Title: Specifications
Author's names:  Eric Evans and Martin Fowler
Email address:  evans@acm.org
Postal address: 3133 Frontera Way, #405, Burlingame, CA 94010
Phone number: 415-692-6305
Key words: domain model, constraint

Abstract:
In development of the domain model of business applications, often you need
to constrain or specify an object without referring to it.  You may need to
select a subset of objects based on some criteria.  You may need to check
that only suitable objects are used for a certain role.  You may need to
describe the requirements for an object in such a way that an object might
be built-to-order to fulfill the requirements.  The specification pattern
solves this problem by creating a separate object for describing such
requirements and giving it the behavior needed to compare itself with
candidate objects and with other specifications.