Airbnb Backend Engineer Interview: Rounds, Questions and Preparation

What the job involves

Backend engineers at global tech companies design, build, and maintain server-side systems that power user-facing features and internal services. They write APIs, manage databases, ensure scalability and reliability, and collaborate with frontend, mobile, and data teams. Day to day involves coding, code reviews, debugging, and improving system performance.

Role and Technical Knowledge

1. 1

   Explain the difference between SQL and NoSQL databases. When would you choose one over the other for a booking system like a travel platform?

   Strong answer covers consistency, scalability, schema flexibility, and gives concrete examples like using SQL for transactions and NoSQL for high-volume user activity logs.

2. 2

   What is a RESTful API? How would you design an endpoint to fetch a list of listings with pagination?

   Mention HTTP methods, status codes, resource naming, and pagination strategies like limit/offset or cursor-based pagination.

3. 3

   How does caching improve backend performance? What are some common caching strategies and eviction policies?

   Discuss cache-aside, write-through, TTL, LRU eviction, and trade-offs between consistency and latency.

4. 4

   Explain the concept of idempotency in APIs. Why is it important for payment or booking operations?

   Define idempotency, give examples like idempotency keys, and explain how it prevents duplicate charges or double bookings.

Practical Scenarios

1. 5

   You need to design a service that sends booking confirmation emails to users. How would you ensure it is reliable and scalable?

   Cover message queues, retries with exponential backoff, dead-letter queues, and decoupling email sending from the main booking flow.

2. 6

   A sudden spike in traffic causes your API to slow down. How would you diagnose and mitigate the issue?

   Mention monitoring tools, database query analysis, caching, horizontal scaling, rate limiting, and load balancing.

3. 7

   You are asked to add a new feature that requires changing the database schema. How would you handle the migration with minimal downtime?

   Discuss backward-compatible changes, dual writes, feature flags, and phased rollouts.

4. 8

   How would you debug a production issue where some users see stale data after an update?

   Consider cache invalidation, read replicas lag, eventual consistency, and logging to trace the data flow.

Behavioural and HR

1. 9

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

   Use STAR method, highlight collaboration, conflict resolution, and your specific contribution.

2. 10

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

   Show initiative, resourcefulness, and ability to apply new knowledge under pressure.

3. 11

   Why do you want to work as a backend engineer at a global tech company?

   Connect personal interest in scalable systems with the company's mission and engineering culture.

4. 12

   How do you handle feedback or criticism on your code?

   Emphasize openness, growth mindset, and using feedback to improve code quality and teamwork.

Problem Solving and Coding

1. 13

   Given a list of bookings with start and end times, find the maximum number of concurrent bookings.

   Use sorting and sweep line algorithm; explain time and space complexity.

2. 14

   Implement a function to detect if a linked list has a cycle. How would you find the starting node of the cycle?

   Use Floyd's cycle detection algorithm; explain the two-pointer technique and its complexity.

3. 15

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

   Discuss token bucket or sliding window, use of Redis, and trade-offs between accuracy and performance.

4. 16

   Write a SQL query to find the top 5 most booked cities in the last month from a bookings table.

   Use GROUP BY, ORDER BY, LIMIT, and date filtering; mention indexing for performance.