Design a Personal Finance / Expense Categorization System

Problem

Design a system that ingests bank/card transactions and auto-categorizes them for budgeting.

Requirements

Functional:

• Import transactions from many institutions
• Auto-categorize (groceries, travel, etc.)
• Let users correct categories (feedback loop)
• Monthly summaries and insights

Non-functional:

• 10s of millions of users
• Accurate, improving categorization
• Privacy and security of financial data

Discussion points

1. Ingestion/normalization across institutions
2. Categorization: rules + ML, feedback loop
3. Idempotent transaction dedup
4. Storage and aggregation for reports
5. Security/PII handling and compliance