Eventual Consistency and Business Alignment

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I recently discovered through eventual consistency that my bounded contexts were not properly aligned with the business.   I won’t lie, it took me quite a while to make this realization. This was most likely the case in many situations I’ve had in the past.  Because of this realization, I wanted to let out some of my thoughts about eventual consistency and business alignment.

Dependent Bounded Context

I’ve often encounter situations where a bounded context requires information that another bounded context is responsible for.  I’d like to use a simple example I’ve heard from Udi Dahan.  In the context of an Ecommerce site.

• A customer can be a defined as a “preferred” customer.
• Preferred customers receive a 10% discount on all orders.

Based on the above, the “preferred” flag and any business rules associated to it, most likely exists in some sort of the CRM bounded context.  However, this detail is required in the Sales bounded context in order to apply a discount if eligible. As you can see, there is information that needs to be shared between bounded contexts.

Publish / Subscribe Domain Events

One approach for decoupling your bounded context is to publish domain events from your domain model.  This allows other bounded contexts to subscribe to those events and handle them accordingly. Let’s use our example above to see how this would be implemented.  In our CRM bounded context, when a customer is defined as preferred in our domain model, we would publish a CustomerIsPreferred event.

class CustomerIsPreferred
{
	public Guid CustomerId { get; private set; }
	public DateTime Date { get; private set; }
	
	public CustomerIsPreferred(Guid customerId, DateTime date)
	{
		CustomerId = customerId;
		Date = date;
	}
}

In our Sales bounded context, we would subscribe to this event and update our customer model with a preferred flag. This piece of information is used as a local cache in our Sales bounded context. During our checkout process in Sales, we would then use the preferred flag on the concept of a customer in Sales to determine if they should receive a 10% discount. However, remember that this preferred flag is not owned by Sales. Because of the publish / subscribe model (assuming asynchronicity), at any given time, our preferred flag in Sales could be out of sync with current state in our CRM bounded context. Eventually consistency doesn’t mean our data is wrong, it just means it is stale.

Business Alignment

There are many situations where data being eventually consistent is totally acceptable.  I’ve found in the real world we often make decisions with stale data all the time. However, there are times where full consistency is required.  When describing the example above to the business, does the eventual consistency of the preferred flag have true business impact?  If it truly does matter and the data must be fully consistent, then you may have bad business alignment with your bounded contexts. Re-evaluate your bounded context and the boundaries as you may have an wrong interpretation of responsibilities. I’ve found that drawing a context map and the events which are published and subscribed with a domain expert should flush out any of these incorrect interpretations and help you re-align boundaries and responsibilities.