Payment handling in LLD - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Architecture

intermediate

Designing a scalable payment processing system

You need to design a payment processing system that can handle thousands of transactions per second. Which architectural component is essential to ensure the system can scale horizontally and handle failures gracefully?

A A centralized monolithic server that processes all payments synchronously

B A distributed message queue to decouple payment requests from processing services

C A single database that stores all payment transactions and processes them sequentially

D A client-side script that processes payments directly without server interaction

💡 Hint

Think about how to handle high volume and failures without blocking the entire system.

Using a distributed message queue allows the system to handle many payment requests asynchronously. It decouples the request from processing, enabling horizontal scaling and better fault tolerance.

❓ scaling

intermediate

Estimating capacity for peak payment loads

Your payment system expects a peak load of 10,000 transactions per minute. Each transaction takes 200ms to process on average. How many concurrent processing workers do you need to handle the peak load without delay?

A 33 workers

B 200 workers

C 100 workers

D 50 workers

💡 Hint

Calculate how many transactions can be processed per second by one worker, then divide total transactions per second by that number.

10,000 transactions per minute equals about 167 transactions per second. Each worker processes 1 transaction in 0.2 seconds, so 5 transactions per second per worker. 167 / 5 = 33.4, so at least 34 workers are needed. To handle overhead and avoid delays, 50 workers is a safe estimate.

❓ tradeoff

advanced

Choosing between synchronous and asynchronous payment confirmation

In a payment system, you can confirm payments synchronously (immediate response) or asynchronously (delayed confirmation). Which tradeoff is true when choosing asynchronous confirmation?

A Asynchronous confirmation increases system scalability but may delay user notification of payment status

B Asynchronous confirmation improves user experience by providing instant success feedback

C Asynchronous confirmation reduces system complexity by avoiding retries and error handling

D Asynchronous confirmation guarantees immediate consistency of payment status across all services

💡 Hint

Think about how asynchronous processing affects user feedback and system load.

Asynchronous confirmation allows the system to handle more requests by processing payments in the background, improving scalability. However, it delays notifying the user about the payment result, which may affect user experience.

❓ component

advanced

Critical components for secure payment handling

Which component is essential to ensure secure handling of sensitive payment data in a payment system?

A A caching layer that stores payment data in plain text for faster access

B A load balancer that distributes payment requests without inspecting data

C An encryption module that encrypts payment data both at rest and in transit

D A logging system that records all payment data including card numbers for audit

💡 Hint

Consider how to protect sensitive data from unauthorized access.

Encrypting payment data at rest and in transit protects it from theft or leaks. Caching in plain text or logging sensitive data violates security best practices. Load balancers do not secure data themselves.

🧠 Conceptual

expert

Ensuring idempotency in payment processing

Why is idempotency critical in payment processing systems, and how is it typically implemented?

A Idempotency prevents duplicate charges by ensuring repeated requests with the same idempotency key are processed only once

B Idempotency allows multiple payments to be processed simultaneously without coordination

C Idempotency improves system speed by caching payment results indefinitely

D Idempotency is used to encrypt payment data before sending it to external services

💡 Hint

Think about what happens if a payment request is sent twice due to network issues.

Idempotency ensures that if the same payment request is received multiple times, only one payment is processed, preventing duplicate charges. This is usually done by assigning a unique idempotency key to each request and checking it before processing.