Hook Discovery Tool

Drupal Hook System — How It Works

Drupal's hook system is the primary way modules intercept and extend core functionality without modifying core files. A hook is simply a function in your module following the naming convention MODULENAME_hookname(). When Drupal invokes a hook, it calls every implementation of that hook in every enabled module.

Hooks are defined in .module files or in dedicated service classes registered through the event system in Drupal 8+. Unlike WordPress hooks, Drupal hooks are discovered at bootstrap time and cached — there is no add_action() equivalent. If your module file defines the function, it will be called automatically.

Implementing a Hook

In your mymodule.module file:

/**
 * Implements hook_node_presave().
 */
function mymodule_node_presave(\Drupal\node\NodeInterface $node) {
  if ($node->bundle() === 'article') {
    // Your logic here.
  }
}

Hooks vs. Event Subscribers

Drupal 8+ introduced Symfony's event system as an alternative to hooks for many subsystems. Event subscribers offer better performance through dependency injection and can be unit tested more easily. However, many Drupal-specific operations (entity operations, form alters, theme preprocessing) still rely on hooks. Use hooks for Drupal-specific integrations and event subscribers for Symfony-level events.