How to implement event sourcing in Laravel applications



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Event sourcing is a pattern where changes to the state of an application are captured as a series of immutable events. These events represent state transitions and can be stored to recreate the state of the application at any point in time. Laravel, being a popular PHP framework, provides the tools needed to implement event sourcing. Here's a basic guide on how to implement event sourcing in Laravel applications:

1. Install Laravel EventSourcing Package

There are several Laravel packages available to help with event sourcing. One popular choice is the spatie/laravel-event-sourcing package. Install it using Composer:

bash
composer require spatie/laravel-event-sourcing

2. Create EventSourcingServiceProvider

Create a service provider to register the necessary services. Run the following command:

bash
php artisan make:provider EventSourcingServiceProvider

Then, in EventSourcingServiceProvider.php, register the necessary services:

php
/ app/Providers/EventSourcingServiceProvider.php

use Illuminate\Support\ServiceProvider;
use Spatie\EventSourcing\Facades\Projectionist;
use Spatie\EventSourcing\Facades\Projectionist as EventSourcingProjectionist;

class EventSourcingServiceProvider extends ServiceProvider
{
    public function register()
    {
        $this->app->singleton(Projectionist::class, function () {
            return EventSourcingProjectionist::forSubject();
        });
    }
}

3. Configure EventSourcingServiceProvider

Register your new service provider in the providers array of your config/app.php configuration file.

php
// config/app.php

'providers' => [
    // ...
    App\Providers\EventSourcingServiceProvider::class,
],

4. Define Events

Create events that represent changes in your application's state. Events should implement the Spatie\EventSourcing\StoredEvents\ShouldBeStored interface.

php
// app/Events/OrderPlaced.php

namespace App\Events;

use Spatie\EventSourcing\StoredEvents\ShouldBeStored;

class OrderPlaced implements ShouldBeStored
{
    public string $orderId;

    public function __construct(string $orderId)
    {
        $this->orderId = $orderId;
    }
}

5. Create Aggregate Roots

Aggregate roots are the objects that apply events to their state. Create a class for your aggregate root and extend Spatie\EventSourcing\AggregateRoots\AggregateRoot.

php
// app/Aggregates/OrderAggregate.php

namespace App\Aggregates;

use Spatie\EventSourcing\AggregateRoots\AggregateRoot;
use App\Events\OrderPlaced;

class OrderAggregate extends AggregateRoot
{
    public function placeOrder(string $orderId)
    {
        $this->recordThat(new OrderPlaced($orderId));
    }

    public function applyOrderPlaced(OrderPlaced $event)
    {
        // Apply the event to the aggregate's state
    }
}

6. Dispatch Events

Dispatch events when state changes occur in your application.

php
// ExampleController.php

use App\Aggregates\OrderAggregate;

public function placeOrder()
{
    $orderAggregate = new OrderAggregate();
    $orderAggregate->placeOrder('123');

    // Persist the aggregate and dispatch events
    $orderAggregate->persist();
}

7. Event Handlers and Projections

Create event handlers and projections to update read models or other data representations based on the events.

php
// app/EventHandlers/OrderEventHandler.php

namespace App\EventHandlers;

use App\Events\OrderPlaced;

class OrderEventHandler
{
    public function onOrderPlaced(OrderPlaced $event)
    {
        // Update read model or perform other actions
    }
}

8. Configure Event Handlers

Configure event handlers to listen for specific events and perform actions.

php
// EventSourcingServiceProvider.php

use App\EventHandlers\OrderEventHandler;

public function register()
{
    $this->app->singleton(OrderEventHandler::class);

    Projectionist::addProjector(OrderEventHandler::class)
        ->listenTo(OrderPlaced::class);
}

9. Run Projections

Run the projections to update read models or other representations based on the events.

bash
php artisan event-sourcing:replay

10. Testing

Write tests to ensure that events are handled correctly and that projections are updated as expected.

Conclusion

Implementing event sourcing in Laravel involves creating events, aggregate roots, and event handlers. The spatie/laravel-event-sourcing package provides useful tools to facilitate event sourcing in Laravel applications. Ensure that you thoroughly test your implementation to guarantee its correctness and reliability.





=== Happy Coding :)

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