系统设计面试：2026 必会的 50 道经典题目（附完整思路与标准解答）

总结了一下一些大厂和独角兽常考的题库题，有不少题目是大家经常碰到的。方便大家准备系统设计面试。求赞求米！

🏦 Finance & Payments

• Digital Wallet System Design  
    Design a highly reliable digital wallet system similar to Venmo or PayPal. The system must handle high-concurrency P2P transfers, deposits, and withdrawals while maintaining strict financial consistency. Focus on the core accounting ledger, idempotency mechanisms to prevent double-spending, and how the system remains resilient during external bank gateway failures. Assume a scale of 10M daily active users and 10,000 peak transactions per second.
• Stock Exchange Matching Engine Design  
    Design a high-performance stock exchange system capable of handling 100,000 orders per second with sub-millisecond matching latency. The system must ensure strict price-time priority, deterministic execution, and zero data loss for accepted orders. Detail the architecture for order sequencing, in-memory matching, and high-fanout market data distribution while addressing fault tolerance and recovery mechanisms.
• Scalable Payment Orchestration & Ledger System  
    Design a globally distributed payment system similar to Stripe. The system must support high-volume transaction processing, ensure strict idempotency to prevent double-charging, and maintain an immutable ledger for financial auditing. Focus on the end-to-end lifecycle of a payment, including interaction with external Payment Service Providers (PSPs), handling asynchronous webhooks, and ensuring system-wide consistency and PCI-DSS compliance constraints.
• Scalable Payment Gateway Architecture  
    Design a global-scale payment processing system similar to Stripe. The system must handle millions of transactions per day, ensure absolute idempotency to prevent double-charging, and maintain a high degree of availability. Key challenges include managing PCI compliance, integrating with multiple third-party payment service providers (PSPs), and maintaining a consistent financial ledger. Discuss how you would handle network failures during the critical charge path and your strategy for reconciliation and merchant notifications.

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💬 Messaging & Real-time Communication

• Real-Time Chess Platform Design  
    Design a high-concurrency, real-time chess platform similar to Chess.com. The system must support ELO-based matchmaking, sub-200ms move latency for blitz play, server-side move validation to prevent cheating, and move-reversion (undo) for casual play. Address how you would handle 100,000 concurrent games, player disconnections, and persistent storage of millions of historical matches in PGN format.
• Global Instant Messaging System  
    Design the backend for a global instant messaging application like WhatsApp. The system must support real-time 1:1 and group chat for 500 million daily active users. Key requirements include low-latency message delivery, presence indicators (online/offline status), delivery and read receipts, and support for offline users via push notifications. Discuss how you handle millions of concurrent persistent connections, message ordering, and data persistence for high-volume writes.
• Real-Time Ride-Sharing System Design  
    Design a real-time ride-sharing platform similar to Uber or Lyft. The system must handle high-frequency GPS updates from hundreds of thousands of drivers, perform efficient geospatial matching to connect riders with the nearest available drivers, and manage the transactional lifecycle of a trip. Address the challenges of low-latency location tracking, concurrency control during driver assignment, and scaling the system to handle millions of daily trips while maintaining high availability and strong consistency for bookings.
• Design a Real-time Collaborative Document Editor  
    Design a scalable, real-time collaborative editing system similar to Google Docs. The system must support millions of users, provide sub-100ms latency for keystroke synchronization, and handle concurrent edits gracefully using a robust conflict resolution strategy. Focus on the core mechanics of synchronization, document versioning, and the challenges of maintaining a consistent state across distributed clients while ensuring high availability and durability.
• Design a Scalable Real-time Messaging Platform  
    Design a real-time communication system similar to Discord that supports massive persistent connections (1M+ PCU), instantaneous message delivery across channels, and a scalable presence system. The system must handle 'megaservers' with hundreds of thousands of members and ensure message durability and ordering. Discuss your strategy for WebSocket management, storage selection for high-write chat logs, and the architectural trade-offs involved in presence broadcasting at scale.
• Real-time Dynamic Pricing System  
    Design a real-time surge pricing engine for a global ride-sharing platform. The system must process millions of concurrent location updates from drivers and riders to calculate localized price multipliers based on supply and demand. Key challenges include handling high-throughput ingestion, ensuring sub-second pricing retrieval, managing geographic data at scale, and maintaining system reliability during massive demand spikes.
• Real-Time End-to-End Encrypted Messaging System  
    Design the backend for a global-scale messaging application similar to WhatsApp. The system must support real-time 1:1 and group text messaging with strict end-to-end encryption (E2EE). Key constraints include supporting 100M daily active users, providing reliable delivery status (sent/delivered/read), and managing user presence (online/offline). Discuss how you would handle asynchronous key exchanges for E2EE, connection management for millions of mobile clients, and the storage strategy for messages that cannot be delivered immediately. Ensure the design addresses network volatility and minimizes latency while maintaining zero visibility into message content for the service provider.
• Scalable Notification System Design  
    Design a global-scale notification system capable of handling 1 billion notifications per day across Push, SMS, and Email. The system must support high-priority alerts (e.g., OTPs) with sub-second latency and bulk marketing messages. Your design should address 3rd-party provider reliability, user preference management, rate limiting to prevent spam, and idempotency guarantees. Discuss how you would handle massive traffic spikes and ensure at-least-once delivery for every notification requested.
• Scalable Multi-Channel Notification System  
    Design a highly available and scalable notification system capable of delivering millions of messages across Push (iOS/Android), SMS, and Email. The system must handle varying priorities (e.g., OTP vs Marketing), ensure at-least-once delivery, manage user preferences, and gracefully handle third-party provider failures or rate limits. Detail the architectural choices for decoupling, message persistence, and handling high-concurrency peak loads.
• Scalable Real-time Chat System Design  
    Design a real-time messaging platform similar to Discord that supports millions of concurrent users. The system must handle persistent message history, real-time delivery with low latency, and a presence system (online/offline status). Specifically address how the architecture handles massive group servers with over 100,000 concurrent members, focusing on storage partitioning, real-time fan-out challenges, and maintaining high availability during peak traffic spikes.
• Real-time Trending Video Ranking System  
    Design a highly scalable system for YouTube to identify and display the top 10 most popular videos globally based on view count within a rolling 24-hour window. The system must handle over 100,000 write events per second with minimal lag and serve the leaderboard to millions of concurrent users with sub-50ms latency. Discuss how you handle high-cardinality data, viral 'hot' keys, and ensure the system remains available even during massive traffic spikes.
• Real-Time Online Chess Platform  
    Design a high-scale online chess platform capable of supporting 10 million daily active users. The system must handle real-time multiplayer gameplay with low latency (<200ms), manage distributed matchmaking based on player skill (Elo), and maintain a server-authoritative game state to prevent cheating. Consider how to handle clock synchronization, game persistence for millions of matches daily, and a global leaderboard that updates accurately as games conclude.

