Domain-Driven Design, Applied

Domain-driven design starts with domain experts and concrete business scenarios, before any code. You practice knowledge-crunching to build a ubiquitous language and surface model assumptions and terminology conflicts. You also sort business capabilities into core, supporting, and generic subdomains, so you know where careful modeling effort is worth spending.

A term like customer can mean one thing to billing and another to shipping. You propose bounded contexts that keep each model and language internally consistent, read boundary signals from organizational and model friction, and separate bounded contexts from modules and deployment units. Then you choose an explicit relationship for every dependency, weighing customer-supplier, conformist, partnership, and shared-kernel options, and protect your model with anti-corruption layers, published language, and open-host service contracts.

Entities, value objects, and aggregates give business rules a clear home. You design small aggregate boundaries that enforce invariants within one transaction, route state changes through the aggregate root, and reference other aggregates by identity, weighing contention trade-offs as you go. You then place the remaining logic deliberately: application services coordinate use cases, domain services hold operations without a natural entity home, factories create valid aggregates, and specifications capture reusable predicates, so the model carries behavior instead of staying anemic.

Repositories act as aggregate-oriented collections, which keeps storage concerns out of domain behavior. You work through unit-of-work boundaries, optimistic concurrency for conflicting updates, and read-model queries that live outside aggregate repositories. Domain events then record facts at successful invariant transitions. You learn to separate in-process dispatch from integration messages published only from committed state, translate events into external contracts, and handle eventual consistency across aggregate and context boundaries, all without requiring event-sourced persistence.