architecture-patternslisted

# Architecture Patterns ## Overview A catalogue of architectural patterns used in Clean Architecture + DDD + Event-Driven systems. **Core principle:** Patterns are tools — select based on problem, not preference. Every pattern added to a design must be justified by a story or a quality attribute requirement. YAGNI applies to architecture. The patterns in this skill compose in a specific order: Event Modeling → DDD Strategic → DDD Tactical → Clean Architecture → CQRS, with Event Sourcing applied at the Domain/Infrastructure boundary when history has value. --- ## Event Modeling BDD-first methodology to model system behaviour as a timeline of events. It is the starting point for every DESIGN session. ### Core Concepts | Concept | Definition | Example | |---|---|---| | **Command** | An expression of user or system intent. Imperative mood. May be rejected. | `CheckEligibility`, `SubmitApplication` | | **Event** | An immutable fact that records a state change. Past tense. Cannot be rejected. | `EligibilityChecked`, `ApplicationSubmitted` | | **Read Model** | A query output optimised for a specific view. No mutation. | `EligibilityResult`, `ApplicationSummary` | ### Timeline Structure Commands cause Events. Events update Read Models. The timeline reads left to right: ``` [Command] → [Event] → [Read Model] ``` Each vertical grouping of Command + Event + Read Model is a **Slice** — the minimum unit of deliverable business value. ### How to Build a Timeline from Stories