then *your API* will process the invoice, and at some point later, send a callback request to the `callback_url` (the *external API*):

```
https://www.external.org/events/invoices/2expen51ve
```

with a JSON body containing something like:

```JSON
{
    "description": "Payment celebration",
    "paid": true
}
```

and it would expect a response from that *external API* with a JSON body like:

```JSON
{

	}
	extractedSignedHeaders := make(http.Header)
	for _, header := range signedHeaders {
		// `host` will not be found in the headers, can be found in r.Host.
		// but its always necessary that the list of signed headers containing host in it.
		val, ok := reqHeaders[http.CanonicalHeaderKey(header)]
		if !ok {
			// try to set headers from Query String
			val, ok = reqQueries[header]
		}
		if ok {

  optional bool namespaced = 2;

  // group is the preferred group of the resource.  Empty implies the group of the containing resource list.
  // For subresources, this may have a different value, for example: Scale".
  optional string group = 8;

  // version is the preferred version of the resource.  Empty implies the version of the containing resource list

    byte max = array[0];
    for (int i = 1; i < array.length; i++) {
      if (array[i] > max) {
        max = array[i];
      }
    }
    return max;
  }

  /**
   * Returns a string containing the supplied {@code byte} values separated by {@code separator}.
   * For example, {@code join(":", 0x01, 0x02, -0x01)} returns the string {@code "1:2:-1"}.
   *

       * need to clear our reference to the TimeoutFuture is the reason we use a *static* nested
       * class with a manual reference back to the "containing" class.)
       *
       * This has the nice-ish side effect of limiting reentrancy: run() calls
       * timeoutFuture.setException() calls run(). That reentrancy would already be harmless, since

    operator fun next(): Route {
      if (!hasNext()) throw NoSuchElementException()
      return routes[nextRouteIndex++]
    }
  }

  companion object {
    /** Obtain a host string containing either an actual host name or a numeric IP address. */
    val InetSocketAddress.socketHost: String get() {
      // The InetSocketAddress was specified with a string (either a numeric IP or a host name). If

 *
 * <p>The size of the data structure used by {@link ArrayBasedCharEscaper} is proportional to the
 * highest valued character that requires escaping. For example a replacement map containing the
 * single character '{@code \}{@code u1000}' will require approximately 16K of memory. If you need
 * to create multiple escaper instances that have the same character replacement mapping consider

  }

  /**
   * {@inheritDoc}
   *
   * <p>Creates an empty {@code LinkedHashSet} for a collection of values for one key.
   *
   * @return a new {@code LinkedHashSet} containing a collection of values for one key
   */
  @Override
  Set<V> createCollection() {
    return Platform.newLinkedHashSetWithExpectedSize(valueSetCapacity);
  }

  /**
   * {@inheritDoc}
   *

    the Modifications made by that particular Contributor.

    1.3. "Covered Software" means (a) the Original Software, or (b)
    Modifications, or (c) the combination of files containing Original
    Software with files containing Modifications, in each case including
    portions thereof.

    1.4. "Executable" means the Covered Software in any form other than
    Source Code.

   *
   * <p><b>{@link java.nio.file.Path} equivalent:</b> {@link java.nio.file.Files#readAllBytes}.
   *
   * @param file the file to read from
   * @return a byte array containing all the bytes from file
   * @throws IllegalArgumentException if the file is bigger than the largest possible byte array
   *     (2^31 - 1)
   * @throws IOException if an I/O error occurs
   */