Domain-Driven Design in PHP

Forcing Invariants

Factory Methods in the Aggregate Root are also a good place for invariants.

In a Domain Model with Forum and Post Entities, where Post is an aggregated part of the Aggregate Root Forum, publishing a Post could look something like this:

class Forum{    // ...    public function publishPost(PostId $postId, $content)    {        $post = new Post($this->id, $postId, $content);        DomainEventPublisher::instance()->publish(            new PostPublished($postId)        );        return $post;    } }

After talking with a Domain Expert, we came to the conclusion that a Post shouldn't be published when the Forum is closed. This is an invariant, and we could force it directly on Post creation, thereby preventing an inconsistent Domain state:

class Forum{     // ... public function ...