PHP Fibers support has been added to Drupal's default and Big Pipe rendering systems.
Code that executes asynchronous operations (such as async database queries, http requests, or file operations) can now call Fiber::suspend(), to allow Drupal to potentially run different code while it's waiting for the operation to return. Drupal's rendering system makes extensive use of HTML placeholders for its caching and BigPipe systems, and the rendering of these placeholders now happens within a Fiber, allowing it to loop over all pending placeholders while any one is suspended, and return to it later.
Note that Drupal does not yet support making asynchronous database queries out of the box, this will be added in a future release.
See https://www.php.net/manual/en/language.fibers.php for more explanation of the underlying feature.
Once Drupal core supports asynchronous database queries, this change record will be update or a new one added to show usage examples.
