A constraint rule is a business rule. They let you to include, exclude, validate, recommend, and replace products based on business logic. At the very centre of it, a constraint rule is like if/then statement. It comprises of three things.Design By Contract (DbC) is a software correctness methodology. It uses preconditions and postconditions to document (or programmatically assert) the change in state caused by a piece of a program. Design by Contract is a trademarked term of BertrandMeyer and implemented in his EiffelLanguage as assertions.