*
   * <p><i>Notes:</i> This is an implementation of the algorithm for Lexicographical Permutations
   * Generation, described in Knuth's "The Art of Computer Programming", Volume 4, Chapter 7,
   * Section 7.2.1.2. The iteration order follows the lexicographical order. This means that the
   * first permutation will be in ascending order, and the last will be in descending order.
   *

But for now, let's check these important **conceptual ideas**. These concepts also apply to any other type of web API. 💡

## Security - HTTPS

In the [previous chapter about HTTPS](https.md){.internal-link target=_blank} we learned about how HTTPS provides encryption for your API.

    Data from forms is normally encoded using the "media type" `application/x-www-form-urlencoded`.

    But when the form includes files, it is encoded as `multipart/form-data`. You'll read about handling files in the next chapter.

     *
     * @see #getModuleInfo(Path)
     */
    private final Map<Path, PathModularization> moduleInfo;

    /**
     * Whether JAR files are modular. This map is redundant with {@link #moduleInfo},
     * but cheaper to compute when the module names are not needed.
     *
     * @see #getPathType(Path)
     */
    private final Map<Path, PathType> pathTypes;

    /**
     * Creates an initially empty cache.
     */

   *
   * <p><i>Notes:</i> This is an implementation of the algorithm for Lexicographical Permutations
   * Generation, described in Knuth's "The Art of Computer Programming", Volume 4, Chapter 7,
   * Section 7.2.1.2. The iteration order follows the lexicographical order. This means that the
   * first permutation will be in ascending order, and the last will be in descending order.
   *

     */
    Set<Throwable> seenExceptionsLocal = seenExceptions;
    if (seenExceptionsLocal == null) {
      // TODO(cpovirk): Should we use a simpler (presumably cheaper) data structure?
      /*
       * Using weak references here could let us release exceptions earlier, but:
       *
       * 1. On Android, querying a WeakReference blocks if the GC is doing an otherwise-concurrent

   to use a memory-hard function (Argon2) that (on purpose) consumes a lot of memory and CPU.
   The new KMS-based approach can use a key derivation function that is orders of magnitudes
   cheaper w.r.t. memory and CPU.
- Root credentials can now be changed easily. Before, a two-step process was required to
   change the cluster root credentials since they were used to en/decrypt the IAM data.

     *
     * @see #getModuleInfo(Path)
     */
    private final Map<Path, PathModularization> moduleInfo;

    /**
     * Whether JAR files are modular. This map is redundant with {@link #moduleInfo},
     * but cheaper to compute when the module names are not needed.
     *
     * @see #getPathType(Path)
     */
    private final Map<Path, PathType> pathTypes;

    /**
     * Creates an initially empty cache.
     */

!!! 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

That defines the metadata about the main response of a *path operation*.

You can also declare additional responses with their models, status codes, etc.

There's a whole chapter here in the documentation about it, you can read it at [Additional Responses in OpenAPI](additional-responses.md){.internal-link target=_blank}.

## OpenAPI Extra