And the sub-application could also have its own mounted sub-applications and everything would work correctly, because FastAPI handles all these `root_path`s automatically.

Routes are declared in a single place, using functions declared in other places (instead of using decorators that can be placed right on top of the function that handles the endpoint). This is closer to how Django does it than to how Flask (and Starlette) does it. It separates in the code things that are relatively tightly coupled.

/// check | Inspired **FastAPI** to

## Dispatch

For synchronous calls, you bring your own thread and are responsible for managing how many simultaneous requests you make. Too many simultaneous connections wastes resources; too few harms latency.

# Using Dataclasses { #using-dataclasses }

FastAPI is built on top of **Pydantic**, and I have been showing you how to use Pydantic models to declare requests and responses.

But FastAPI also supports using <a href="https://docs.python.org/3/library/dataclasses.html" class="external-link" target="_blank">`dataclasses`</a> the same way:

{* ../../docs_src/dataclasses/tutorial001.py hl[1,7:12,19:20] *}

  private boolean checkFilter(int c) {
    return ((filter >> c) & 1) == 1;
  }

  // This is all essentially copied from ImmutableSet, but we have to duplicate because
  // of dependencies.

  // Represents how tightly we can pack things, as a maximum.
  private static final double DESIRED_LOAD_FACTOR = 0.5;

  /**
   * Returns an array size suitable for the backing array of a hash table that uses open addressing

 * {@code NtlmServlet}, and {@code NetworkExplorer} to negiotiate password
 * hashes via NTLM SSP with MSIE. It might also be used directly by servlet
 * containers to incorporate similar functionality.
 * <p>
 * How NTLMSSP is used in conjunction with HTTP and MSIE clients is
 * described in an <A HREF="http://www.innovation.ch/java/ntlm.html">NTLM
 * Authentication Scheme for HTTP</A>.  <p> Also, read <a

    }

    // Write the length. This takes 1 byte if length is less than 128.
    val length = content.size
    if (length < 128) {
      sink.writeByte(length.toInt())
    } else {
      // count how many bytes we'll need to express the length.
      val lengthBitCount = 64 - java.lang.Long.numberOfLeadingZeros(length)
      val lengthByteCount = (lengthBitCount + 7) / 8
      sink.writeByte(0b1000_0000 or lengthByteCount)

But as you passed the `HTMLResponse` in the `response_class` too, **FastAPI** will know how to document it in OpenAPI and the interactive docs as HTML with `text/html`:

<img src="/img/tutorial/custom-response/image01.png">

## Available responses { #available-responses }

And **your users** define in some way (for example in a web dashboard somewhere) the **URL** where your app should send those requests.

All the **logic** about how to register the URLs for webhooks and the code to actually send those requests is up to you. You write it however you want to in **your own code**.

      private fun evictToRecoverBytes(bytesToRecover: Int): Int {
        var bytesToRecover = bytesToRecover
        var entriesToEvict = 0
        if (bytesToRecover > 0) {
          // determine how many headers need to be evicted.
          var j = dynamicTable.size - 1
          while (j >= nextHeaderIndex && bytesToRecover > 0) {
            val toEvict = dynamicTable[j]!!
            bytesToRecover -= toEvict.hpackSize