Klaviyo India Backend Engineer Interview: Rounds, Questions and Preparation

What the job involves

Backend engineers at global product companies build and maintain the server-side systems that power customer-facing features, APIs, and data pipelines. Day to day, they write clean, testable code, design database schemas, debug production issues, and collaborate with frontend, product, and infrastructure teams. Freshers typically start with well-scoped tasks like adding endpoints, fixing bugs, or improving service reliability under mentorship.

Role and technical knowledge

1. 1

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

   Cover schema flexibility, consistency models, scaling patterns, and give a concrete example like user profiles vs. event logs.

2. 2

   What happens when you type a URL in a browser and hit enter? Focus on the backend aspects.

   Mention DNS lookup, TCP handshake, HTTP request, load balancer, application server, database query, and response rendering.

3. 3

   How would you design a REST API for a simple task management app? List the endpoints and methods.

   Use resource-based URLs like /tasks and /tasks/{id}, map HTTP methods correctly, and discuss status codes and pagination.

4. 4

   What is the difference between authentication and authorization? Give an example of each in a backend system.

   Define authentication as identity verification and authorization as permission checks, then mention JWT or session tokens and role-based access.

Practical scenarios

1. 5

   A production API endpoint suddenly starts returning 500 errors. How would you debug and resolve the issue?

   Describe checking logs, reproducing the error, reviewing recent deployments, isolating the faulty component, and rolling back if needed.

2. 6

   You need to add a new feature that sends email notifications when a user signs up. Outline your approach.

   Discuss using a message queue, an email service, handling failures with retries, and ensuring the signup flow is not blocked.

3. 7

   Your database queries are slow as the dataset grows. What steps would you take to improve performance?

   Mention indexing, query analysis with EXPLAIN, caching, pagination, and possibly denormalization or read replicas.

4. 8

   How would you ensure your backend service is secure against common attacks like SQL injection and XSS?

   Cover parameterized queries, input validation, output encoding, HTTPS, and keeping dependencies updated.

Behavioural and HR

1. 9

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

   Use the STAR method: describe the situation, the technology, your learning steps, and the outcome with a measurable result.

2. 10

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

   Show respect, focus on data and trade-offs, and emphasize reaching a consensus or escalating constructively.

3. 11

   Why do you want to work as a backend engineer at a global product company like Klaviyo India?

   Connect your interest in scalable systems and data-driven products to the company's domain, and mention specific technologies or values.

4. 12

   Where do you see yourself in three years, and how does this role fit into your career goals?

   Highlight growth in backend depth, ownership of services, and mentorship, while aligning with the company's engineering culture.

Coding and problem solving

1. 13

   Given an array of integers, find two numbers that add up to a target sum. Return their indices.

   Explain the brute-force approach first, then optimize to O(n) using a hash map, and discuss edge cases like duplicates.

2. 14

   Write a function to reverse a linked list iteratively and recursively.

   Show pointer manipulation for iterative, recursion base case for recursive, and mention time and space complexity.

3. 15

   Design a rate limiter for an API. How would you implement it?

   Discuss token bucket or sliding window algorithms, in-memory vs. distributed stores like Redis, and handling bursts.

4. 16

   How would you find the first non-repeating character in a string?

   Use a hash map to count frequencies in one pass, then scan again to find the first unique character, noting O(n) time.