AiSensy 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 build and maintain server-side logic, APIs, and databases that power customer-facing features like messaging, analytics, and integrations. You will work with product managers and frontend engineers to ship reliable, scalable services, often handling high-volume data and third-party APIs. Expect to debug production issues, optimise queries, and contribute to system design discussions in a fast-paced, iterative environment.

Role and technical knowledge

1. 1

   What is the difference between a monolithic and a microservices architecture? Which would you choose for a fast-growing SaaS product and why?

   A strong answer explains trade-offs: monoliths are simpler to start, microservices scale independently but add complexity. Relate to a B2B SaaS like a messaging platform where different services (e.g., campaign sender, analytics) may need separate scaling.

2. 2

   How would you design a REST API for a feature that lets customers schedule bulk WhatsApp messages?

   Cover endpoints (e.g., POST /campaigns, GET /campaigns/{id}), request/response formats, status codes, validation, and how to handle scheduling (e.g., using a job queue). Mention idempotency and rate limiting.

3. 3

   Explain how you would implement authentication and authorisation for a B2B SaaS product with multiple organisations.

   Discuss JWT or session-based auth, role-based access control (RBAC), and multi-tenancy (e.g., org_id in tokens). Mention secure password storage and token expiry.

4. 4

   What are database indexes and how do they improve performance? Give an example of when an index might slow things down.

   Explain that indexes speed up reads but slow writes and take space. Example: a write-heavy table with many indexes can degrade insert performance. Mention composite indexes and query patterns.

Practical scenarios

1. 5

   Your API is suddenly responding slowly. How would you diagnose and fix the issue?

   Outline steps: check logs and metrics (e.g., response times, error rates), identify slow queries or external API calls, use profiling, add caching, optimise database queries, or scale horizontally. Mention tools like New Relic or CloudWatch.

2. 6

   You need to integrate a third-party messaging API that has rate limits and occasional downtime. How would you design your system to handle this?

   Propose a queue-based retry mechanism with exponential backoff, circuit breakers, and fallback logic. Discuss idempotency to avoid duplicate messages and monitoring for failures.

3. 7

   How would you ensure data consistency between your primary database and a cache like Redis?

   Explain cache-aside pattern, write-through vs write-behind, and strategies like TTL and invalidation on writes. Mention eventual consistency trade-offs and how to handle stale data.

4. 8

   A customer reports that they are not receiving scheduled messages. How would you debug this from a backend perspective?

   Walk through checking job queue status, database records for the campaign, logs for errors, third-party API responses, and timezone handling. Suggest adding better logging and alerts.

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 STAR method: describe the situation, the technology (e.g., a new framework), your learning strategy (docs, tutorials, small prototypes), and the outcome. Show adaptability and self-learning.

2. 10

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

   Focus on respectful communication, data-driven discussion, and willingness to compromise. Highlight that you prioritised project success over being right.

3. 11

   Why do you want to work in a SaaS or B2B product company like AiSensy rather than a service-based IT firm?

   Show understanding of product-driven development, faster iteration, direct impact on customers, and exposure to scaling challenges. Mention interest in building reusable features and working closely with product teams.

4. 12

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

   Express desire to deepen technical expertise (e.g., system design, cloud), take ownership of critical services, and mentor juniors. Align with the company's growth and learning opportunities.

Coding and problem solving

1. 13

   Write a function to find the first non-repeating character in a string. What is the time and space complexity?

   Use a hash map to count frequencies, then iterate to find first with count 1. Time O(n), space O(n). Discuss edge cases like empty string or all repeating.

2. 14

   Given a list of message events with timestamps, how would you find the busiest hour of the day?

   Parse timestamps, extract hour, count occurrences, and find max. Consider time zones and efficient aggregation. Mention using a hash map or array of size 24.

3. 15

   How would you detect if a linked list has a cycle? Implement it.

   Explain Floyd's cycle-finding algorithm (tortoise and hare). Code two pointers moving at different speeds. Discuss O(n) time and O(1) space.

4. 16

   Design a simple rate limiter for an API endpoint. What data structures would you use?

   Describe token bucket or sliding window with Redis. Use counters and timestamps. Discuss distributed rate limiting and handling bursts.