In computer programming, the specification pattern is a particular software design pattern, whereby business rules can be recombined by chaining the business rules together using Boolean logic. The pattern is frequently used in the context of domain-driven design.
A specification pattern outlines a business rule that is combinable with other business rules. In this pattern, a unit of business logic inherits its functionality from the abstract aggregate Composite Specification class. The Composite Specification class has one function called IsSatisfiedBy that returns a Boolean value. After instantiation, the specification is "chained" with other specifications, making new specifications easily maintainable, yet highly customizable business logic. Furthermore, upon instantiation the business logic may, through method invocation or inversion of control, have its state altered in order to become a delegate of other classes such as a persistence repository.
As a consequence of performing runtime composition of high-level business/domain logic, the Specification pattern is a convenient tool for converting ad-hoc user search criteria into low level logic to be processed by repositories.
Since a specification is an encapsulation of logic in a reusable form it is very simple to thoroughly unit test, and when used in this context is also an implementation of the humble object pattern.
In the next example, invoices are retrieved and sent to a collection agency if:
This example is meant to show the result of how the logic is 'chained' together.
This usage example assumes a previously defined <code>OverdueSpecification</code> class that is satisfied when an invoice's due date is 30 days or older, a <code>NoticeSentSpecification</code> class that is satisfied when three notices have been sent to the customer, and an <code>InCollectionSpecification</code> class that is satisfied when an invoice has already been sent to the collection agency. The implementation of these classes isn't important here.
Using these three specifications, we created a new specification called <code>SendToCollection</code> which will be satisfied when an invoice is overdue, when notices have been sent to the customer, and are not already with the collection agency.