Lenskart Backend Engineer Interview: Rounds, Questions and Preparation

What the job involves

A backend engineer at an Indian unicorn or high-growth startup builds and maintains the server-side systems that power consumer-facing apps, internal tools, and data pipelines. Day to day you write APIs, design database schemas, debug production issues, and optimise for scale as user traffic grows rapidly. You work closely with product, frontend, and DevOps teams in fast iteration cycles.

Role and technical knowledge

1. 1

   How would you design a REST API for a product catalog that supports filtering by category, price range, and availability?

   Cover endpoint structure, query parameters, pagination, and how you would index the database for fast filtering. Mention versioning and error response format.

2. 2

   Explain how you would handle a sudden spike in traffic on an order placement service during a flash sale.

   Discuss horizontal scaling, load balancing, caching, queueing non-critical tasks, and rate limiting. Show awareness of database connection pools and idempotency.

3. 3

   What is the difference between SQL and NoSQL databases, and when would you choose one over the other for a user profile service?

   Compare schema flexibility, consistency, scaling patterns, and query needs. Give a clear recommendation with trade-offs for a profile service that reads often and writes rarely.

4. 4

   How do you ensure that an API you built is secure from common attacks like SQL injection and broken authentication?

   Mention parameterised queries, input validation, JWT or session best practices, HTTPS, and least privilege for database users. Show you know OWASP basics.

Practical scenarios and problem solving

1. 5

   You deploy a new feature and see a 20 percent increase in API latency. How do you debug and fix it?

   Walk through checking logs, metrics, and traces, isolating the slow endpoint, and testing recent changes. Mention rollback strategy and adding monitoring if missing.

2. 6

   A background job that syncs inventory with a warehouse system fails every night at 2 AM. How would you investigate and prevent it?

   Suggest checking job logs, dependency availability, timeouts, and retry logic. Propose idempotent jobs, alerting, and a dead letter queue for failed items.

3. 7

   You need to add a new field to a large production table without downtime. How do you approach the migration?

   Discuss adding nullable columns, backfilling in batches, using online schema change tools, and testing on a replica first. Mention feature flags to decouple deploy from release.

4. 8

   How would you design a simple notification service that sends email and SMS to users without blocking the main order flow?

   Cover using a message queue, separate worker services, retries with exponential backoff, and a provider fallback. Mention tracking delivery status and rate limits.

Behavioural and teamwork

1. 9

   Tell me about a time you had to learn a new technology quickly for a project. How did you manage it?

   Use a specific example from college projects or internships. Show a structured learning approach, time management, and a concrete outcome.

2. 10

   Describe a situation where you disagreed with a teammate on a technical decision. What did you do?

   Focus on listening, presenting data or trade-offs, and reaching a decision without personal conflict. Show maturity and willingness to commit to the final call.

3. 11

   How do you prioritise when you have multiple deadlines, like a college exam and a production bug?

   Explain how you assess impact and urgency, communicate early with stakeholders, and avoid silent delays. Show you can balance academics and work responsibly.

4. 12

   Give an example of a mistake you made in a project and what you learned from it.

   Pick a real, low-stakes mistake and focus on the fix and prevention. Avoid blaming others and show you added a check or process to avoid repeating it.

HR and company fit

1. 13

   Why do you want to work as a backend engineer at a fast-growing Indian startup rather than a large service company?

   Talk about ownership, speed of learning, direct impact on users, and comfort with ambiguity. Connect it to your own working style and career goals.

2. 14

   What do you know about our company and how does our product use backend systems?

   Research the company's products and mention specific backend challenges like inventory, recommendations, or order management. Show genuine interest, not generic praise.

3. 15

   Where do you see yourself in three years, and how does this role fit into that plan?

   Show ambition to grow technically, take ownership of services, and mentor juniors. Align your plan with the company's need for reliable, scalable systems.

4. 16

   Do you have any questions for us about the team, tech stack, or engineering culture?

   Ask about code review practices, deployment frequency, on-call rotation, or how engineers collaborate with product. Avoid questions only about salary or leave in early rounds.