ZeePay API

The Challenge

Traditional banking systems struggle with audit requirements, data consistency, and the ability to reconstruct historical states. The challenge was to build a system that maintains complete transaction history, prevents data loss, handles concurrent operations safely, and provides instant access to current account states while supporting complex financial regulations.

The Solution

Implemented Event Sourcing with CQRS (Command Query Responsibility Segregation) architecture. Instead of updating account balances directly, every operation (deposit, withdrawal, transfer) is stored as an immutable event in an append-only event store. The current state is derived by replaying these events. Separate read and write models optimize for their specific use cases, with MongoDB transactions ensuring ACID guarantees.

Impact

• •Achieved 100% audit compliance with complete immutable transaction history
• •Enabled time-travel debugging by replaying events to any point in history
• •Prevented data loss through append-only event store architecture
• •Implemented optimistic concurrency control preventing race conditions
• •Built idempotency support ensuring duplicate operations are safely handled
• •Delivered sub-100ms query performance through optimized read projections

Key Features

Tech Stack

Lessons Learned

• →Event Sourcing provides unparalleled audit capabilities but adds complexity
• →CQRS enables independent scaling of read and write operations
• →Optimistic concurrency is essential for handling concurrent financial operations
• →Idempotency keys are critical for reliable distributed systems
• →Event replay is powerful for debugging and data recovery scenarios
• →MongoDB transactions require replica set configuration even in development