* You can then further modify the `response` before returning it.

{* ../../docs_src/middleware/tutorial001.py hl[8:9,11,14] *}

/// tip

Keep in mind that custom proprietary headers can be added <a href="https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers" class="external-link" target="_blank">using the 'X-' prefix</a>.

Just use any kind of model, any kind of class, any kind of database that you need for your application. **FastAPI** has you covered with the dependency injection system.

## Code size { #code-size }

This example might seem verbose. Keep in mind that we are mixing security, data models, utility functions and *path operations* in the same file.

But here's the key point.

The security and dependency injection stuff is written once.

* data validation
* data serialization
* data documentation, etc.

This works the same way as with Pydantic models. And it is actually achieved in the same way underneath, using Pydantic.

/// info

Keep in mind that dataclasses can't do everything Pydantic models can do.

So, you might still need to use Pydantic models.

 * number of streams, dynamically configured with `SETTINGS_MAX_CONCURRENT_STREAMS`. A connection
 * currently carrying zero streams is an idle stream. We keep it alive because reusing an existing
 * connection is typically faster than establishing a new one.
 *
 * When a single logical call requires multiple streams due to redirects or authorization

  private static void executeListener(Runnable runnable, Executor executor) {
    try {
      executor.execute(runnable);
    } catch (Exception e) { // sneaky checked exception
      // Log it and keep going -- bad runnable and/or executor. Don't punish the other runnables if
      // we're given a bad one. We only catch Exception because we want Errors to propagate up.
      log.get()
          .log(

	// As long as the compressed size is not also 2³²-1 (implausible)
	// and the header is not also 2³²-1 (equally implausible),
	// accept the uncompressed size 2³²-1 as valid.
	// If nothing else, this keeps archive/zip working with 42.zip.
	_ = needUSize

	if needCSize || needHeaderOffset {
		return ErrFormat
	}

	return nil
}

func readDataDescriptor(r io.Reader, f *File) error {

Including how to handle lifespan state that can be used in other areas of your code.

///

## Sub Applications { #sub-applications }

		case <-ctx.Done():
			return
		case <-keepAliveTicker.C:
			// if previous result is set keep writing the
			// previous result back to the client
			if prevResult.Version != "" {
				if err := enc.Encode(prevResult); err != nil {
					return
				}
			} else {
				// first result is not yet obtained, keep writing
				// empty entry to prevent client from disconnecting.

 * newHasher().putByte(b1).putBytes(new byte[] { b2, b3 }).hash()
 * newHasher().putBytes(new byte[] { b1, b2, b3 }).hash()
 * }
 *
 * <p>If you wish to avoid this, you should either prepend or append the size of each chunk. Keep in
 * mind that when dealing with char sequences, the encoded form of two concatenated char sequences
 * is not equivalent to the concatenation of their encoded form. Therefore, {@link

	maxUnconsumedHealResultItems = 1000

	// if no heal-results are consumed (via the heal-status API)
	// for this timeout duration, the heal sequence is aborted.
	healUnconsumedTimeout = 24 * time.Hour

	// time-duration to keep heal sequence state after it
	// completes.
	keepHealSeqStateDuration = time.Minute * 10

	// nopHeal is a no operating healing action to
	// wait for the current healing operation to finish
	nopHeal = ""
)