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

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.

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.

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.

Design Microsoft Teams' presence system. How do you propagate "online / away / busy" status to millions of users with sub-second latency without overwhelming the backend?

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.

Design the backend for a live cricket score app. 50 million users are watching simultaneously, score updates every ball. Users must see the score update within 2 seconds.

Design Facebook's News Feed ranking system. How do you score posts from thousands of friends and pages? Cover: candidate generation, feature extraction, ML ranking model, how you handle near-real-time freshness, and edge vs server-side ranking for mobile clients.

Design a hotel booking system. Rooms must not be double-booked. Searches should show real-time availability. Handle 10,000 concurrent booking attempts for popular dates.

What is the Saga pattern in microservices? When would you choose choreography over orchestration?

Design YouTube's video upload and processing pipeline. A user uploads a 4K video of 10GB. How does it become available in 360p, 720p, and 4K for streaming?

Design a system that detects fraudulent credit card transactions in real-time (decision in under 50ms). The system processes 10,000 transactions per second.

Design a distributed job scheduler like AWS Batch. Jobs have dependencies (job B starts only after job A completes). The system must handle 100K jobs per day.

Design Stripe's webhook delivery system. Stripe sends millions of webhooks per day to customer endpoints that can be slow, down, or buggy. Cover: at-least-once delivery guarantees, retry with exponential backoff, ordering guarantees (or lack thereof), how to handle a customer endpoint that is down for 24h, and the dead-letter strategy.

Design Google Search autocomplete. As a user types "wea", suggest "weather", "weather today", "weather London". Cover: trie vs inverted index, personalisation, freshness, latency budget (<100ms), and how you handle offensive query suggestions.

How would you implement search-as-you-type autocomplete for 10 million product names with <50ms response time?

You're designing an API for external developers. What versioning strategy would you use and why?

What is the difference between a monorepo and polyrepo? What are the real trade-offs?

Design Flipkart's flash sale system. The system must handle 1M concurrent users trying to buy a limited inventory of 10K items without overselling. Walk through the inventory locking, queueing, and database isolation strategy.

Design Uber's dynamic pricing (surge) system. When supply is low and demand is high in a geofenced zone, prices increase automatically. Explain the data pipeline, the pricing model, how you prevent oscillations (price going up → drivers rush in → price drops → drivers leave), and regulatory constraints.

Design Netflix's adaptive bitrate streaming (ABR) system. How does the client decide which video quality to stream second by second based on available bandwidth? Explain DASH vs HLS, the segment request pipeline, buffer-based rate adaptation algorithms, and CDN architecture.

Design Razorpay's payment retry system. When a payment fails due to a transient error (bank timeout, network blip), how do you retry safely without double-charging the customer? Cover: idempotency keys, exponential backoff with jitter, dead-letter queues, and reconciliation with bank statements.

Design Google Maps ETA calculation. How do you compute the fastest route for 1B+ users in real time? Cover: graph representation of road network, Dijkstra vs A* vs Contraction Hierarchies, live traffic data ingestion, and how you handle road closures.

Design Facebook Live - a system to stream live video from one user to potentially millions of concurrent viewers with low latency. Cover: ingest pipeline, transcoding, CDN edge caching, adaptive bitrate, and how you handle a celebrity going live.

Design Netflix's content delivery pipeline. A new movie is added. Walk through transcoding into 50+ formats, uploading to CDN edge nodes globally, and ensuring playback starts within 200ms for any device anywhere.

Explain rate limiting algorithms: token bucket, leaky bucket, fixed window, sliding window. Which is best for APIs?

Design Stripe's fraud detection system. For every payment, you must decide accept/decline in under 100ms using hundreds of signals. Cover: feature engineering, real-time ML scoring, rule engine, and how you minimise false positives (declined legitimate payments).

What is idempotency? Why is it critical for payment APIs and how do you implement it?

Design LinkedIn's feed. Each user sees posts from their connections, pages they follow, and sponsored content. Posts must be ranked by relevance. Cover: fan-out-on-write vs fan-out-on-read, feed generation for users with 30k connections, and freshness.

Explain what happens when you type 'google.com' in a browser. Cover DNS, TCP, TLS, HTTP in sequence.

Design Instagram Stories. A story disappears after 24 hours and is shown in a ring around profile pictures. Cover: upload pipeline, storage with TTL, efficient delivery to followers, and view count tracking at scale.