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📹 Video & Streaming

• Scalable Video Feed & Recommendation System  
    Design a high-scale short-form video feed system similar to TikTok's 'For You' page. The system must support 500 million daily active users, provide personalized content with sub-200ms latency, and handle massive ingestion of user interaction signals. Explain how you would architect the multi-stage recommendation pipeline (Retrieval and Ranking), manage global video distribution, and ensure seamless playback across varying network conditions while maintaining high availability.
• Global Video Streaming & Sharing System  
    Design a high-scale video sharing platform similar to YouTube. The system must support reliable video uploads from a global user base, asynchronous processing/transcoding of videos into multiple formats, and low-latency global playback for billions of users. Address how you would handle massive storage requirements, bandwidth costs, and the consistency of video metadata and view counts under high concurrency.
• Scalable Video Sharing and Streaming Platform  
    Design a global video platform similar to YouTube. The system must support high-volume video uploads (500 hrs/min), asynchronous transcoding into multiple resolutions, and low-latency global streaming for billions of users. Focus on the end-to-end lifecycle of a video from upload to playback, detailing the storage strategy for petabyte-scale data, the transcoding pipeline orchestration, and the content delivery architecture. Address challenges related to massive egress bandwidth, metadata scalability, and adaptive bitrate streaming.
• Ultra-Scale Metadata Storage for Video Streaming  
    Design a highly-available metadata storage system for a global video streaming platform with 100M+ DAUs. The system must handle massive write loads for watch progress tracking (1M+ events/sec) and provide low-latency access to the content catalog. Specifically, address how you would scale a relational database architecture (Postgres-based) to meet these requirements while handling connection management, data partitioning, and write-heavy bottlenecks. Discuss the trade-offs between consistency and performance for playback features.
• Scalable Video Sharing Platform  
    Design a high-scale video sharing platform similar to YouTube. The system must support millions of daily uploads, billions of daily views, and provide a seamless, low-latency playback experience globally. Key requirements include handling massive video files, asynchronous video processing for various devices/resolutions, and managing highly concurrent metadata reads and view count updates. Discuss the trade-offs between storage costs, processing latency, and delivery performance.

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🌐 Social Networks & Feeds

• Scalable Social Media Feed Design  
    Design a high-scale social media platform similar to Instagram that allows users to upload photos, follow others, and view a real-time feed. The system must support 500 million daily active users and handle the 'celebrity problem' where certain accounts have millions of followers. Focus on the end-to-end architecture for media storage, metadata management, and the feed generation strategy (push vs. pull). Detail how you would ensure sub-second latency for feed retrieval and high availability for uploads.
• Scalable Social Media Photo Sharing Service  
    Design a mobile-first social photo-sharing application. The system must support photo uploads, a social graph (following/followers), and a personalized home feed showing recent content from followed users. Key challenges include handling high read-to-write ratios, managing the 'fan-out' of content to millions of followers, and ensuring low latency for global content delivery. Discuss trade-offs between push and pull models for feed generation and how to handle celebrity users with massive follower counts.

