Create events and commands

Last updated at 25 September 2019 | Published at 25 February 2018

Events are the core of any event sourced system. They are the payload, the message, they allow our system to communicate in a meaningful way. Events and commands are very simple objects. They should be modeled as “read-only” objects. This means they have to be instantiated with all the data they need and only expose that data. In EventSauce, they have but one technical requirement:

All events must be objects.

Depending on your serialization strategy your events may need to implement more methods or indicate they implement a certain interface.

Event serialization

In order to persist events they must be serializable. You can create your own serialization strategy, or use the default ones provided.

By default the MessageSerializer uses the PayloadSerializer to serialize events. This serializer requires events to implement the SerializablePayload interface. This interface requires you to implement 2 public functions:

  1. toPayload(): array
  2. fromPayload(array $payload): SerializablePayload

To and From payload

The toPayload and (static) fromPayload methods are used in the serialization process. The toPayload method is expected to return an array that’s serializable as JSON. The fromPayload method is expected to create an instance from a deserialized JSON array.

To illustrate:

$event1 = new MyEvent();
$event2 = MyEvent::fromPayload($event1->toPayload());

assert($event1 == $event2);

Defining events (and commands)

Defining events and commands can be done in 2 ways.

  • Defining them in YAML (code generation).
  • Creating classes by pressing keys on your keyboard.

Manually creating classes.

EventSauce provides interfaces for events and commands. You can create implementations of this. Here are minimal examples.

Event

<?php

use EventSauce\EventSourcing\Serialization\SerializablePayload;

class SomeEvent implements SerializablePayload
{
    public function toPayload(): array
    {
        return ['property' => $this->property];
    }

    public static function fromPayload(array $payload): SerializablePayload
    {
        return new SomeEvent($payload['property']);
    }
}

As you can see in the examples above, there are just 2 required methods. The from and to payload methods are used in the serialization process. This ensures the events can be properly stored. Values returned in the toPayload method should be json_encode-able. Additional required properties for an event should be injected into the constructor and properly formatted in the payload methods.

Defining commands and events using YAML.

Find out how to define commands and events using YAML

Frank de Jonge

EventSauce is a project by Frank de Jonge.