Nikolaos Papagrigoriou

Solutions Architect and DevOps Consultant

DDD - Modelling the Domain - Value Objects

There are elements of the Domain Model without any conceptual identity which are typically used to characterise Entities (Evans, 2003). Those elements are called Value Objects and their significance is often neglected (Vernon, 2013). Examples of Value Objects are: Zip Code, Order Number, Phone Number, Money, Currency, Date, Date Range, E-mail Address, URL etc. Value Objects express domain concepts and are part of the Ubiquitous Language (Evans 2003; Vernon, 2013).

DDD - Modelling the Domain - Entities

Entities are objects of the domain model with a distinct identity and a life cycle (Evans, 2003). Moreover, the key defining characteristic is not the values of their properties which could change over time (Evans, 2003). The uniqueness of an Entity's identity is defined by the model and does not change during its lifetime (Evans, 2003).


Software applications are typically composed of a User Interface (UI), code to process input from a user or another system and a database that acts as the persistence mechanism. Python applications could incorporate the UI, the business logic and the data access into a single file. For very simple applications this might make sense. However, as the complexity of the domain rises there is a need to divide code intro separate files within many directories and create separate code packages. The aim of this high level code organization is separation of concerns.

Domain Driven Design

Domain-Driven Design (DDD) is an approach to drive complex software projects using a set of principles, software design practices and techniques by directly aligning the code with the concepts in the domain (Evans2003). The concept was established by Eric Evans in his book “Domain-Driven Design, tackling complexity in the heart of software”. Vaughn Vernon notes that Evans' book “is essentially a large pattern language” (Vernon2013).