System Architecture Document Template (Project Planning)

System Architecture Document Template.md

1. Overview

Purpose

[Provide a brief description of the system, its objectives, and its intended users.]

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2. System Context Diagram

[Provide a high-level diagram illustrating how the entire system interacts with external entities, such as APIs, databases, and user interfaces.]

Description

[Explain the key components and their roles in the system context diagram.]

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3. Interfaces

[Define key external and internal interfaces for the system (e.g., APIs, external services, modules). Describe their contracts and interaction boundaries.]

Interface List

[List and briefly describe all interfaces.]

Inputs/Outputs

[For each interface, define:]

• Inputs: [What the interface expects to receive]
• Outputs: [What the interface provides back]

Example format:

Interface: Payment API
Inputs:
  - POST /charge
    - amount: number
    - currency: string
Outputs:
  - 200 OK { success: boolean, transactionId: string }
  - 400 Bad Request { error: string }

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4. Technology Stack

[Specify technologies, frameworks, and tools chosen for the project, along with justifications for their selection.]

Programming Languages & Frameworks

[List chosen languages, frameworks, and libraries, and explain why they were selected.]

Example:

### Frontend
- React Native - Mobile application framework
- Expo - Development platform and tools
- Expo Router - Navigation solution
- Nativewind - Tailwind CSS for React Native
- React Hook Form - Form handling
- Zod - Schema validation
- react-native-async-storage - Local storage management
- expo-secure-store - Secure data storage
- react-native-reusables - UI component library

### Backend
- Supabase - Backend as a Service platform

### Development Tools
- TypeScript - Programming language
- Expo CLI - Command line interface
- ESLint - Code linting
- Prettier - Code formatting
- Babel - JavaScript compiler

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5. Data Modeling

[Think about what data the system handles:

• What entities exist? (e.g., User, Report, Comment)
• What are the relationships? (e.g., A Report belongs to a User)
• What fields are important? (e.g., Report = title, content, date)]

[Define the structure of data used by the system, including schemas, entity-relationship diagrams, and data flow descriptions.]

Database Schema

[Outline the database schema with tables, fields, and relationships.]

Example ER diagram format:

User
 - id
 - name
 - email

Report
 - id
 - userId
 - title
 - content
 - createdAt

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6. Data Flow Design

[Map how data moves:

• Input → user types/clicks
• Local State → temporarily holds data
• Global/Shared State → shared across components
• Persistence Layer → localStorage, database, API
• UI Update Cycle → re-render on state change]

[Explain how data moves through the system (using mermaid diagram), including transformations, relationships and storage.]

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7. State Management Strategy

[Decide:

• What needs to be local (inside one component)?
• What needs to be global (shared across app)?
• How is state updated? (events? actions?)]

[General Rule:

• Local = Component’s own small concerns (e.g., form input)
• Global = App-wide concerns (e.g., user login status)]

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8. Component Architecture (UI Breakdown)

[Break system into components:

• What components do I need?
• What should they own (state/props)?
• How do they talk (props, context, events)?]

High-Level Architecture

[Provide a diagram showcasing system components and their relationships.]

Subsystems & Modules

[List the major subsystems or modules, along with their descriptions.]

Example component tree:

<App>
  ├── <Header>
  ├── <Sidebar>
  └── <MainContent>
       ├── <ReportForm>
       └── <ReportList>

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9. Persistence and APIs

[Define how system components communicate, including API specifications, protocols, and message formats.]

APIs

[List key APIs with request/response structures and authentication mechanisms.]

Data Contracts

[Describe expected data formats, payload structures, and validation rules.]

Inputs/Outputs

[For each API, define:]

• Inputs: [Request structure, parameters, headers]
• Outputs: [Response structure, status codes, error formats]

Example format:

API: POST /login
Inputs:
  - body: { email: string, password: string }
Outputs:
  - 200 OK { token: string }
  - 401 Unauthorized { error: string }

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10. Edge Cases and Error Handling

[Think about what could go wrong:

• Network failure?
• User leaves page halfway?
• Invalid inputs?
• API returns error?]

[Plan defensive code paths and fallback UI (loading states, error banners).]

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11. Performance and UX Optimization

[Finally:

• How big is the data being loaded?
• Should you lazy-load anything?
• Should you debounce input?
• Should you cache API responses?]

[Think in optimizations after correctness.]

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12. Security and Scalability Considerations

[Optional for bigger apps:

• How to protect user data? (XSS, CSRF, Auth)
• How to grow system cleanly? (modular architecture)]

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Guidelines

• Use UML Mermaid diagrams for components, relationships and systems