Simplify your application with a command bus

Introduction to the command bus pattern, and how to set it up on your Symfony application.
Talk given at SymfonyCon Cluj 2017

Transcript

1. Simplify your
   application with a
   command bus
   SymfonyCon Cluj 2017
   

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2. Romaric Drigon
   Software engineer @ netinfluence,
   Switzerland
   

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3. What we want to avoid
   

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4. How our code
   should feel
   like
   

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5. What is a command bus?
   A command...
   "...an object is used to encapsulate all information
   needed to perform an action..."
   ...and a bus
   "...a communication system that transfers data
   between components..."
   

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6. To wrap up...
   

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7. A use case
   A Finder-like application in javascript
   On each file, you can operate commands: create a folder, rename a file,
   remove...
   Requests are sent from the JS application to a Symfony backend
   

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8. Setting up a command bus
   We will use Tactician (https://tactician.thephpleague.com/)
   Developed by Ross Tuck
   A Symfony bundle is available
   Notable alternatives include SimpleBus, by Matthias Noback
   

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9. A Command
   namespace AppBundle\Model\Command;
   use AppBundle\Entity\Item;
   class RemoveCommand
   {
   private $item;
   public function __construct(Item $item)
   {
   $this->item = $item;
   }
   public function getItem(): Item
   {
   return $this->item;
   }
   }
   

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10. The corresponding handler
    namespace AppBundle\Handler;
    use AppBundle\Entity\Item;
    use AppBundle\Model\Command\RemoveCommand;
    class RemoveHandler
    {
    private $entityManager; // To inject
    public function handle(RemoveCommand $removeCommand): string
    {
    $item = $removeCommand->getItem();
    $this->entityManager->remove($item);
    $this->entityManager->flush(); // We can find a better way
    return $item->getUuid(); // It can return a result!
    }
    }
    

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11. Declaring the handler
    Declared as a Symfony service:
    Note: handlers now support autowiring, check out Tactician bundle documentation
    app.remove_handler:
    class: AppBundle\Handler\RemoveHandler
    tags:
    - { name: tactician.handler, command: AppBundle\Model\Command\RemoveComman
    arguments:
    - '@doctrine.orm.default_entity_manager'
    

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12. Sending the command to the bus
    From the Symfony API controller:
    Great, Romaric... but what is the interest?
    /**
    * @ParamConverter("item")
    */
    public function apiAction(Item $item)
    {
    $command = new RemoveCommand($item);
    $uuid = $this->get('tactician.commandbus')->handle($command);
    return new JsonResponse(['uuid' => $uuid]);
    }
    

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13. It scales well!
    

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14. The application is getting complex...
    After one month, we had 25 commands and handlers, totaling 3500 lines
    of code.
    

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15. Adding middlewares
    Huge interest: middlewares help to simplify common tasks.
    

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16. Middleware examples
    Doctrine (https://tactician.thephpleague.com/plugins/doctrine/) : wraps handlers
    in a DB transaction
    Logger (https://tactician.thephpleague.com/plugins/logger/) : can log everything
    to Monolog (audit!)
    Validation (shipped with Symfony Tactician bundle)
    Security, authorization
    ...
    

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17. Going further
    Testing: handlers and middlewares are easier to unit test
    Commands could be sent to a message queue (RabbitMQ...) instead of
    staying in memory, or scheduled to be executed later (delayed task...).
    

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18. romaric@netinfluence.ch
    http://blog.netinfluence.ch
    

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