system-architecture-document

## Purpose Produce a System Architecture Document (SAD) that gives every stakeholder - from non-technical product leadership to infrastructure engineers - the right level of detail about the system's structure. This document answers: - **What** components make up the system? - **How** do they communicate? - **Why** was the architecture designed this way? - **Where** does data flow and where are the trust boundaries? - **What** are the non-functional constraints the architecture must satisfy? ## Input **Works best with:** The name of the system or subsystem being documented. **Also valuable:** Existing diagrams, technology stack decisions, known integration points, existing ADRs, NFR targets. **Example invocation:** `Create a system architecture document for PayFlow, a multi-brand payment gateway with a PHP 8.3 backend, MySQL database, custom plugin system, and white-label domain routing.` ## Key Concepts ### C4 Model (Simon Brown) The C4 Model provides a hierarchical, developer-friendly approach to architecture documentation - like a "Google Maps" with multiple zoom levels: - **Level 1: System Context** - The system's place in the world. Users and external systems. Intended for all stakeholders. - **Level 2: Container** - The major deployable units (web apps, APIs, databases, queues). Intended for technical leads and architects. - **Level 3: Component** - The internal structure of a container (controllers, services, repositories). Intended for developers. - **Level 4:...