Razorpay Backend Engineer Interview: Rounds, Questions and Preparation

What the job involves

Backend engineers at Indian unicorns and high-growth startups build and maintain the server-side systems that power consumer-facing products, often handling high traffic, payments, and real-time data. Day to day, they write APIs, design database schemas, debug production issues, and optimise performance for scale. They work closely with product, frontend, and DevOps teams to ship features quickly in fast-paced environments.

Role and Technical Knowledge

1. 1

   What happens when you type a URL in a browser and hit enter? Explain the backend components involved.

   Cover DNS resolution, HTTP request, load balancer, web server, application server, database queries, and response. Highlight caching and CDN if relevant.

2. 2

   How would you design a URL shortener like bit.ly for a high-traffic Indian startup?

   Discuss generating unique short codes, database choice (SQL vs NoSQL), handling collisions, caching, and scaling reads. Mention analytics and rate limiting.

3. 3

   Explain the difference between SQL and NoSQL databases. When would you choose one over the other for a payments system?

   Compare ACID compliance, schema flexibility, scaling. For payments, emphasise transactions and consistency, so SQL or NewSQL is often preferred.

4. 4

   What are the key considerations for designing a REST API for a mobile app used by millions?

   Cover versioning, authentication, rate limiting, pagination, error handling, and response formats. Mention idempotency for payment APIs.

Practical Scenarios

1. 5

   Your API response time suddenly spikes to 2 seconds. How do you debug and fix it?

   Outline steps: check logs, monitor metrics, profile database queries, inspect recent code changes, and use tools like APM. Suggest caching or indexing fixes.

2. 6

   You need to implement a feature that sends real-time payment notifications to users. How would you approach it?

   Discuss using webhooks, message queues (like RabbitMQ or Kafka), and push notifications. Mention retry mechanisms and idempotency to avoid duplicates.

3. 7

   How would you handle a situation where your database is reaching its storage limit and queries are slowing down?

   Propose sharding, archiving old data, adding read replicas, and optimising indexes. Discuss trade-offs and monitoring.

4. 8

   You are asked to integrate a third-party payment gateway. What are the key technical and security considerations?

   Cover API authentication, encryption, PCI compliance, error handling, and fallback mechanisms. Mention testing in sandbox and handling timeouts.

Behavioural and HR

1. 9

   Tell me about a time you worked in a team to solve a technical problem. What was your role?

   Use STAR method: describe situation, task, action, result. Highlight collaboration, communication, and impact.

2. 10

   Why do you want to work at a high-growth startup like Razorpay rather than a large MNC?

   Show enthusiasm for fast-paced environments, ownership, and learning. Connect to company mission and your career goals.

3. 11

   Describe a situation where you had to learn a new technology quickly for a project.

   Emphasise self-learning, resourcefulness, and applying knowledge effectively. Mention specific resources and outcome.

4. 12

   How do you handle tight deadlines and multiple priorities?

   Discuss prioritisation, time management, communication with stakeholders, and staying calm under pressure. Give an example.

Coding and Problem Solving

1. 13

   Given an array of integers, find two numbers that add up to a specific target. How would you optimise it?

   Start with brute force O(n^2), then explain hash map approach O(n). Discuss edge cases and space-time trade-offs.

2. 14

   Write a function to detect a cycle in a linked list. Explain your approach.

   Use Floyd's cycle detection (tortoise and hare). Explain why it works and its O(n) time, O(1) space complexity.

3. 15

   How would you design a rate limiter for an API? Implement a simple version.

   Discuss token bucket or sliding window algorithms. Code a basic in-memory rate limiter and mention distributed considerations.

4. 16

   Given a string, find the longest substring without repeating characters.

   Use sliding window with a hash set. Explain O(n) time and space, and walk through an example.