Infobip India Backend Engineer Interview: Rounds, Questions and Preparation

What the job involves

As a Backend Engineer at an Indian SaaS or B2B product company, you will design, build, and maintain server-side logic, APIs, and databases that power multi-tenant applications used by businesses. Day to day, you will write scalable code, optimise queries, handle integrations with third-party services, and ensure high availability for enterprise customers. You will also collaborate with frontend, DevOps, and product teams to ship features that meet strict security and compliance needs.

Role and Technical Knowledge

1. 1

   What happens when a client sends a request to a REST API endpoint in a SaaS product?

   Cover DNS resolution, load balancer, API gateway, authentication, routing, business logic, database query, and response serialisation. Mention logging and error handling.

2. 2

   How would you design a database schema for a multi-tenant B2B application where each customer has users and projects?

   Discuss tenant isolation strategies like shared schema with tenant_id, separate schemas, or separate databases. Explain trade-offs in cost, security, and maintenance.

3. 3

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

   Compare consistency, scalability, schema flexibility, and query patterns. Give examples like PostgreSQL for transactional data and MongoDB for flexible event logs.

4. 4

   What are idempotent APIs and why are they important in B2B integrations?

   Define idempotency and explain how it prevents duplicate operations during retries. Mention idempotency keys, HTTP methods, and real-world payment or order APIs.

Practical Scenarios

1. 5

   An API endpoint is timing out under load. How would you debug and fix it?

   Start with monitoring and logs, check database slow queries, add indexes, introduce caching, or scale horizontally. Discuss load testing and profiling.

2. 6

   You need to integrate a third-party payment gateway into your B2B SaaS product. What backend steps would you take?

   Cover API authentication, webhook handling for callbacks, idempotency, retry logic, secure storage of keys, and reconciliation of transactions.

3. 7

   How would you handle a situation where a bug in production affects only one large enterprise customer?

   Prioritise customer impact, reproduce the issue with their data, apply a hotfix or feature flag, communicate transparently, and add regression tests.

4. 8

   Describe how you would implement rate limiting for your public APIs.

   Explain algorithms like token bucket or sliding window, where to enforce limits (API gateway or application), and how to return 429 status with headers.

Behavioural and HR

1. 9

   Tell me about a time you had to learn a new technology quickly for a project.

   Use the STAR method. Highlight the learning approach, resources used, and how you applied it to deliver results under a deadline.

2. 10

   Why do you want to work as a backend engineer in a SaaS or B2B product company?

   Show interest in building scalable systems that solve business problems. Mention exposure to multi-tenancy, integrations, and enterprise customers.

3. 11

   How do you handle feedback or criticism on your code during a review?

   Demonstrate openness, focus on learning, and willingness to improve. Give an example of incorporating review comments to enhance code quality.

4. 12

   Where do you see yourself in three years in this role?

   Align your growth with deeper technical expertise, ownership of services, and possibly mentoring juniors. Show commitment to the company's domain.

Coding and Problem Solving

1. 13

   Given a list of API request logs, find the top 3 endpoints with the highest average response time.

   Explain data structures like hash maps and heaps. Discuss time complexity and how to handle large input streams.

2. 14

   Write a function to validate if a string is a valid JSON.

   Cover parsing logic, edge cases like nested objects, arrays, and escaped characters. Mention using a stack or recursive descent.

3. 15

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

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

4. 16

   Implement a rate limiter using a token bucket algorithm.

   Explain the token bucket concept, refill rate, capacity, and how to store state. Code the logic with timestamps and thread safety if needed.