Core Domain

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October 24, 2010 by huionn

Core Domain is my favourite subchapter in Domain-Driven Design by Eric Evans. The book is the seminal work in domain design.

“In designing a large system, there are so many contributing components, all complicated and all absolutely necessary to success, that the essence of the domain model, the real business asset, can be obscured and neglected.

A system that is hard to understand is hard to change. The effect of a change is hard to foresee. A developer who wanders outside his or her own area of familiarity gets lost. (This is particularly true when bringing new people into a team, but even an established member of the team will struggle unless code is very expressive and organized.) This forces people to specialize. When developers confine their work to specific modules, it further reduces knowledge transfer. With the compartmentalization of work, smooth integration of the system suffers, and flexibility in assigning work is lost. Duplication crops up when a developer does not realize that a behavior already exists elsewhere, and so the system becomes even more complex.

Those are some of the consequences of any design that is hard to understand, but there is another, equally serious risk from losing the big picture of the domain:

The harsh reality is that not all parts of the design are going to be equally refined. Priorities must be set. To make the domain model an asset, the model’s critical core has to be sleek and fully leveraged to create application functionality. But scarce, highly skilled developers tend to gravitate to technical infrastructure or neatly definable domain problems that can be understood without specialized domain knowledge.

Such parts of the system seem interesting to computer scientists, and are perceived to build transferable professional skills and provide better resume material. The specialized core, that part of the model that really differentiates the application and makes it a business asset, typically ends up being put together by less skilled developers who work with DBAs to create a data schema and then code feature-by-feature without drawing on any conceptual power in the model at all.

Poor design or implementation of this part of the software leads to an application that never does compelling things for the users, no matter how well the technical infrastructure works, no matter how nice the supporting features are. This insidious problem can take root when a project lacks a sharp picture of the overall design and the relative significance of the various parts.”

When I was reading this many years ago, it has essentially changed my perspective in software development. Domain specific application can be developed in either simple or simplistic way. Taking an example from the book:

In a booking application to associate each Cargo with a Voyage, the original code is:

public int makeBooking(Cargo cargo, Voyage voyage) {
   int confirmation =;
   voyage.addCargo(cargo, confirmation);
   return confirmation;

Because there are always last-minute cancellations, standard practice in the shipping industry is to accept more cargo than a particular vessel can carry on a voyage. This is called "overbooking." Sometimes a simple percentage of capacity is used, such as booking 110 percent of capacity. In other cases complex rules are applied, favouring major customers or certain kinds of cargo.

This is a basic strategy in the shipping domain that would be known to any businessperson in the shipping industry, but it might not be understood by all technical people on a software team.

The requirements document contains this line:

Allow 10% overbooking.”

Simplistic solution:

public int makeBooking(Cargo cargo, Voyage voyage) {
   double maxBooking = voyage.capacity() * 1.1;
   if ((voyage.bookedCargoSize() + cargo.size()) > maxBooking)
       return –1;
   int confirmation =;
   voyage.addCargo(cargo, confirmation);
   return confirmation;

Simple solution:

public int makeBooking(Cargo cargo, Voyage voyage) {
if (!overbookingPolicy.isAllowed(cargo, voyage)) return –1;
   int confirmation =;
   voyage.addCargo(cargo, confirmation);
   return confirmation;

The new OverbookingPolicy class contains this method:

public boolean isAllowed(Cargo cargo, Voyage voyage) {
   return (cargo.size() + voyage.bookedCargoSize()) <=
         (voyage.capacity() * 1.1);

Both solution works exactly the same. But which one is easier to understand?


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