Laravel development: How to build event-driven applications using Laravel Event Sourcing?

Laravel Development: How to build event-driven applications using Laravel Event Sourcing?

An event-driven application is an application implemented using events and event handlers (Event Handler). An event-driven architecture makes applications easier to expand and maintain, more flexible, and easier to adapt to changes.

Laravel is a popular PHP framework that provides a feature called Event Sourcing that can help us build event-driven applications. This article will introduce how to use Laravel Event Sourcing to build a simple event-driven application.

1.What is Laravel Event Sourcing?

Laravel Event Sourcing is an event-driven modeling framework, which is a suite provided by Laravel to help us build event-driven applications. It stores and restores events, allowing us to reproduce the state in the application and go back to previous states.

2. Why use Laravel Event Sourcing?

The benefit of using Laravel Event Sourcing is that it can improve the scalability and maintainability of the application. When we use event-driven applications, it is easier to understand and modify different parts of the application, and the application is more robust.

Using Laravel Event Sourcing, we can easily implement multiple modes, including CQRS (Command Query Responsibility Segregation) mode and ES (Event Sourcing) mode.

3. How to build an event-driven application using Laravel Event Sourcing?

In this example, we will build a simple task management application where users can create and complete tasks.

Step 1: Create a task

We can demonstrate how to use Laravel Event Sourcing by creating a task. First, we need to create a "TaskCreated" event to handle the behavior of creating a task.

php artisan make:event TaskCreated

Step 2: Create an event handler for the task

Once we create an event, we need to create an event handler to handle the event. Now we need to create an event handler to handle the "TaskCreated" event.

php artisan make:listener CreateTaskListener --event=TaskCreated

Step 3: Bind the event and event handler together

Now we need to bind the event and event handler together. We can achieve this in Laravel's EventServiceProvider file.

protected $listen = [
    TaskCreated::class => [
        CreateTaskListener::class,
    ],
];

Step 4: Use event handler to handle task creation event

Now we can use our event handler to handle task creation event. The first event handler we will implement is CreateTaskListener, which will actually create the new task.

public function handle(TaskCreated $event)
{
    $task = new Task;
    $task->name = $event->name;
    $task->save();
}

Step 5: Use Laravel Event Sourcing to store events

Using Laravel Event Sourcing allows us to store and restore events. We need to use Event Sourcing library in Laravel, such as Broadway library.

We can use Laravel's composer.json file to add the Broadway library:

"require": {
    "broadway/broadway": "^1.0",
    "broadway/serializer": "^1.0",
    "broadway/event-store": "^1.0"
}

Then run the following command to install the Broadway library:

composer install

Step 6: Use Laravel Event Sourcing

Now we can use Laravel Event Sourcing to store events.

We need to create an event store to store and retrieve events. We can implement it by creating a class called TaskEventStore.php in Laravel's app folder:

use BroadwayEventStoreEventStore;
use BroadwayEventSourcingEventSourcingRepository;

class TaskEventStore extends EventSourcingRepository
{
    public function __construct(EventStore $eventStore)
    {
        parent::__construct(
            $eventStore,
            new TaskAggregateRootEventSourcedFactory(),
            new TaskAggregateRootEventSourcedRepository()
        );
    }
}

We need to create a new event store in the constructor of the TaskEventStore class and use the Broadway library EventSourcingRepository in EventSourcingRepository to store events. We also need to define an aggregate root factory and aggregate root repository to manage our aggregate roots.

Now we can use the TaskEventStore class to store events. We can add the following code in the CreateTaskListener event handler:

$eventStore = $this->app->make(TaskEventStore::class);
$eventStream = new DomainEventStream([$event]);

$aggregateRoot = $eventStore->load($command->taskId);
$aggregateRoot->handle($event);

$eventStore->save(
    $aggregateRoot->getUncommittedEvents(),
    $aggregateRoot->getId()
);

This code snippet gets an instance of the TaskEventStore class, creates an event stream, loads the aggregate root, calls the handle method and saves uncommitted events.

We also need to bind the TaskEventStore class in Laravel's ServiceProvider class:

$this->app->singleton(TaskEventStore::class, function ($app) {
    $eventStore = new InMemoryEventStore;
    return new TaskEventStore($eventStore);
});

Step 7: Find and display tasks

Now we have created a new task, we We can modify our query to display all tasks to the user.

Create a command called ShowTasks:

php artisan make:command ShowTasks

The first command processor we will implement is ShowTasks, which will return all tasks for list display.

public function handle()
{
    $tasks = Task::all();
    foreach ($tasks as $task) {
        $this->info("Name: {$task->name}");
    }
}

Step 8: Mark the task as completed

Now we want to simulate the behavior of marking the task as completed. We can use a "TaskCompleted" event to track this behavior.

First, we need to create a "TaskCompleted" event:

php artisan make:event TaskCompleted

Then, we will create an event handler named CompleteTaskHandler to handle this event.

php artisan make:listener CompleteTaskHandler --event=TaskCompleted

Next, we bind the "TaskCompleted" event and the CompleteTaskHandler event handler:

protected $listen = [
    TaskCreated::class => [
        CreateTaskListener::class,
    ],
    TaskCompleted::class => [
        CompleteTaskHandler::class,
    ],
];

Finally, the second event handler we want to implement is the CompleteTaskHandler, which will set the task status as completed.

public function handle(TaskCompleted $event)
{
    $task = Task::where('name', $event->name)->firstOrFail();
    $task->completed = true;
    $task->save();
}

At this point, we have successfully created an event-driven application where users can create, complete and display task lists.

in conclusion

Using Laravel Event Sourcing can help us build event-driven applications. Event-driven applications are more scalable, maintainable, and more flexible. With Laravel Event Sourcing, we can easily implement multiple patterns, including CQRS and ES patterns, so we recommend developers to use event-driven architecture while building applications.

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