AHAH callback is not working I have a form element in d8 as follows :

$form['cluster_dropdown'] = array(
    '#type' =&gt; 'select',
    '#title' =&gt; 'Cluster',
    '#options' =&gt; $cluster_options,
    '#default_value' =&gt; $cluster_selection,
    '#ahah' =&gt; array(
      'path' =&gt; 'module_name/admin5/callback',
      'wrapper' =&gt; 'dependent-dropdown-wrapper',
      // 'event' =&gt; 'change', // default value: does not need to be set explicitly.
    ),
    '#attributes' =&gt; array('class' =&gt; 'cluster-dropdown'),
  );


But in this case ahah callback is not working , is there gasf/admin5/callback want to replace with routing path as am using this in d6? any idea ?
Neither #ahah nor path are used in Drupal 8. Instead of the first, Drupal 8 uses #ajax; instead of the second, it uses url which must be a \Drupal\Core\Url to which to submit the Ajax request.

Finally, callback is a PHP callback, which generally is a full qualified (using a PSR-4 namespace) method name.

Reference


Ajax API
PSR-4 namespaces and autoloading in Drupal 8