Sleepy Owl Coffee Backend Engineer Interview: Rounds, Questions and Preparation

What the job involves

As a Backend Engineer at an Indian unicorn or high-growth startup like Sleepy Owl Coffee, you will build and maintain server-side logic, APIs, and databases that power consumer apps, order management, inventory, and delivery systems. You will work closely with frontend, product, and DevOps teams to ship features fast, handle scale during peak demand, and ensure system reliability. Expect to write clean, testable code, debug production issues, and continuously optimise for performance and cost.

Role and Technical Knowledge

1. 1

   How would you design a REST API for a coffee subscription service that handles recurring orders and inventory checks?

   Outline endpoints for users, subscriptions, orders, and inventory, and mention authentication, error handling, and database transactions to avoid overselling.

2. 2

   Explain the difference between SQL and NoSQL databases. Which would you choose for storing user profiles and order history in a fast-growing coffee startup, and why?

   Compare schema flexibility, scaling, and consistency; justify choice based on read/write patterns and need for ACID transactions in orders.

3. 3

   What are database indexes and how can they improve query performance? Give an example relevant to an e-commerce backend.

   Explain how indexes speed up lookups on columns like user_id or order_date, and mention trade-offs like slower writes and storage overhead.

4. 4

   How do you handle authentication and authorisation in a backend API? Discuss JWT vs session-based auth.

   Cover token-based stateless auth with JWT, refresh tokens, and role-based access control; mention security best practices like HTTPS and token expiry.

Practical Scenarios

1. 5

   Your API is experiencing high latency during peak hours. How would you diagnose and fix the issue?

   Describe checking logs and metrics, identifying slow database queries, adding caching (Redis), optimising indexes, and scaling horizontally.

2. 6

   You need to integrate a third-party payment gateway for online orders. What steps would you take to ensure reliability and handle failures?

   Mention idempotency keys, retry logic with exponential backoff, webhook handling, and storing transaction states for reconciliation.

3. 7

   How would you design a system to send real-time order status notifications to customers via email and SMS?

   Propose a message queue (e.g., RabbitMQ, Kafka) to decouple services, with workers for email/SMS, and a retry mechanism for failures.

4. 8

   A bug causes duplicate orders in the database. How do you debug and prevent it from happening again?

   Trace the code path, check for race conditions, implement database unique constraints or idempotent operations, and add tests.

Behavioural and HR

1. 9

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

   Use STAR method: describe the situation, your learning strategy (docs, tutorials, small prototypes), and the successful outcome.

2. 10

   Describe a situation where you disagreed with a teammate on a technical decision. How did you resolve it?

   Show empathy, data-driven discussion, and willingness to compromise or escalate constructively; emphasise team goal over personal preference.

3. 11

   Why do you want to work at a fast-growing startup like Sleepy Owl Coffee rather than a large MNC?

   Highlight desire for ownership, impact, learning across the stack, and excitement for building products that reach consumers directly.

4. 12

   How do you prioritise tasks when you have multiple deadlines and limited time?

   Mention using urgency/importance matrix, communicating with stakeholders, and breaking tasks into smaller deliverables.

Coding and Problem Solving

1. 13

   Write a function to find the first non-repeating character in a string. Discuss time and space complexity.

   Use a hash map to count frequencies, then iterate to find first with count 1; aim for O(n) time and O(1) space for fixed character set.

2. 14

   Given an array of order amounts, find two orders that sum to a target amount. How would you optimise for large datasets?

   Use a hash set for O(n) time; discuss trade-offs with sorting and two-pointer approach, and handling duplicates.

3. 15

   Design a simple URL shortener like bit.ly. What database schema and API endpoints would you use?

   Cover generating unique short codes, storing mappings, handling redirects, and using caching for popular URLs.

4. 16

   How would you implement a rate limiter for an API to prevent abuse?

   Explain token bucket or sliding window algorithms, using Redis for distributed rate limiting, and returning 429 status codes.