Trending Interview Questions 2026

Design Instagram feed — a user follows 500 people, each posts 2-3 times daily. How do you generate and serve a personalized, ranked feed at scale?

Design a ride-sharing location tracking system. 10 million drivers update their GPS location every 5 seconds. How do you store and query nearby drivers efficiently?

Your company needs 99.99% uptime for a critical web application. How do you architect a multi-region active-active setup on AWS?

Design a notification system (push/email/SMS) that sends 10 million notifications per day to users across time zones with delivery guarantees.

Design a URL shortening service (like bit.ly) that handles 1 billion shortened URLs and 100K redirects per second.

Design a data pipeline to ingest CDC (Change Data Capture) events from a MySQL database into a Snowflake data warehouse in near real-time.

Design a distributed rate limiter that works across 100 API servers. A user is allowed 1000 requests per minute.

Design a serverless ETL pipeline on AWS that processes files uploaded to S3 and loads them into Redshift.

Design an AI content moderation system for a social platform with 10 million daily posts. How do you balance accuracy, cost, and latency?

Your React app initial load time is 8 seconds. Users complain it is too slow. Walk me through your optimization strategy.

You need to run a DAG that processes 10,000 files daily — one task per file. How do you design this in Airflow without creating 10,000 static tasks?

How would you reduce the latency of a RAG pipeline from 4 seconds to under 1 second in production?

Your RDS MySQL database is becoming a bottleneck. Read queries are slow. How do you scale it on AWS without re-architecting the entire application?

Your PySpark job has data skew on a join key (user_id) where 1% of users account for 60% of the data. How do you handle it?

A React dashboard with 20 charts re-renders every time any filter changes, causing a 3-second lag. How do you optimize it?

Design Uber's driver-rider matching system. How do you match the nearest available driver to a rider within 200ms at global scale? Cover: geospatial indexing (H3/S2), the dispatch algorithm, how you handle supply-demand imbalances, and what happens when a matched driver rejects the trip.

You have a 10TB Snowflake table queried daily by 50 analysts with very different filter patterns. How do you optimize it?

Our RAG pipeline responses are accurate in staging but hallucinate in production on long user queries. Walk me through how you debug and fix this.

Design Netflix's video recommendation system. Netflix serves 250 million subscribers. How do you generate personalised top-10 rows for each user in real time? Cover: collaborative filtering, content-based signals, contextual signals (time of day, device), and how you handle the cold-start problem for new users.

A list of 50,000 items renders in a React component. Users scroll and experience jank. How do you fix it without pagination?

You have a Spark job that ingests 500GB of clickstream data daily. It is failing with OOM errors on the executor. How do you debug and fix it?

Design a distributed rate limiter that can handle 10M requests per second across multiple data centers. Discuss consistency tradeoffs, the choice between token bucket vs leaky bucket, and how you would handle clock skew across regions.

Your Snowflake query is slow even with a large warehouse. How do you investigate and optimize it?

What is the difference between useMemo and useCallback? Give concrete examples of when each actually helps vs. when it hurts.

Your application on EC2 experiences a spike to 10x normal traffic every Friday at 6 PM. How do you architect for this predictable load?

Your fine-tuned LLM performs well on your eval set but users report degraded responses 2 weeks after deployment. What is happening and what do you do?

How do you prioritize a backlog when you have 50 features requested by 10 different stakeholders?

Design PhonePe's UPI transaction system to handle 1 billion transactions per day. Cover: request routing to the correct bank, NPCI integration, handling duplicate transactions, consistency vs availability trade-offs, and how you build a reliable audit trail for regulatory compliance.

Your team discovers that 20% of user_id values in the orders table do not match any record in the users table. How do you handle this as a data engineer?

Predict the output of this code: console.log(1); setTimeout(() => console.log(2), 0); Promise.resolve().then(() => console.log(3)); console.log(4); Explain the event loop mechanism that determines this order.