BigBasket Now 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 apps, internal tools, and data pipelines. Day to day, you write APIs, design database schemas, debug production issues, and optimise for scale and latency under tight deadlines. In quick-commerce or e-commerce setups like BigBasket Now, your code directly affects order processing, inventory sync, delivery slotting, and payment flows.

Role and technical knowledge

1. 1

   What happens when a user places an order on a quick-commerce app like BigBasket Now from the backend perspective?

   Walk through API call, inventory check, payment gateway, order creation, and notification. Mention idempotency and failure handling.

2. 2

   How would you design a database schema for a grocery delivery service with products, inventory, and orders?

   Cover tables like products, warehouses, inventory, orders, order_items. Discuss indexes, foreign keys, and denormalisation for read-heavy workloads.

3. 3

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

   Compare consistency, scaling, schema flexibility. Give examples: PostgreSQL for transactions, MongoDB or Cassandra for high write throughput or flexible catalogs.

4. 4

   What is an API rate limit and why would a backend engineer implement it?

   Explain protecting services from abuse, ensuring fair usage, and maintaining uptime. Mention token bucket or sliding window algorithms.

Practical scenarios and problem solving

1. 5

   Your service is suddenly slow during peak hours. How do you debug and fix it?

   Start with monitoring and logs, check database queries, cache hit ratio, thread pools, and downstream dependencies. Suggest short-term fixes and long-term improvements.

2. 6

   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, or atomic decrement operations. Mention handling race conditions.

3. 7

   How would you handle a payment gateway timeout in an order flow?

   Cover retries with idempotency keys, webhook reconciliation, and user notification. Emphasise not double-charging or double-shipping.

4. 8

   You need to add a new feature that reads from a third-party API. How do you ensure it does not bring down your main service?

   Mention circuit breakers, timeouts, fallbacks, caching, and asynchronous processing. Show awareness of resilience patterns.

Coding and algorithms

1. 9

   Given a list of delivery orders with timestamps, find the maximum number of orders delivered in any one-hour window.

   Use sliding window or two-pointer after sorting. Explain time complexity O(n log n) due to sorting.

2. 10

   Write a function to detect if a string of parentheses is balanced. How would you extend it to validate a JSON-like structure?

   Use a stack. For JSON-like, handle nested objects, arrays, and strings. Discuss edge cases like escaped quotes.

3. 11

   How would you find the top K most frequently ordered products from a large log file?

   Use a hash map for counts, then a min-heap of size K. Mention memory constraints and possible distributed counting.

4. 12

   Implement a simple LRU cache. Where would you use it in a quick-commerce backend?

   Use doubly linked list and hash map. Relate to caching product details, user sessions, or delivery slot availability.

Behavioural and HR questions

1. 13

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

   Use STAR method. Highlight resourcefulness, hands-on practice, and delivering under time pressure.

2. 14

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

   Show respectful communication, data-driven reasoning, and willingness to compromise for the team's goal.

3. 15

   Why do you want to work at a high-growth startup like BigBasket Now instead of a large IT services company?

   Connect to ownership, fast-paced learning, direct impact on users, and interest in solving scale problems. Be specific about the sector.

4. 16

   How do you handle working on multiple urgent tasks with tight deadlines?

   Talk about prioritisation, communication with stakeholders, and breaking work into smaller deliverable chunks. Mention staying calm and focused.