Protecting domain integrity through validation

Validating inputs to preserve business integrity in our domain

In this post, I’m going to discuss how to validate inputs to maintain the integrity of our business domain. Specifically, I’ll assume we are working with entities and aggregates, which are tactical patterns in Domain-Driven Design (DDD).

There are situations, where we might need to pass an entity directly to a method within our aggregate. Let’s look at an example:

 
    
    class Order { 
        constructor(
            private id: OrderId, 
            private requestId: RequestId, 
            // Other properties... 
        ) {} 
    } 

Consider a method like this:

 
    class Order {
        processRequest(request: Request): void {
            if (!this.belongsToRequest(request)) {
                throw new RequestDoesNotBelongToOrder(request.getId());
            }
    
            // Rest of the code
        }
    }

    private belongsToRequest(request: Request): boolean {
        return this.requestId.equal(request.getId())
    }

As mentioned, there are cases where we might need to pass another entity directly (in our case Request) into the aggregate method. It’s crucial in such situations to ensure that our domain maintains its integrity by validating the entity before proceeding.

In the example above, the method processRequest first checks if the given request actually belongs to the order. This is achieved through the belongsToRequest method, which compares the requestId of the order with the requestId of the incoming request. If they don’t match, an exception is thrown, indicating that the request does not belong to the order.

This guard clause is essential to maintain the integrity of our domain. By encapsulating this logic within the aggregate, we ensure that any operation on the aggregate respects the domain's invariants and business rules.

Why is This Important?

Ensuring the integrity of our domain models is critical for several reasons:

• Consistency: It prevents inconsistent states within the aggregate, which could lead to bugs or unexpected behaviors in the system.
• Protection of Business Rules: By validating inputs and operations within the aggregate, we safeguard the core business logic that the aggregate is responsible for.
• Encapsulation: The aggregate is the guardian of its own state, ensuring that all modifications happen in a controlled and valid manner.

By following these principles, we maintain a clean and robust domain model.

Remember, always validate inputs and operations within your aggregates to uphold the business rules and keep your domain model in a consistent state.