Value Object

DDD tactical pattern for core subdomain business logic: value objects (immutable, identified by values), entities, aggregates (consistency boundary, one-per-transaction rule, aggregate root, OCC), domain events, domain services; Evans elaborations on associations, entity identity, context-dependent entity/VO classification, service partitioning, modules as model elements

predicate VALUE OBJECT for expressing domain rules: three uses (validation, selection/querying, building to order); composite Specification with AND/OR/NOT; subsumption; integrates with REPOSITORY via selectSatisfying()

Evans' six patterns for design that is easy to work with and extend: Intention-Revealing Interfaces, Side-Effect-Free Functions, Assertions, Conceptual Contours, Standalone Classes, Closure of Operations; declarative style as outcome