Learning Domain-Driven Design — Chapter 3

The fourth session turns DDD into practice. Vlad Khononov's decision tree is a heuristic, not a recipe: it connects subdomains, bounded contexts, and implementation. An architectural choice should be justified by business conditions and tradeoffs rather than by citing a book.

Subdomains evolve with the company through business, organizational, knowledge, and growth changes. A supporting capability can become a core product, while a former core becomes generic, as navigation did after smartphones. When knowledge is scarce, broad bounded contexts are safer.

Tactical design evolves from Transaction Script to Active Record, a domain model, and an event-sourced model. Repeated rules suggest moving behavior into an aggregate. Transitions stay incremental; new team structures reshape partnerships, while Conway's Law can turn growth into a big ball of mud.

Big Picture EventStorming maps events, commands, policies, read models, and pain points to reveal bounded contexts. In brownfield work, teams first map subdomains, current patterns, and dependencies. A Strangler Fig retains a facade while requests gradually move from the old system to the new one.