Design PhonePe's switch - the internal routing layer that forwards UPI requests to the correct bank. It must be highly available (99.999%), handle 50k TPS, and complete each request within 30 seconds (NPCI timeout). Cover: circuit breakers, retries, and bank-specific timeouts.

What is a circuit breaker pattern? How does it prevent cascade failures in microservices?

Design LinkedIn's "People You May Know" feature. How do you compute second and third-degree connections for 900 million users efficiently? Discuss graph storage (adjacency list vs matrix), batch vs real-time computation, and how to refresh recommendations when someone adds a new connection.

Design WhatsApp's end-to-end encrypted messaging system. How do you exchange keys, store messages server-side without reading them, and handle multi-device sync? Cover the Signal Protocol basics.

Explain the SOLID principles in software design. Give a real code example of violating Single Responsibility Principle.

Design Airbnb's search and availability system. A guest searches for "Paris, 3 nights, 2 guests" and sees ranked listings. Cover: availability calendar storage, real-time inventory updates (when host blocks dates), geospatial search, ranking signals (price, reviews, Superhost status), and how you handle the thundering herd on popular dates.

Your team has been writing code for 6 months with no documentation. How do you introduce documentation without disrupting velocity?

Design Swiggy's real-time order tracking system. A customer should see their delivery agent's location update every 5 seconds on a map. Cover: location ingestion from agents, WebSocket vs SSE for pushing updates to clients, data store choices, and how you scale to 100k concurrent orders during peak hours.

Design Flipkart's product search and ranking system. A user searches "wireless earphones under 2000". How do you retrieve, rank, and filter 10,000+ matching products in under 200ms?

Design Stripe's subscription billing system (Stripe Billing). Customers are charged on recurring intervals (monthly/annual) in different currencies, with proration for plan upgrades. Cover: invoice generation, dunning for failed payments, and SCA (Strong Customer Authentication) compliance.

Design Netflix's A/B testing platform. Engineers run 100+ experiments simultaneously on different features. How do you assign users to buckets consistently, collect metrics, measure statistical significance, and avoid interaction effects?

Design Uber Eats' order batching system. A single delivery partner picks up orders from multiple nearby restaurants and delivers to multiple customers in one trip. How do you optimise the route and assignment?

Design Zomato's delivery time prediction system. Before placing an order, show "Delivers in 35 minutes". The estimate must account for restaurant preparation time, distance, traffic, and number of parallel orders the partner is handling.

Explain gRPC vs REST API. When would you choose gRPC?

Design a library management system. Include entities (Book, Member, Loan), relationships, and key operations: borrow, return, search, fine calculation. Discuss how to handle concurrent borrows of the last copy of a book.

Design Swiggy's delivery partner assignment algorithm. When an order is placed, how do you assign the optimal nearby delivery partner within 2 minutes considering distance, current load, and restaurant prep time?

Design Razorpay's payout system (RazorpayX). Businesses send bulk payouts to thousands of vendor accounts simultaneously. Cover: batching, bank API rate limits, real-time status tracking, and failure handling.

What is event sourcing and how does it differ from CRUD? What are the trade-offs?

Design an autocomplete/typeahead system for a search engine handling 100K queries per second. Discuss the trie-based approach, how you would rank suggestions, and how you would keep the trie updated with trending queries without hurting read latency.

Design Uber's payment reconciliation system. Uber collects payments in 70+ countries with different currencies, payment methods, and regulations. How do you reconcile driver payouts against rider charges and detect discrepancies?

Design Bitbucket's code review system (Pull Requests). Multiple reviewers comment on specific lines of code. The PR is mergeable when all reviewers approve. Cover: diff computation, inline comments anchored to code lines, and what happens when new commits invalidate existing comments.

Explain the difference between horizontal and vertical scaling. Give examples where vertical scaling is NOT the solution.

Design iCloud Photo Library — 2 billion photos synced across iPhone, iPad, and Mac. Cover: deduplication (perceptual hashing), delta sync (only changed chunks), offline-first with conflict resolution, and privacy — how Apple performs server-side processing without seeing unencrypted photos.

Design Flipkart's order management system. From "Place Order" to "Delivered", walk through every state transition, the microservices involved, how you handle partial failures (payment succeeds but inventory deduct fails), and eventual consistency.

