Mobile App Architecture Documentation
Architecture documentation matters in three cases: new developers understand the system in a day, not two weeks; the team discusses expansion and everyone sees different mental models of the same app; the project transfers to another team or client.
What We Document and How
Don't write "App uses MVVM" in README. Document decisions and their rationale.
C4 Model—practical standard for mobile systems. Four levels of detail:
- Context—mobile app in context of users, external systems, backend
- Containers—iOS app, Android app, API backend, push notification service, CDN
- Components—within iOS app: Authentication Module, Feed Module, Payment Module
- Code—classes and interfaces within a specific module (only for complex parts)
Tools: Structurizr (DSL for C4, renders diagrams from code—diagrams stay current if updated with code), draw.io / Miro for informal diagrams.
ADR (Architecture Decision Records)—short documents in format "we chose X over Y because Z." File in repo: docs/adr/0001-use-swiftui-over-uikit.md. New developers read ADR and understand why, not rediscover decisions.
Data flow diagrams. How data flows from API to screen: network layer → cache → ViewModel → View. For offline mode: local database → ViewModel → sync queue → API. Draw sequence diagrams for non-trivial flows (Apple Sign-In, background sync).
Minimal Necessary Set
For medium mobile app (2–4 developers, 50+ screens):
- README with onboarding instructions (how to run project, Xcode/Android Studio/Flutter SDK versions, environment variables)
- Architecture diagram at C4 Containers level (one page)
- Description of key architecture patterns: "we use Clean Architecture, data flows Repository → UseCase → ViewModel → View"
- ADR for each non-obvious technical decision
- CI/CD documentation: what runs on PR, what runs on merge to main
Don't document the obvious. No diagram for every class—IDE shows it. Document decisions whose logic isn't visible in code.
Timeline: audit existing codebase and create documentation from scratch—1–2 weeks. Maintenance stays current—built into process: new ADR for each architecture decision, diagrams updated when structure changes.