Moglix Backend Engineer Interview: Rounds, Questions and Preparation

What the job involves

Backend engineers in Indian unicorns and high-growth startups build and maintain the server-side systems that power products at scale, such as order management, payment processing, inventory sync, and logistics tracking. Day to day they write APIs, design database schemas, optimise queries, debug production issues, and collaborate with frontend, product, and DevOps teams. In a company like Moglix, this often means working on systems that handle thousands of transactions per minute across B2B commerce and supply chain workflows.

Role and technical knowledge

1. 1

   What happens when you type a URL in a browser and hit enter, from a backend perspective?

   Cover DNS resolution, TCP handshake, HTTP request, server routing, database query, and response. Mention load balancers and caching if relevant.

2. 2

   How would you design a REST API for a B2B order placement system?

   Show resource modelling (orders, items, users), HTTP methods, status codes, idempotency for payments, and pagination for listing orders.

3. 3

   Explain database indexing and when you would avoid using an index.

   Describe B-tree indexes, trade-offs with write performance and storage, and cases like low-cardinality columns or small tables where indexes hurt.

4. 4

   What is the difference between SQL and NoSQL databases, and when would you choose one over the other for a supply chain product?

   Contrast schema flexibility, ACID compliance, scaling patterns, and give examples like orders needing strong consistency versus product catalogues needing flexible schemas.

Practical scenarios

1. 5

   An API that usually responds in 100 ms is now taking 3 seconds. How do you debug it?

   Check logs and metrics, look for slow database queries, missing indexes, N+1 queries, external service latency, and recent deployments. Suggest profiling and caching.

2. 6

   You need to send order confirmation emails to 10,000 customers after a flash sale. How do you design this without blocking the main order flow?

   Use a message queue to decouple email sending, a worker service to process jobs asynchronously, and retry logic with dead letter queues for failures.

3. 7

   Two users try to buy the last unit of a product at the same time. How do you prevent overselling?

   Discuss database transactions, row-level locking, optimistic concurrency control with version numbers, or atomic decrement operations in Redis.

4. 8

   Your service depends on a third-party payment gateway that is intermittently down. How do you make your system resilient?

   Mention circuit breakers, retries with exponential backoff, fallback queues, idempotency keys, and monitoring alerts for gateway failures.

Behavioural and HR

1. 9

   Tell me about a time you had to learn a new technology quickly for a project or hackathon.

   Use STAR format, highlight the learning approach, time constraints, and the outcome. Show curiosity and ability to self-learn.

2. 10

   Describe a situation where you disagreed with a teammate on a technical approach. How did you handle it?

   Focus on listening, presenting data or trade-offs, and reaching a decision. Avoid blaming others and show willingness to compromise.

3. 11

   Why do you want to work at a high-growth startup like Moglix instead of a large service company?

   Talk about ownership, fast-paced learning, direct impact on products, and interest in supply chain or B2B commerce. Avoid generic answers about brand name.

4. 12

   Where do you see yourself in three years as a backend engineer?

   Show a growth path from individual contributor to owning modules or mentoring juniors, with emphasis on deepening system design and scalability skills.

Problem solving and coding

1. 13

   Given a list of order IDs, find the first order ID that appears more than once.

   Use a hash set to track seen IDs in one pass. Discuss time and space complexity, and edge cases like empty input or no duplicates.

2. 14

   How would you find the top 5 most frequently ordered products from a large order history table?

   Explain SQL GROUP BY with COUNT and ORDER BY DESC LIMIT 5, or a hash map approach for streaming data. Mention handling ties and large data using partitioning.

3. 15

   Write a function to validate if a string is a valid Indian GST number format.

   Show character-by-character validation using regex or manual checks for length, digits, and pattern. Discuss test cases with valid and invalid inputs.

4. 16

   Given a binary tree representing a category hierarchy, how do you find the lowest common ancestor of two categories?

   Use recursion or parent pointers. Explain base cases, time complexity O(n), and how this applies to category navigation in an e-commerce backend.