Design Apple Pay's tap-to-pay system. When a user taps their iPhone, a payment must complete in under 500ms even with no internet connection. Cover: secure element, tokenisation, NFC protocol, and offline authorisation.

Design Paytm's wallet system that supports deposits, withdrawals, peer-to-peer transfers, and merchant payments. Ensure ACID properties for all transfers, discuss optimistic vs pessimistic locking, and explain how you handle a split-brain scenario where two nodes disagree on a wallet balance.

Design Amazon's recommendation engine (customers who bought X also bought Y). Cover: collaborative filtering vs content-based, item-item similarity at scale, real-time vs batch computation, and how you handle the cold-start problem for new products.

Design Airbnb's trust and safety system. How do you detect fraudulent listings, scam messages, and fake reviews at scale without blocking legitimate hosts and guests?

Design Azure Blob Storage. How do you store, replicate, and serve petabytes of unstructured data with 11 nines of durability? Cover: chunking, erasure coding vs 3-way replication, geo-redundancy, and handling hot blobs.

Design Paytm's cashback system. When a user completes a transaction, they may receive cashback based on complex rules (merchant, time, amount, user segment). The cashback is credited to their wallet. Cover: rule engine, deduplication, and fraud prevention.

Design Oracle Cloud's IAM (Identity and Access Management) system. Thousands of customers, each with their own users, groups, and policies. How do you evaluate a permission check in under 10ms for every API call?

Design Adobe Experience Cloud's customer data platform. Unify customer profiles from website visits, email clicks, and CRM data across devices and sessions. Cover identity resolution (same person on mobile and desktop), real-time segment computation, and privacy (GDPR right-to-erasure).

Design Confluence's page version history. Every edit creates a new version. Users can view any historical version and restore it. Cover: delta vs full-snapshot storage, efficient diff computation, and how you handle concurrent edits creating a branch.

Design Airbnb's review system. Guests and hosts review each other after a stay. Reviews are revealed simultaneously to prevent bias. Cover: the blind reveal mechanism, preventing fake reviews, and abuse detection.

Design PhonePe's merchant settlement system. Thousands of merchants receive daily settlements of their collected payments minus fees. How do you compute net payables, batch into bank transfers, and provide a reconciliation report?

Design Salesforce Flow (visual process automation). Users define business workflows (if-then rules, send emails, create records) in a UI. How do you store, version, execute, and debug millions of active flows in a multi-tenant environment?

Design Adobe Sign (e-signature platform). A contract is sent to 5 signatories who must sign in a specific order. Cover: document integrity (hash verification), audit trail, legal compliance (e-SIGN Act), and what happens if a signatory refuses.

Design a library management system with online book reservation. Students can search, reserve, and borrow books. Librarians can manage inventory. Cover the database schema, REST APIs, and how you handle concurrent reservations of the last copy.

Design Bigtable or a wide-column store from scratch. Explain SSTable, memtable, compaction strategies (size-tiered vs levelled), bloom filters for read optimisation, and how you handle tombstones for deletes.

Design Swiggy's notification system. When an order status changes (Confirmed, Picked Up, Arriving), send push notification, SMS, and in-app notification reliably with at-least-once delivery. Cover fan-out and handling inactive devices.

Design Siri's speech recognition pipeline. Audio is captured on-device, processed, and a response is returned. How do you split on-device vs server processing to balance latency, privacy, and accuracy?

Design LinkedIn's job recommendation engine. Given a user's profile, skills, and activity, rank the most relevant open positions. Cover candidate retrieval, feature engineering (skills overlap, company affinity, recency), and how you A/B test ranking changes at scale.

Design Swiggy's restaurant search and ranking system. How do you index 200k restaurants? Discuss full-text search (Elasticsearch), geospatial filtering, personalised ranking signals (past orders, ratings, delivery time), and how to refresh the index when a restaurant updates its menu.

Design a ride-sharing system like Uber. Focus on matching riders to drivers in real-time.

Design Zomato Pro (subscription). Members get free delivery, discounts, and priority support. Cover: subscription lifecycle, prorated upgrades/downgrades, cohort-based analytics to measure retention, and how you handle payment failures.

Design Salesforce's governor limits enforcement system. Every Apex code execution is limited (SOQL queries, CPU time, heap). How do you track and enforce these limits in real time across millions of concurrent org executions?

Design a URL shortener like bit.ly that handles 100K writes and 10M reads per day. The shortened URLs should never expire.

