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4 Strategies for future events with Event Sourcing

Craig Morris  |  10 November 2021

Most systems will have to deal with future or deferred events. Even the most common example of software on the web—a blog—will have the ability to “publish” a post in the future at a certain time.

When that time is reached, the read model should return “published”. How does the read model do this? Compute the current time and override the status? What if we need a Reactor to do something once the post is published? Following are some strategies that can be used, depending on the system and the complexity required.

But first, this problem is not exclusive to event sourcing, take the world’s most popular blogging software, WordPress, it has two key fields in the wp_posts table—post_status and post_date.

When the editor sets a post to go live in the future, the post_status is “future” and the post_date is given the timestamp of when the post will go live—simple!

How does the post_status change from future to publish? WordPress seems to run a check on every request to update the status where post_date is in the past (it is WordPress after all!). This actually brings us to our first strategy.

Note: As I explore each strategy, it solves a problem created from the previous strategy, let’s gooo!

Strategy 1: Update our read model

Time processing is deferred to a dedicated CRON script. This could be as simple as executing the following SQL query every minute/hour/day.

UPDATE posts SET status = "published" WHERE status = "future" 
AND post_date < NOW()

Strategy 2: Compute in the Read Model

Relying on a cron job means our read model will be outdated until the cron job runs. Running the script on every request (ala WordPress) will come with its own performance problems.

We can get around this problem, by computing the status on the fly. The read model can introduce a new state field called “current_status”. When retrieving our model, it’s simply calculated on the fly. Here’s an example of a Laravel computed attribute.

<?php

public function getCurrentStatusAttribute()
{
    if ($this->status === 'published') {
        return $this->publish_date?->lessThan(now()) ? 'published' : 'future';
    }

    return $status;
}

Fantastic! Now our status field is super dynamic and always up to date!

But what if we need to grab all published posts, we’d have to retrieve all records from the database into memory and then display only the posts which are published.

An alternate implementation of this strategy is doing the computation in the database. This can be achieved via Generated Columns which is available in recent versions of Postgres and MySQL. This moves the business logic into the database layer which may or may not suit the culture of your business.

Strategy 3: Scheduled Commands / Events

We’re getting to a pretty good place here, however, there’s a key thing missing from an application standpoint. What happens when we need to do something once the post is published? We might need to ping iTunes if a Podcast is published, charge a client if an order is placed, etc...

We can schedule a job to be fired at the appropriate time. This is quite easy with something like Laravel’s queue system:

<?php

dispatch(function () use ($uuid) {
    PostAggregateRoot::retrieve($uuid)
        ->publish()
        ->persist();
})->delay($request->publish_date);

This doesn’t cause any state change in the database until the desired time. Once the time is reached, the job is executed, the event is fired and the system can react to the event at the appropriate time. We might need to do some extra work here to inform users of scheduled commands for a particular post.

Strategy 4: Verbose Events

We’re getting better and more flexible, and not too complex just yet. We could probably stop here, but what if we needed a report on upcoming posts. The read model keeps the Post in the draft state until the PostPublished event fires.

The answer is in providing more verbose events. The first event indicates that a post has been scheduled to be published (past, present, or future it doesn’t matter) and another event to indicate that the post is published. This gives the system two hooks in the post lifecycle, very flexible indeed.

With the PublishPost command, we can immediately fire one event after the other which keeps the event listeners simple.

<?php

class PostAggregateRoot
{
    public function publishPost(DateTime $when) {
        $this->recordThat(new PostPublishScheduled($when));

        if ($when < now()) {
            $this->recordThat(new PostPublished());
        }

        return $this;
    }

    public function publishPostNow() {
        $this->recordThat(new PostPublished());

        return $this;
    }
}

class PublishPostsConsole
{
    public function run() {
        $posts = Post::toBePublished(); // query in the model

        foreach ($posts as $post) {
            PostAggregateRoot::retrieve($post->id)
                ->publishPostNow()
                ->persist();
        }
    }
}

Bonus Strategy: Time is a Command

At this point, we’re in a pretty good state. We’ve got verbose events, giving us a lot of flexibility in what the system should do when posts are scheduled to be published and when the posts are actually published.

There’s just one small design flaw with our final strategy. We’ve mixed our read models and write models together, we’re dependent on a read model to execute things in our read model. We’ve also got some duplicated business logic in the aggregate root, and our toBePublished query.

In a very real-world scenario, what if we had thousands of scheduled posts which were causing a performance problem on our blog, we’d update our read projection to only store published posts to optimize the query. toBePublished would now always return an empty result and no further posts would be published!

To keep our business logic in the write model, we can treat Time as a Command. The aggregate root, looks at the time, its current state and determines what events it needs to publish.

<?php

class PostAggregateRoot
{
    public function applyPostPublished(PostPublished $event) {
        $this->status = 'future';
        $this->publishDate = $event->publishDate;
    }

    public function timeElapsed(DateTime $now) {
        if ($this->status == 'future' && $now->greaterThan($this->publishDate)) {
            $this->recordThat(new PostPublished());
        }

        // other time events to execute..

        return $this;
    }
}

class PublishPostsConsole
{
    public function run() {
        $posts = StoredEvents::postUuids(); // get all posts

        foreach ($posts as $post) {
            PostAggregateRoot::retrieve($post->id)
                ->timeElapsed(now())
                ->persist();
        }
    }
}

This does add a little complexity, and I don’t think it adds any more flexibility to the system. Personally, I’m willing to trade some simplicity with a dependency, everything is a trade-off, But you already know that if you’re using Event Sourcing ;)

Bi-Temporal Models

As soon as we introduce an effective date into our events, we now have 2-time data points in our events—the effective date (when the event should take effect) and the recorded date (when the event was recorded).

For scheduling posts, the PostPublishScheduled might be an effective date of February and a recorded date of January. For backdating posts, it could have PostPublishScheduled events in January and a recorded date of February.

This is venturing into the world of bi-temporal event sourcing, which is the concept of viewing the state on two-time dimensions—what was published on 14 January vs. what did we think was published on 14 January.

Conclusion

Which strategy you choose entirely depends on your project, if you need maximum flexibility in your project then you might need a more complex strategy. Otherwise, keep it simple as possible!


This post has been reposted from a version that appeared on Medium with the author's permission.


Photo of Craig Morris

Craig Morris is currently Head of Engineering at Superscript, implementing the Event Sourcing paradigm in the world of insurance. Craig is tasked with designing software that aligns with an industry that has evolved with weird and wonderful behaviours that evolved over the last few centuries.


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