What type of data should be passed to domain events?

Question

I've been struggling with this for a few days now, and I'm still not clear on the correct approach. I've seen many examples online, but each one does it differently. The options I see are:

• Pass only primitive values
• Pass the complete model
• Pass new instances of value objects that refer to changes in the domain/model
• Create a specific DTO/object for each event with the data.

This is what I am currently doing, but it doesn't convince me. The example is in PHP, but I think it's perfectly understandable.

MyModel.php

    class MyModel {
    
         //...
         private MediaId $id;
         private Thumbnails $thumbnails;
         private File $file; 
         //...
    
        public function delete(): void
        {
            $this->record(
                new MediaDeleted(
                    $this->id->asString(),
                    [
                        'name' => $this->file->name(),
                        'thumbnails' => $this->thumbnails->toArray(),
                    ]
                )
            );
        }
}

MediaDeleted.php

final class MediaDeleted extends AbstractDomainEvent
{
    public function name(): string
    {
        return $this->payload()['name'];
    }

    /**
     * @return array<ThumbnailArray>
     */
    public function thumbnails(): array
    {
        return $this->payload()['thumbnails'];
    }
}

As you can see, I am passing the ID as a string, the filename as a string, and an array of the Thumbnail value object's properties to the MediaDeleted event.

How do you see it? What type of data is preferable to pass to domain events?

Updated

The answer of @pgorecki has convinced me, so I will put an example to confirm if this way is correct, in order not to change too much. It would now look like this.

public function delete(): void
{
    $this->record(
        new MediaDeleted(
            $this->id,
            new MediaDeletedEventPayload($this->file->copy(), $this->thumbnail->copy())
        )
    );
}

I'll explain a bit:

The ID of the aggregate is still outside the DTO, because MediaDeleted extends an abstract class that needs the ID parameter, so now the only thing I'm changing is the $payload array for the MediaDeletedEventPayload DTO, to this DTO I'm passing a copy of the value objects related to the change in the domain, in this way I'm passing objects in a reliable way and not having strange behaviours if I pass the same instance.

What do you think about it?

Answer 1

A domain event is simply a data-holding structure or class (DTO), with all the information related to what just happened in the domain, and no logic. So I'd say Create a specific DTO/object for each event with the data. is the best choice. Why don't you start with the less is more approach? - think about the consumers of the event, and what data might they need.

Also, being able to serialize and deserialize the event objects is a good practice, since you could want to send them via a message broker (although this relates more to integration events than domain events).