Jeff Bailey | Domain-Driven Design

How Do I Use a Software Ontology?

Jeff Bailey — Wed, 03 Jun 2026 00:00:00 +0000

The interviews are done. Someone sat with the domain expert and committed ontology.md next to the code. Then the quarter got busy. Six months later a report counts the same Customer three times, and the model built to prevent exactly that has not changed since launch.

An ontology earns its keep only when a team uses it. A software ontology is a shared model of a domain: its entities, their attributes, and their relationships, written so people and code agree on what each term means. This guide assumes the model already exists. The goal is to put it to work: in workflows, design sessions, database schemas, and on every screen where a teammate looks something up. Pick the section that matches the task in front of you. The command examples assume PostgreSQL and psql, but the practice maps to any engine that supports comments and constraints. The examples use the Subscription Billing context from the creation guide, so the terms line up across both articles.

How Do I Create a Software Ontology?

Jeff Bailey — Fri, 29 May 2026 00:00:00 +0000

Two senior engineers argued for an hour over a bug. The fix took ten minutes, but the argument lasted fifty. One meant “active subscription” when saying customer, the other meant “any account.” Both were correct but lacked a shared model.

A software ontology is an explicit, agreed-upon domain model, including concepts, meanings, and relationships. In DDD, it covers ubiquitous language, bounded contexts, and aggregates. This guide creates one.

Goal

Create a software ontology for a domain: a model that identifies key concepts, defines them once, classifies (entity, value object, or aggregate), and maps relationships and boundaries. The final artifact is an agreed-upon model reflected in the code.

What Is a Software Ontology?

Jeff Bailey — Thu, 28 May 2026 00:00:00 +0000

Open a mature codebase and grep for Customer. In billing, it means an active, paid account; in support, anyone who has sent an email; in analytics, a deduplicated household. The bug isn’t in one module, but in three teams misunderstanding they’re referring to the same thing.

A software ontology is a clearly defined, shared model of a domain that includes concepts, meanings, and relationships, documented and integrated with code. This article explains its role in Domain-Driven Design (DDD), why it exists, and how it functions as a mental model.