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+## Fitness Tracker MVP - Architecture Overview
+
+This document outlines the architectural design of the Fitness Tracker MVP, focusing on simplicity, scalability, and rapid development. It prioritizes core features that validate the product's value proposition and lays the foundation for future growth.
+
+### 1. High-Level Architecture
+
+The Fitness Tracker MVP employs a client-server architecture with a web application frontend and a backend API to manage data and logic.
+
+**Frontend:**
+
+- Built using Next.js 14 with the App Router.
+- Leverages React.js for building dynamic UI components.
+- Utilizes Tailwind CSS 3.x with JIT mode for rapid styling and visual iteration.
+- Employs Zustand for simple and efficient state management.
+
+**Backend:**
+
+- Implemented using Node.js with the Express.js framework.
+- Communicates with the frontend via a RESTful API.
+- Stores user data, goals, and progress in a PostgreSQL database.
+- Uses Prisma ORM for efficient and type-safe database interaction.
+- Integrates with NextAuth.js for authentication and authorization.
+- Leverages Sentry for centralized error tracking and reporting.
+
+**Data Flow:**
+
+1. User interactions in the frontend trigger API requests.
+2. The API Gateway (Express.js) receives requests and routes them to the appropriate API endpoints.
+3. API endpoints (e.g., `/api/goals`, `/api/progress`) interact with the PostgreSQL database using Prisma ORM.
+4. Data is retrieved or updated in the database.
+5. API endpoints return responses to the frontend.
+6. The frontend updates the UI based on the API responses.
+
+**Architectural Principles:**
+
+- **Modularity:** The application is divided into separate modules (e.g., components, pages, services) for easier maintenance and scalability.
+- **Loose Coupling:** Components and modules are designed to have minimal dependencies on each other, allowing for independent development and updates.
+- **Separation of Concerns:** Frontend and backend responsibilities are clearly separated to ensure code organization and maintainability.
+
+### 2. Key Components and Interactions
+
+**Frontend:**
+
+- **`pages/_app.tsx`**: The application's root component. Initializes global styling, state management, and renders the main layout (`Layout.tsx`).
+- **`pages/api/auth/[...nextauth].ts`**: Handles user authentication, integrating with Google and Facebook OAuth providers.
+- **`components/ui/`**: Reusable UI components (e.g., `Button.tsx`, `Card.tsx`, `Modal.tsx`, `Input.tsx`, `Select.tsx`, `Spinner.tsx`) for constructing the application interface.
+- **`components/features/`**: Feature-specific components, including login/signup forms, goal management, progress tracking, and dashboard elements.
+- **`lib/hooks/`**: Custom hooks for managing user authentication (`useUser`), goals (`useGoals`), and progress (`useProgress`).
+- **`lib/api/client.ts`**: Centralized API client for interacting with the backend API.
+
+**Backend:**
+
+- **`pages/api/`**: API endpoints for handling user authentication, goal management, progress tracking, and other core functionalities.
+- **`lib/prisma/schema.prisma`**: Defines the database schema using Prisma's schema language.
+- **`services/`**: Backend services (e.g., `goalService.ts`, `progressService.ts`, `userService.ts`) for interacting with the database and handling business logic.
+
+**Data Model:**
+
+- **`lib/types/goal.ts`**: Defines the data structure for a goal, including name, target date, progress, and target value.
+- **`lib/types/progress.ts`**: Defines the data structure for a progress entry, including value, date, and associated goal ID.
+- **`lib/types/user.ts`**: Defines the data structure for a user, including email, password, and associated goals.
+
+### 3. Implementation Considerations
+
+**Performance Optimization:**
+
+- **Code Splitting:** Next.js's App Router automatically splits code into smaller chunks, improving initial load times.
+- **Image Optimization:** Next.js's image optimization features automatically resize and compress images based on device and screen size.
+- **Caching:** Implement caching for API responses to reduce server load and improve response times.
+- **Lazy Loading:** Load components and resources on demand to minimize the initial page load.
+- **Database Query Optimization:** Optimize SQL queries for performance, particularly when dealing with large datasets.
+
+**Security:**
+
+- **Authentication:** Implement secure authentication using NextAuth.js and JWTs.
+- **Authorization:** Control access to specific resources based on user roles and permissions.
+- **Input Validation and Sanitization:** Validate user inputs and sanitize them to prevent cross-site scripting (XSS) attacks and other vulnerabilities.
+- **Data Encryption:** Encrypt sensitive data (e.g., user passwords) both in transit and at rest.
+- **Security Audits:** Regularly conduct security audits to identify and address potential vulnerabilities.
+
+**Scalability:**
+
+- **Microservices Architecture:** Consider adopting a microservices architecture for greater scalability and flexibility.
+- **Database Sharding:** Partition the database into smaller, independent shards to handle increasing data volume.
+- **Load Balancing:** Distribute traffic across multiple servers to handle high request volumes.
+- **Cloud Hosting:** Utilize cloud-based infrastructure (e.g., AWS, Azure) for scalability and fault tolerance.
+- **Efficient Database Design:** Design the database schema for optimal performance and scalability.
+
+### 4. Development and Testing
+
+**Testing:**
+
+- **Unit Testing:** Implement unit tests for components, hooks, services, and API endpoints.
+- **Integration Testing:** Ensure seamless integration between frontend, backend, and database.
+- **End-to-End Testing:** Test complete user flows from end to end, including authentication, goal creation, progress tracking, and visualization.
+
+**Development Tools:**
+
+- **Linting:** Use ESLint to enforce code style and prevent common errors.
+- **Code Formatting:** Utilize Prettier for consistent code formatting.
+- **Version Control:** Employ Git for version control and branching for efficient development.
+- **Debugging Tools:** Leverage browser developer tools and logging to identify and fix issues.
+- **Performance Monitoring:** Use tools to monitor application performance and identify bottlenecks.
+
+### Conclusion
+
+This architectural specification provides a blueprint for developing a robust, scalable, and secure Fitness Tracker MVP. By focusing on core features, implementing a modular design, and utilizing best practices for performance and security, this MVP will be well-positioned for future growth and success in the competitive fitness tracking market.