Dunzo Backend Engineer Interview: Rounds, Questions and Preparation

What the job involves

Backend engineers at Indian unicorns and high-growth startups like Dunzo build and maintain the server-side systems that power core products such as order management, delivery routing, payments, and notifications. Day to day, they write APIs, design database schemas, debug production issues, and collaborate with product and mobile teams to ship features quickly. The role demands strong fundamentals in data structures, databases, and system design, along with comfort in a fast-paced, iterative environment.

Role and technical knowledge

1. 1

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

   Cover DNS resolution, TCP handshake, HTTP request/response cycle, server-side routing, and database interaction. Mention how a backend like Dunzo's order service would process the request.

2. 2

   Explain the difference between SQL and NoSQL databases. When would you choose one over the other for a food delivery app?

   Discuss ACID vs BASE, schema flexibility, scaling, and use cases. For a delivery app, SQL for orders and transactions, NoSQL for real-time tracking or logs.

3. 3

   What is a REST API? How would you design an endpoint to place an order on a platform like Dunzo?

   Cover HTTP methods, status codes, resource naming, request/response structure, and idempotency. Mention validation, authentication, and error handling.

4. 4

   Explain indexing in databases. How does it improve performance, and what are the trade-offs?

   Describe how indexes speed up reads but slow down writes and consume space. Give an example of indexing a user_id column in an orders table.

Practical scenarios

1. 5

   A delivery order is stuck in 'processing' state for hours. How would you debug this in a production system?

   Outline checking logs, database state, queue backlogs, and external service dependencies. Mention adding monitoring and alerts to prevent recurrence.

2. 6

   You need to design a system to assign delivery partners to orders in real time. What factors would you consider?

   Discuss location proximity, partner availability, order priority, load balancing, and minimizing delivery time. Mention using a geospatial index and a matching algorithm.

3. 7

   How would you handle a sudden spike in traffic during a festival sale on a Dunzo-like platform?

   Cover horizontal scaling, caching, rate limiting, database read replicas, and queue-based load leveling. Mention monitoring and graceful degradation.

4. 8

   Your API is slow for a particular endpoint. How would you diagnose and improve its performance?

   Talk about profiling, query optimization, adding indexes, caching, and reducing payload size. Mention using tools like explain plans and APM.

System design and architecture

1. 9

   Design a simple URL shortener like bit.ly. What are the key components and database choices?

   Cover hashing or base62 encoding, key generation, database schema, caching, and redirection. Discuss handling collisions and scaling reads.

2. 10

   How would you design a notification service that sends push, SMS, and email alerts for order updates?

   Discuss message queues, third-party integrations, retry logic, user preferences, and idempotency. Mention decoupling via a pub/sub system.

3. 11

   What is a load balancer and why is it important for a high-growth startup?

   Explain distribution of traffic, health checks, session persistence, and horizontal scaling. Give examples like Nginx or AWS ELB.

4. 12

   Explain how you would implement a rate limiter for an API endpoint.

   Cover token bucket or sliding window algorithms, Redis for distributed counters, and returning 429 status codes. Discuss per-user and per-IP limits.

Behavioural and HR questions

1. 13

   Why do you want to work at a fast-paced startup like Dunzo rather than a large MNC?

   Show enthusiasm for ownership, rapid learning, and impact. Connect personal goals to the startup environment and mention specific aspects of the company's product or culture.

2. 14

   Tell me about a time you faced a technical challenge in a project. How did you overcome it?

   Use the STAR method. Focus on the problem, your approach, the solution, and what you learned. Highlight debugging or collaboration skills.

3. 15

   How do you prioritize tasks when you have multiple deadlines?

   Discuss using urgency and impact, communicating with stakeholders, and breaking down work. Give an example from college projects or internships.

4. 16

   Where do you see yourself in three years?

   Align your growth with the company's needs, such as becoming a strong backend engineer or taking on more ownership. Avoid vague answers like 'in a managerial role' without context.