Perfios Backend Engineer Interview: Rounds, Questions and Preparation

What the job involves

A backend engineer at an Indian SaaS or B2B product company builds and maintains server-side logic, APIs, and data pipelines that power financial or business workflows. Day to day you write code in Java or Python, design REST APIs, work with SQL and NoSQL databases, and ensure systems handle high transaction volumes reliably. You collaborate with product, frontend, and DevOps teams to ship features that clients like banks and NBFCs depend on.

Role and technical knowledge

1. 1

   What happens when a client sends a request to a REST API in a B2B SaaS product, and how would you structure the response?

   Walk through routing, authentication, business logic, database call, and JSON response with proper HTTP status codes. Mention idempotency for payment or data-fetch APIs.

2. 2

   How would you design a database schema for storing loan application data that multiple financial institutions can query?

   Cover tables for applications, applicants, documents, and status history. Discuss indexing on institution ID and application date, and why you avoid storing everything in one wide table.

3. 3

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

   Give clear use cases: SQL for transactional, relational data like invoices; NoSQL for logs or unstructured documents. Mention consistency and scaling trade-offs.

4. 4

   What is an API gateway and why do B2B products use one?

   Explain routing, rate limiting, authentication, and logging. Relate it to multi-tenant SaaS where different clients need different access rules.

Practical scenarios

1. 5

   A client reports that an API is slow during peak hours. How do you debug and fix it?

   Start with logs and metrics, check database query plans, add caching or indexes, and consider read replicas. Mention load testing before and after the fix.

2. 6

   You need to add a new field to an existing API response without breaking old clients. How do you handle it?

   Use versioning or additive changes only. Discuss backward compatibility, documentation, and gradual rollout with feature flags.

3. 7

   How would you handle a failed payment callback in a system that processes loan repayments?

   Cover retry logic with exponential backoff, idempotency keys, dead letter queues, and alerting. Mention reconciliation jobs to fix mismatches.

4. 8

   Your service needs to send daily reports to 500 business clients. How do you design this without blocking the main application?

   Use a message queue or scheduled job, generate reports asynchronously, store them in object storage, and email links. Mention failure handling and retries.

Behavioural and project questions

1. 9

   Tell me about a project where you worked in a team. What was your specific contribution?

   Use STAR format. Focus on your code, how you coordinated with others, and what you learned about backend work.

2. 10

   Describe a time you faced a bug you could not solve for hours. What did you do?

   Show persistence, systematic debugging, and willingness to ask for help. Mention what you learned from the root cause.

3. 11

   Why do you want to work in a B2B SaaS company rather than a consumer app?

   Talk about solving complex business problems, working with enterprise clients, and learning about scalability and reliability.

4. 12

   How do you keep your backend skills updated? Give an example of something you learned recently.

   Mention a specific topic like Docker, a database feature, or a design pattern. Show curiosity and self-learning.

HR and company fit

1. 13

   What do you know about our company and the kind of products we build?

   Research the company's domain, such as financial data aggregation or lending SaaS. Mention clients, scale, and why the work interests you.

2. 14

   Are you comfortable working with legacy code and gradually improving it?

   Say yes and give an example of refactoring or fixing old code. Show patience and respect for existing systems.

3. 15

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

   Focus on deepening technical skills, owning modules, and contributing to design discussions. Avoid vague answers about management.

4. 16

   Do you have any questions for us?

   Ask about tech stack, code review process, or how the team handles production incidents. Shows genuine interest in the role.