backend_design.txt

The backend will be a Python Flask server with the following components:

1. Database:
   - Use SQLite for simplicity
   - Two tables:
     a. Expenses: id, date, amount, description, category, is_recurring
     b. Categories: id, name

2. API Endpoints:
   - GET /expenses: Retrieve all expenses
   - POST /expenses: Add a new expense
   - PUT /expenses/<id>: Update an existing expense
   - DELETE /expenses/<id>: Delete an expense
   - GET /categories: Retrieve all categories
   - POST /categories: Add a new category

3. Data Processing:
   - Calculate total expenses by category
   - Calculate monthly recurring expenses
   - Calculate one-time expenses

4. Dependencies:
   - Flask
   - SQLAlchemy (for database ORM)
   - Flask-RESTful (for API endpoints)

5. Main app structure:
   - app.py: Main Flask application
   - models.py: Database models
   - routes.py: API route definitions
   - utils.py: Helper functions for data processing

The server will handle all CRUD operations for expenses and categories, as well as provide processed data for the frontend to display in charts and tables.