Design Zomato's surge pricing engine. When demand spikes (rain, IPL final), delivery fees increase automatically. Cover: demand signals, pricing formula, fairness constraints, A/B testing pricing strategies, and how to avoid customer backlash.

Design a collaborative code editor like VS Code Live Share. Multiple developers edit the same file simultaneously with cursors visible to all. Cover: OT vs CRDT, undo/redo in a collaborative context, and low-latency sync across regions.

Design Razorpay's dashboard for merchants. A merchant wants to see real-time revenue, transaction success rates, and refund trends. How do you aggregate millions of transactions into live charts with under 5-second latency?

Design Paytm's KYC (Know Your Customer) pipeline. Users must submit Aadhaar/PAN documents for verification. How do you store documents securely (encrypted at rest), verify them (ML + manual review), and handle status callbacks?

Design a real-time collaborative whiteboard like Miro where multiple users draw simultaneously.

Design Airbnb's host payout system. Hosts in 220+ countries receive payouts in local currency via bank transfer, PayPal, or Payoneer. Cover: currency conversion, scheduled payouts, failed payout handling, and tax compliance.

Design LinkedIn's InMail spam filtering system. Premium users can message strangers. How do you detect and block spam campaigns while allowing legitimate cold outreach? Cover: sender reputation, content analysis, and feedback loops.

Design a distributed task scheduler for 10 million scheduled tasks (delayed, recurring, one-time).

Design the booking system for a home-rental marketplace, ensuring two guests can never double-book the same property for overlapping dates under concurrent requests.

Design a multi-tenant SaaS database architecture like Salesforce's. Thousands of enterprise customers share the same infrastructure but must never see each other's data. Compare silo (one DB per tenant), pool (shared DB, tenant_id column), and bridge models. Discuss query performance, schema customisation, and compliance isolation.

Design the backend for a Google Maps-like routing service with real-time traffic.

Design the "who viewed your profile" feature for a professional network with 500M users. Discuss write amplification, retention policy, and privacy controls for who can see the data.

Design a REST API for an e-commerce cart system. Define the endpoints (add item, remove item, get cart, checkout), explain authentication with JWT, handle race conditions on inventory, and discuss idempotency for the checkout endpoint.

Design a system to send 1 billion push notifications per day.

Design the rider-driver matching system for a ride-hailing app. How would you balance minimizing rider wait time against fairness in driver earnings distribution?

Design Jira's real-time collaborative issue editor (multiple teammates editing the same ticket simultaneously). Cover: Operational Transformation vs CRDTs, conflict resolution, last-write-wins vs merge, and how you persist the final state while keeping latency low for users on slow connections.

Design an API gateway for 100K req/sec: authenticates, rate limits per user, routes to 20 microservices, adds observability.

What is the difference between a message broker and an event streaming platform? Kafka vs RabbitMQ?

Design a distributed counter for 1 million increments per second across 100 servers tracking daily active users.

Design Adobe's document collaboration system (similar to Google Docs for PDFs). Multiple users can annotate and comment on a PDF simultaneously. Cover: conflict-free annotation merging, storing annotation deltas vs full snapshots, real-time sync via WebSockets, and how you export the final annotated PDF.

Your team uses feature flags but they're managed as hardcoded constants in code. This means a code deploy is needed to toggle a feature. How do you improve this?

Explain the Twelve-Factor App methodology. Pick 3 factors and explain how violating them causes production problems.

Design the video encoding pipeline that takes a raw uploaded file and produces multiple bitrate/resolution variants for adaptive streaming. How do you make this fault-tolerant and cost-efficient at scale?

A developer asks: 'Should I use PUT or PATCH for updating a resource?' What's the correct answer?

Design a distributed SQL database that supports ACID transactions across shards. Explain two-phase commit (2PC), its failure modes, and how Google Spanner uses TrueTime to replace 2PC with a more scalable approach.

Design a URL shortener that must handle 100M new URLs per day and 10x that in reads. Walk through your ID generation strategy and why a simple auto-increment counter breaks at this scale.

Your production database has a table missing an index. Adding the index on 1 billion rows. How do you do this safely?

Design a distributed lock service. How does it ensure mutual exclusion across 10 servers?

What is the Strangler Fig pattern for migrating from a monolith to microservices?

Design a leaderboard system for a game with 10 million players updated in real-time.

