Domain-Driven Design and Architectural Patterns: From DDD to Hexagonal and Onion Architecture

The author, a senior architect, introduces the question of how to begin a new project by applying Domain‑Driven Design (DDD) from scratch.

Terminology : IAAS (Infrastructure‑as‑a‑Service), PAAS (Platform‑as‑a‑Service), and SAAS (Software‑as‑a‑Service) are defined to set the context for cloud‑based service models.

Architecture Evolution : The article illustrates the progression from monolithic applications (SAAS) to modular systems using MVC/SSM, then to micro‑service style isolation (PAAS) with technologies like mesh and SOA, and finally to infrastructure scaling (IAAS) employing VMs, Docker, and Kubernetes.

Bounded Context : The concept of bounded context (BC) is explained, showing how business domains like Order and Inventory are split into separate contexts, each with its own model and technical solutions such as synchronous calls or asynchronous messaging.

DDD Four‑Layer Boundaries :

First boundary – define vision, core, generic and supporting sub‑domains.

Second boundary – treat each bounded context as a physical process isolation.

Third boundary – within each context, separate interface, domain, application, and infrastructure layers.

Fourth boundary – use aggregates in the domain layer to ensure consistency.

Clean Layered Architecture : A diagram (shown below) demonstrates separating implementation from interfaces so the domain layer remains independent of technical details.

Hexagonal Architecture : The article describes the hexagonal (ports‑and‑adapters) style, distinguishing active adapters (UI, CLI) from passive adapters (databases, external services).

Onion Architecture : Building on the hexagonal model, the onion architecture further separates the core domain model from application services and infrastructure, emphasizing dependency direction toward the center.

Conclusion : Domain‑Driven Design is a popular architectural approach; by following its four‑boundary methodology, teams can achieve clear domain separation, facilitate vertical and horizontal scaling, and build maintainable backend systems.