# Extending OpenAPI

There are some cases where you might need to modify the generated OpenAPI schema.

In this section you will see how.

## The normal process

The normal (default) process, is as follows.

A `FastAPI` application (instance) has an `.openapi()` method that is expected to return the OpenAPI schema.

  // it never skips the webhook.
  //
  // For example, to run the webhook on any objects whose namespace is not
  // associated with "runlevel" of "0" or "1";  you will set the selector as
  // follows:
  // "namespaceSelector": {
  //   "matchExpressions": [
  //     {
  //       "key": "runlevel",
  //       "operator": "NotIn",
  //       "values": [
  //         "0",
  //         "1"

 *   <li>the hash codes of any two equal objects are equal
 * </ul>
 *
 * <p>When a test fails, the error message labels the objects involved in the failed comparison as
 * follows:
 *
 * <ul>
 *   <li>"{@code [group }<i>i</i>{@code , item }<i>j</i>{@code ]}" refers to the
 *       <i>j</i><sup>th</sup> item in the <i>i</i><sup>th</sup> equality group, where both equality

A middleware doesn't have to be made for FastAPI or Starlette to work, as long as it follows the ASGI spec.

In general, ASGI middlewares are classes that expect to receive an ASGI app as the first argument.

So, in the documentation for third-party ASGI middlewares they will probably tell you to do something like:

    /**
     * Proxy server port
     */
    private int port;

    /**
     * Type of the proxy
     */
    private String protocol;

    /**
     * The non-proxy hosts. Follows Java system property format: <code>*.foo.com|localhost</code>.
     */
    private String nonProxyHosts;

    /**
     * For NTLM proxies, specifies the NTLM host.
     */
    private String ntlmHost;

    /**

  }

  static final class Crc32cHasher extends AbstractStreamingHasher {

    /*
     * The striding algorithm works roughly as follows: it is universally the case that
     * CRC(x ^ y) == CRC(x) ^ CRC(y).  The approach we take is to break the message as follows,
     * with each letter representing a 4-byte word: ABCDABCDABCDABCD... and to calculate

  // field must be between 0 and 100, inclusive, and it defaults to 0.
  // The number of seats that other levels can borrow from this level, known
  // as this level's LendableConcurrencyLimit (LendableCL), is defined as follows.
  //
  // LendableCL(i) = round( NominalCL(i) * lendablePercent(i)/100.0 )
  //
  // +optional
  optional int32 lendablePercent = 3;

  // `borrowingLimitPercent`, if present, configures a limit on how many

   * <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.
   *

   * <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.
   *

            Hexdump.hexdump( log, b, 0, b.length );
            log.flush();
        }
        updates = 0;

        return b;
    }

    /**
     * Performs MAC signing of the SMB.  This is done as follows.
     * The signature field of the SMB is overwritted with the sequence number;
     * The MD5 digest of the MAC signing key + the entire SMB is taken;
     * The first 8 bytes of this are placed in the signature field.