Loom India Backend Engineer Interview: Rounds, Questions and Preparation

What the job involves

Backend engineers in Indian SaaS and B2B product companies build and maintain the server-side systems that power multi-tenant web applications, APIs, and data pipelines. Day to day, they write code in languages like Java, Python, or Node.js, design database schemas, optimise queries, and ensure that features like user authentication, billing, and analytics work reliably for enterprise customers. They also collaborate with frontend, product, and DevOps teams to ship features that scale across thousands of organisations.

Role and Technical Knowledge

1. 1

   What happens when a user logs into a multi-tenant SaaS application like a video messaging platform?

   Cover authentication flow (e.g., JWT or session tokens), tenant identification, database lookup, and how you would keep tenant data isolated.

2. 2

   How would you design a REST API for a feature that lets users upload and share videos?

   Discuss endpoints, HTTP methods, status codes, pagination, and how to handle large file uploads with chunking or pre-signed URLs.

3. 3

   Explain the difference between SQL and NoSQL databases. When would you choose one over the other for a B2B product?

   Mention ACID properties, schema flexibility, scaling patterns, and give examples like PostgreSQL for transactional data and MongoDB for unstructured logs.

4. 4

   What is caching and how would you use it to improve performance in a SaaS backend?

   Talk about Redis or Memcached, cache invalidation strategies, and where to place caches (e.g., database query results, session data).

Practical Scenarios

1. 5

   A customer reports that their dashboard is loading slowly. How would you debug and fix this?

   Walk through checking logs, profiling database queries, looking for N+1 problems, and adding indexes or caching.

2. 6

   You need to add a new field to a database table that is used by thousands of tenants. How do you roll out the change without downtime?

   Describe backward-compatible migrations, adding the column as nullable, deploying code that writes to it, then backfilling and enforcing constraints.

3. 7

   How would you handle a sudden spike in API traffic during a product launch?

   Mention rate limiting, auto-scaling, load balancers, queueing, and monitoring to avoid cascading failures.

4. 8

   You are asked to integrate a third-party payment gateway for subscription billing. What are the key considerations?

   Cover idempotency, webhook handling, secure storage of API keys, retry logic, and reconciliation of failed payments.

Behavioural and HR

1. 9

   Tell me about a time you worked in a team to build a software project. What was your role?

   Use the STAR method, highlight your specific contribution, and mention how you handled disagreements or deadlines.

2. 10

   Why do you want to work as a backend engineer in a SaaS company like Loom India?

   Show understanding of the company's product, the scale of B2B SaaS, and how your skills align with building reliable backend systems.

3. 11

   Describe a situation where you had to learn a new technology quickly to solve a problem.

   Focus on your learning process, resources used, and the outcome. Emphasise adaptability and self-driven learning.

4. 12

   Where do you see yourself in three years as a backend engineer?

   Express interest in deepening technical expertise, taking ownership of services, and contributing to system design and mentoring.

Problem Solving and Coding

1. 13

   Given a list of API request logs, write a function to find the top 3 most frequently called endpoints.

   Use a hash map to count frequencies, then a heap or sorting to extract top 3. Discuss time and space complexity.

2. 14

   How would you detect a cycle in a dependency graph of microservices?

   Explain depth-first search with recursion stack or topological sorting. Mention real-world impact like deployment deadlocks.

3. 15

   Implement a rate limiter that allows at most N requests per user per minute.

   Cover sliding window or token bucket algorithms, in-memory vs distributed stores like Redis, and thread safety.

4. 16

   Write a SQL query to find the second highest paying customer from a transactions table.

   Use a subquery with MAX or window functions like DENSE_RANK. Explain how you handle ties and null values.