Design the database schema and queries for a Twitter-like 'trending topics' feature. Topics trend based on tweet volume in the last 1 hour, weighted by user influence.

Explain the differences between REST, GraphQL, and gRPC. Which would you choose for a mobile app backend?

Design a search engine for a 1 million product catalog with faceted filtering (category, price range, brand) and full-text search.

Design the "presence" feature in Teams/Slack — showing whether a user is online, away, or in a meeting — that scales to tens of millions of concurrent users with near real-time updates.

How do you implement database read replicas and what queries should you route where?

Design a video streaming service like Netflix. Focus on: video storage, encoding, CDN delivery.

Design an offline-first note-taking app that syncs across a phone, tablet, and laptop, resolving conflicts when the same note was edited on two devices while offline.

What is the difference between a process and a thread? How does Node.js achieve concurrency with a single thread?

Explain eventual consistency. Design a system where a user updates their profile picture and it shows consistently everywhere.

Design the news feed ranking pipeline for a professional network. How do you balance freshness, relevance, and diversity of content sources without the feed feeling stale or repetitive?

Design a double-entry ledger system for tracking money movement between accounts. Why is this pattern preferred over simply updating a balance column, especially for auditability?

You're building a multi-tenant SaaS application. How do you isolate data between tenants?

Explain the difference between synchronous and asynchronous communication between microservices. When would each fail you?

What is database connection pooling? Why would you NOT want too large a connection pool?

Explain how a CDN handles cache invalidation. What happens if you update a file and CDN serves the old version?

Design the search feature for a workspace tool (like Confluence) that indexes millions of documents across thousands of teams, ensuring users only see search results from spaces they have permission to view.

Design a rate limiter for a public API that must work correctly across multiple application server instances, not just per-instance.

Design the driver-passenger matching system for a ride-hailing app operating across Southeast Asia, where network connectivity is inconsistent in parts of the region. How does that constraint change your design versus a similar system in a market with reliable connectivity?

Your microservices communicate via synchronous REST calls. Service A calls B, B calls C, C calls D. D is slow and times out, causing a cascade failure that takes down A. How do you redesign this to be resilient?

Design a search autocomplete system (like Google's search bar). It must return results in under 50ms, handle 100,000 queries per second, and personalize results based on user history.

Design Google Drive's file sync system. A file changed on desktop must appear on mobile within 5 seconds. Files can be up to 10GB. Handle 100 million users.

Design Uber's surge pricing system. It must calculate real-time multipliers per area, update every minute, handle 1 million location events per second from drivers, and serve the price to 10 million concurrent users.

Design a notification system that can send push, email, and SMS, where a user's preferences determine which channels are used, and a failed channel should fall back to another.

Design a notification system for Flipkart that sends 10 million push notifications during a sale event in 5 minutes. Some notifications are personalized (user-specific deals), some are broadcast.

Design a distributed rate limiter that works across 50 geographically distributed servers without a centralized bottleneck. It must allow 1000 requests per user per minute globally.

Two data centers each think they're the primary for a user's data after a network partition heals — a classic split-brain. How would you design the system to detect and resolve this safely?

Design a dynamic (surge) pricing system that adjusts fares in near real-time based on local supply and demand, without causing prices to flicker wildly for a rider mid-booking.

Your company's monolith handles 10,000 requests/second. The checkout service is the bottleneck. How do you extract it into a microservice without downtime and without breaking existing functionality?

Design a caching layer for a product catalog service that serves 50,000 reads/sec with occasional price updates. Walk through your cache invalidation strategy.

Design WhatsApp's message delivery system. Messages must be delivered exactly once, in order, and the sender must know when the recipient read the message. Handle 50 billion messages per day.

Design a URL shortener like bit.ly. It needs to handle 100 million URLs, 10 billion reads per day, and the short URLs must never collide. Walk through your complete design.

Design the "recently viewed items" feature for a project management tool, capped at the last 20 items per user, updated in real time across tabs.

How would you design a highly scalable and reliable application on GCP?

Design Instagram's newsfeed. 500 million users, each follows up to 2000 accounts. Feed must load in under 200ms. How do you architect this?

If an entire zone becomes unavailable, how would you ensure no data loss?

Which design patterns would you choose while designing an event-driven architecture on GCP?

If data volume increases significantly, how would you modify your service architecture?

Explain the architecture of your Java application on GCP.

What is eventual consistency in distributed systems? How does Amazon S3 handle it?