Laravel Event Sourcing Laravel Package

title: Making sure events get handled in the right order weight: 4

By default all events are handled in a synchronous manner. This means that if you fire off an event in a request, all projectors will get called in the same request.

Handling events in a queue

A queue can be used to guarantee that all events get passed to projectors in the right order. If you want a projector to handle events in a queue then simply add the Illuminate\Contracts\Queue\ShouldQueue interface to your projector just like you would a Job.

A useful rule of thumb is that if your projectors aren't producing data that is consumed in the same request as the events are fired, you should let your projector implement Illuminate\Contracts\Queue\ShouldQueue.

You can set the name of the queue connection in the queue key of the event-sourcing config file. You should make sure that the queue will process only one job at a time.

In a local environment, where events have a very low chance of getting fired concurrently, it's probably ok to just use the sync driver.

Tweaking projector order

You can add a getWeight method to a projector to tweak the order projectors are run in. Projectors with a lower weight run first. When no explicit weight is provided, the weight is considered 0.

namespace App\Projectors;

use Spatie\EventSourcing\EventHandlers\Projectors\Projector;
use Spatie\EventSourcing\StoredEvents\StoredEvent;

class MyProjector extends Projector
{
    public function getWeight(?StoredEvent $event): int 
    {
        return 5;
    }
    
    //
}

Alternatively, you can determine the weight dynamically based on the event being processed. This allows you to prioritize certain events over others. The $event parameter will be null when the resetState() method is called (during projector reset operations).

namespace App\Projectors;

use App\Events\MoneyAddedEvent;
use App\Events\MoneySubtractedEvent;
use Spatie\EventSourcing\EventHandlers\Projectors\Projector;
use Spatie\EventSourcing\StoredEvents\StoredEvent;

class MyProjector extends Projector
{
    public function getWeight(?StoredEvent $event): int
    {
        return match ($event?->event_class) {
            MoneyAddedEvent::class => 2,
            MoneySubtractedEvent::class => -2,
            default => 0,
        };
    }
    
    //
}

Note that providing a weight on a queued projector won't guarantee execution order.

Want to know more?

We discuss projections and complex patterns such as CQRS in depth in our Event Sourcing in Laravel course. In practice, you want to check out these chapters:

• 5. Storing and Projecting Events
• 6. Projectors in Depth
• 14. CQRS