M2P Fintech Backend Engineer Interview: Rounds, Questions and Preparation

What the job involves

A backend engineer in Indian SaaS and B2B products builds and maintains server-side logic, APIs, and databases that power multi-tenant applications for business customers. Day to day, you write code in languages like Java, Python, or Node.js, design RESTful APIs, optimise database queries, and ensure high availability for clients such as fintechs and enterprises. You collaborate with frontend, DevOps, and product teams to ship features that handle payments, user management, and data processing at scale.

Role and Technical Knowledge

1. 1

   What happens when a user sends a request to a REST API endpoint in a SaaS application?

   Trace the request from client to load balancer, application server, business logic, database, and back. Mention authentication, routing, and response serialisation.

2. 2

   How would you design a database schema for a multi-tenant SaaS product where each tenant has users and transactions?

   Discuss tenant isolation strategies (shared schema with tenant ID vs separate schemas). Cover indexing, foreign keys, and data privacy.

3. 3

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

   Highlight ACID compliance, scalability, and data model. For fintech, SQL is often preferred for transactions, but NoSQL can be used for logs or analytics.

4. 4

   What are idempotent APIs and why are they important in payment or transaction systems?

   Define idempotency and explain how it prevents duplicate operations. Give an example like idempotency keys in payment gateways.

Practical Scenarios

1. 5

   Your API response time suddenly increases from 100ms to 2 seconds. How do you debug and fix it?

   Check logs, database query performance, external service calls, and server resource usage. Suggest profiling, caching, or query optimisation.

2. 6

   You need to add a new feature to an existing API without breaking current clients. How do you handle versioning?

   Discuss URL versioning, header versioning, or backward-compatible changes. Mention deprecation policy and communication with clients.

3. 7

   How would you implement rate limiting for an API used by multiple business customers?

   Explain algorithms like token bucket or sliding window. Mention per-tenant limits, Redis for counters, and returning 429 status codes.

4. 8

   A customer reports that their data is visible to another tenant. What steps do you take?

   Prioritise security incident response: investigate access logs, fix tenant isolation bug, notify stakeholders, and add tests to prevent recurrence.

System Design and Scalability

1. 9

   How would you design a notification service that sends emails and SMS to users of a SaaS product?

   Cover queue-based architecture, third-party providers, retry logic, and tracking delivery status. Mention scaling with workers.

2. 10

   What is horizontal and vertical scaling? Which one would you prefer for a backend service and why?

   Define both and discuss trade-offs. Horizontal scaling with load balancers is common for stateless services in cloud environments.

3. 11

   How do you ensure high availability and fault tolerance in a backend system?

   Mention redundancy, health checks, failover, database replication, and graceful degradation. Give examples like multi-AZ deployment.

4. 12

   Explain how caching can improve performance in a B2B application. What are cache invalidation strategies?

   Discuss Redis or Memcached, cache-aside pattern, TTL, and invalidation on writes. Warn about stale data and consistency.

Behavioural and HR

1. 13

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

   Use STAR method. Highlight collaboration, your specific contribution, and the outcome. Mention tools like Git and code reviews.

2. 14

   Why do you want to work as a backend engineer in a SaaS company like M2P Fintech?

   Show interest in fintech and B2B products. Connect your skills to building scalable systems that solve real business problems.

3. 15

   Describe a situation where you faced a difficult bug. How did you resolve it?

   Explain the debugging process, resources used, and lessons learned. Emphasise persistence and systematic approach.

4. 16

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

   Align with company growth: deepening technical expertise, leading small projects, and contributing to architecture decisions.