Understanding Domain Entities

Now let's try to do some structural modeling for the system.Before we start with this exercise let's do a quick warm up.

Structurally any system is made up of Components and their Inter-connectors nested at different levels of granularity.

The structural components at the lowest level of granularity are called as Entities, while the structural components at the intermediate level of granularity are called as Subsystems, Modules or Packages.

There are three different types of entities in any system.

  • Domain Entities or Business Modeling Entities
  • Control Entities or Process Entities or Work-flow Modeling Entities
  • Boundary Entities

These are the different types of entities that you will find in any well designed system today.

Let's talk about Domain Entities.

Domain Entities

Domain Entities are the fundamental structural building blocks of any system. They can be either Physical or Conceptual.

If the requirements are given in the form of a word document, every Noun in the document representing some information will be modeled as Domain Entity.

Whether the Customer describes the system verbally or physically, the rule remains the same. All the information that is a Noun should be modeled as a domain entity.


Every Noun which goes into describing a system should be captured as the domain entity of the system. This is true irrespective of the way system requirements are given; either in the form of a Word document or Orally.

Let's start the structural modeling for Railway Reservation System.

First let's find the Domain entities for the RRS by finding out the nouns which have been used to describe the system.

The following are the nouns that you will come across in this system:

  • Train
  • Coach
  • Berth
  • Station
  • Schedule
  • Passenger
  • Payment
  • Ticket, etc.

There could be many other domain entities involved, but for this discussion we will only consider these domain entities.

Different Domain Entities for the railway reservation - Different Domain Entities for the railway reservation system.

Hemant Jha
Founder - VPlanSolutions
Researcher, Trainer