectedModCount) {

     */
    @Override
    public void prepare ( CommonServerMessageBlockRequest next ) {
        CommonServerMessageBlockResponse n = getNextResponse();
        if ( n != null ) {
            n.prepare(next);
        }
    }


    /**
     * {@inheritDoc}
     *
     * @see jcifs.internal.smb2.ServerMessageBlock2#reset()
     */
    @Override

		tr := NewReader(&buf)
		h, err := tr.Next()
		if err != ErrInsecurePath {
			t.Errorf("tr.Next for file %q: got err %v, want ErrInsecurePath", path, err)
			continue
		}
		if h.Name != path {
			t.Errorf("tr.Next for file %q: got name %q, want %q", path, h.Name, path)
		}
		// Error should not be sticky.
		h, err = tr.Next()
		if err != nil {

     */
    public static AvPair get ( List<AvPair> pairs, int type ) {
        Iterator<AvPair> it = pairs.iterator();
        while ( it.hasNext() ) {
            AvPair p = it.next();
            if ( p.getType() == type ) {
                return p;
            }
        }
        return null;
    }


    /**
     * Remove all occurances of the given type
     * 

///

/// info

In this example we use invented custom headers `X-Key` and `X-Token`.

But in real cases, when implementing security, you would get more benefits from using the integrated [Security utilities (the next chapter)](../security/index.md){.internal-link target=_blank}.

///

## Dependencies errors and return values

You can use the same dependency *functions* you use normally.

### Dependency requirements

     */
    void add(Problem.Severity severity, String message, int line, int column, Exception cause);

    /**
     * The next messages will be bound to this source. When calling this method again, previous messages keep
     * their source, but the next messages will use the new source.
     *
     * @param source a source
     */
    void setSource(String source);

    /**
     *

 * acquire(1000)} will result in exactly the same throttling, if any), but it affects the throttling
 * of the <i>next</i> request. I.e., if an expensive task arrives at an idle RateLimiter, it will be
 * granted immediately, but it is the <i>next</i> request that will experience extra throttling,
 * thus paying for the cost of the expensive task.
 *
 * @author Dimitris Andreou
 * @since 13.0
 */

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