Presentation of the command dispatcher pattern I made during one of the chapter meetings in HelloFresh. Companion article: https://olvlvl.com/2018-04-command-dispatcher-pattern

1. The Command Dispatcher Pattern
26th
April, 2018
Author: Olivier Laviale

2. The command dispatcher pattern
- Remove input concerns
- Decouple the command from its implementation
- Implementation easy to replace or refactor
- Implementation is isolated, easier to test
- Perfect fit for DDD
- Advanced usage : caching, event sourcing
Command ImplementationInput

3. Concepts
Command Dispatcher Handler
Carries parameters Matches the command to the handler Takes care of the execution

4. Disambiguation
Command pattern
Command query responsibility segregation pattern
Event dispatcher pattern
≠

5. Example implementation
Dispatcher
class Dispatcher {
private $handlerResolver;
public function __constructor(array $handlerResolver) {
$this->handlerResolver = $handlerResolver;
}
public function dispatch(object $command) {
$handler = $this->handlerResolver->resolve($command);
return $handler($command);
}
}

6. Example implementation
Command
final class CreateRecipe {
public $id;
public $payload;
public function __construct(string $id, array $payload) {
$this->id = $id;
$this->payload = $payload;
}
}

7. Example implementation
Handler
final class CreateRecipeHandler {
// …
public function __invoke(CreateRecipe $command) {
// …
}
}

8. The life of a command
The life of a command could be summarised as follows:
1. The application receives an input (HTTP, console, message…)
2. A command is instantiated using information from that input
3. The command is passed to a dispatcher
4. The dispatcher finds the handler for that command
5. The command is passed to the handler
6. The handler executes that command
7. A result is returned (depending on the strictness of the implementation)

9. Testing
Unit testing
final class DeleteRecipeHandler {
private $repository;
public function __construct(RecipeRepository $repository) {
$this->repository = $repository;
}
public function __invoke(DeleteRecipe $command): void {
$this->repository->delete($command->id);
}
}
class DeleteRecipeHandlerTest extends TestCase {
public function testHandler() {
$command = new DeleteRecipe($id = uniqid());
$repository = $this->prophesize(RecipeRepository::class);
$repository->delete($id)->shouldBeCalled();
$handler = new DeleteEntityHandler($repository->reveal());
$handler($command);
}
}

10. Testing
Integration testing
class RecipeIntegrationTest extends TestCase {
// ...
public function testDelete() {
$id = $this->fixtures['recipe']->getId();
$command = new DeleteRecipe($id);
$this->dispatchCommand($command);
$this-assertFalse($this->repository->exists($id));
}
}

11. The Command Dispatcher pattern and DDD
The following classes could be used to create a recipe:
- ApplicationCommandCreateRecipe
- ApplicationDomainRecipeHandlerCreateRecipeHandler
- ApplicationDomainRecipeRecipeRepository (interface)
- ApplicationInfrastructurePersistanceRepositoryRecipeRepository (implementation)
- PresentationHTTPActionCreateRecipeAction

12. Advanced usage of the Command Dispatcher pattern
- Implementing CQS
- Implementing caching
- Implementing event sourcing
- Implementing restrictions

13. tl;dr
Because it decouples the command from its execution, the command dispatcher pattern increases the
flexibility of applications by enabling their services to be changed at any point in time without having to
modify the application itself. It removes input concerns and facilitate testing. It enables advanced
implementations such as response caching or event sourcing.

14. References
1. Command pattern – wikipedia.org
2. Paramore's Brighter – brightercommand.github.io
3. The Command Dispatcher Pattern – hillside.net
4. Commands, Command Handlers and Command Dispatcher – weblogs.asp.net
5. Command Buses Demystified: A Look at the Tactician Package – sitepoint.com
6. Event Dispatcher pattern – symfony.com
7. Command Query Responsibility Segregation pattern – martinfowler.com
8. Event Sourcing – martinfowler.com
9. Domain-driven design – wikipedia.org

15. Thanks!
https://olvlvl.com/2018-04-command-dispatcher-pattern