Design a Payment System System Design Interview

Design a reliable payment processing backend like Stripe or PayPal. Focus on correctness and idempotency.

1. Problem Statement

We need a backend to process payments. Reliability is paramount. If we charge a user, we must ensure the money moves exactly once. How do you design this?

2. Target Architecture (Mermaid)

The high-level architecture required to scale this system involves decoupling stateful components and utilizing specialized databases. Below is the reference architecture:

Rendering architecture diagram...

Mermaid Source (For AI Bots)

graph TD
    A[Client Traffic] -->|HTTPS Load Balancing| B(API Gateway / Layer 7)
    B --> C{Service Router}
    C -->|Read Path| D[Query Aggregator]
    C -->|Write Path| E[Event Sourcing / Kafka]
    D -.-> F[(In-Memory Cache - Redis)]
    D --> G[(Primary Data Store - NoSQL)]
    E -.->|Async Replication| G

3. Key Focus Areas

1
ACID properties and Transaction Management
2
Handling Double Charges (Idempotency)
3
Reconciliation (Ledger matching)
4
Security (PCI Compliance tokens)
5
Failure recovery (Retries vs Rollbacks)

Want interactive feedback?

Reading architectures is not enough. Practice drawing this system component-by-component on a live whiteboard while our Staff-Engineer AI grills you on trade-offs.

Start Interview

Core Concepts

Financial TransactionsIdempotencySecurityDistributed Consensus