I want to do something when a node gets updated. To keep it simple, lets say display a drupal message. 

**hello_world.routing.yml**

    hello_world:
        path: /node/{node}
        defaults:
            _controller: Drupal\hello_world\Controller\HelloWorldController::hello_world_entity_update
        requirements:
            _permission: 'access content'

Not sure if the path should be `/node/{node}/edit` instead.

**HelloWorldController.php**

    <?php
    /**
     * @file
     * Contains \Drupal\hello_world\Controller\HelloController.
     */
    
    namespace Drupal\hello_world\Controller;
    
    use Drupal\Core\Controller\ControllerBase;
    
    class HelloWorldController extends ControllerBase {

      public function hello_world_entity_update(Drupal\Core\Entity\EntityInterface $entity) { 
         
        drupal_set_message(t('Something @var just happened.', array('@var' => 'cool')));  
    	  
      }
    }

Since hook_node_update() was depreciated, I assume we now have to use hook_entity_update(). 

I am not getting the message upon updating a node, what I'm I missing or doing wrong? 

<hr>

###Update

> Just so I don't confuse anyone. 
> I was brainwashed into thinking I needed a routing file and a
> controller. All the D8 examples out there use them.
>
> **Since I was not creating any pages**, 
> turns out **I did not need a routing.yml file nor a Controller.php file**
> nor the /src folder. All I had to do is approach this like Drupal 7,
> all that I needed was the .info.yml file and a .module file
> to put the hook.