Using policy and specification to deal with business rules

One of the most valuable things a system possesses is its codified business rules. Those rules represent a vital effort to understand a real-world problem and translate that understanding into working software. That’s not a trivial task, for sure. In DDD, we learn how crucial it is to work closely with domain experts to model our problem domain correctly. If domain experts are not available, we should seek developers with knowledge of a business. If none of them is available, we have no choice but to embark on a knowledge-seeking journey through books and any other resources that can help us grasp our problem domain’s inner workings.

Once the business knowledge is acquired and we have enough relevant information about the problem domain’s steps and processes, we can then start the adventure to transform that knowledge into code. At first glance, this process to understand business needs and transform...