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🔗 Infrastructure & Storage

• Distributed Cron Job Scheduler  
    Design a highly available and scalable distributed system to schedule and execute millions of recurring jobs (cron tasks). The system must support high-precision triggering (within seconds of the scheduled time), handle massive execution spikes at specific time boundaries (e.g., top of the hour), and ensure that jobs are triggered even in the event of partial system failure. Detail your strategy for job metadata storage, the mechanism for polling or triggering due jobs, and how you ensure tasks are distributed to workers efficiently while maintaining at-least-once execution guarantees.
• Distributed Rate Limiter Design  
    Design a distributed rate limiting system capable of handling 50,000 peak QPS. The system must support various algorithms (e.g., Sliding Window), provide sub-5ms latency, and ensure high availability across multiple application nodes. Discuss how you would handle race conditions, rule management, and system failures without significantly impacting the user experience.
• Scalable Distributed Rate Limiter  
    Design a distributed rate limiting system capable of handling 1 million requests per second. The system must support various limiting strategies (e.g., sliding window, token bucket) and allow for per-user or per-API-key quotas. Key constraints include sub-5ms latency overhead, high availability with fail-open semantics, and the ability to scale horizontally as traffic grows. Explain your choice of storage, consistency models, and how you would handle 'hot keys' for extremely popular users.
• Design a Distributed High-Throughput Message Queue  
    Design a distributed message queue system capable of handling 1 million messages per second. The system should support multiple producers and consumers, provide partition-level ordering, and guarantee at-least-once delivery. Discuss how you would handle persistence, horizontal scaling, and broker failures while maintaining low latency for real-time applications.
• Distributed High-Throughput Message Queue Design  
    Design a distributed message queue system capable of handling over 1 million writes per second with high durability and partition-level ordering. The system must support multiple independent consumer groups, allow for message retention up to 7 days, and maintain high availability even during broker failures. Discuss how you would optimize for sequential I/O, manage consumer offsets, and handle data replication across a cluster while ensuring minimal latency.
• Scalable Distributed Key-Value Store  
    Design a globally distributed, sharded data store capable of handling hundreds of terabytes of data and millions of concurrent requests. The system must support tunable consistency, automatic rebalancing as nodes are added or removed, and maintain high availability even during regional network partitions. Detail your strategy for data partitioning, conflict resolution, and intra-cluster coordination.

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🔍 Search & Discovery

• Design a Scalable Typeahead Suggestion System  
    Design a search autocomplete system (Typeahead) similar to Google or Amazon. The system should support 500 million daily searches with a p99 latency under 100ms. Discuss how you would store and query prefix data efficiently, how the system handles the massive write volume of search logs, and how you would ensure the suggestions remain relevant as search trends change. Focus on data structure selection, caching strategies, and the end-to-end data pipeline.
• Scalable Autocomplete System for Web Search  
    Design a high-scale autocomplete/typeahead system similar to Google Search. The system must provide real-time suggestions (top-k) based on user prefixes with a latency under 100ms. Consider a scale of 500 million DAU and 5 billion daily queries. Discuss the trade-offs between live Trie traversal versus pre-computed storage, how to handle massive write-heavy analytics for frequency updates, and strategies for global low-latency delivery.
• Design a High-Performance Search Service  
    Design a scalable search service for an e-commerce platform with 100 million items. The system must support full-text search, faceted filtering (e.g., by category or price), and near real-time updates. The design should handle 10,000 queries per second with sub-200ms latency, while ensuring high availability and fault tolerance. Discuss how you would handle data ingestion from a primary database, indexing strategies, and query optimization.

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🔗 URL & Crawling

• Scalable Web Crawler Design  
    Design a distributed web crawler capable of processing 1 billion pages per month. The system should efficiently discover new URLs, handle duplicate content detection, respect robots.txt and politeness constraints, and store both raw HTML and metadata. Address challenges such as DNS resolution bottlenecks, URL frontier management, and fault tolerance at scale.
• Scalable URL Shortening Service Design  
    Design a high-scale URL shortening service similar to Bitly or TinyURL. The system must support at least 100 million new URLs per day and handle 10 billion redirections daily with extremely low latency. You need to address unique ID generation without collisions, efficient storage of hundreds of billions of records, and a caching strategy for viral links. Consider how the system would handle custom short-links and automatic link expiration, while ensuring high availability even during regional failures.
• Global URL Shortener Design  
    Design a globally distributed URL shortening service similar to Bitly. The system must support 10 billion persistent mappings, handle 1 million read requests per second with sub-20ms latency, and 10,000 write requests per second. Focus specifically on a collision-free ID generation strategy that works across multiple data centers and how to maintain high availability and performance during regional outages.
• Scale-Out URL Shortener  
    Design a globally distributed URL shortening service capable of handling 1 million redirection requests per second and 10,000 shortening requests per second. The system must ensure unique short-code generation without collisions, provide sub-50ms latency for redirects, and scale to store over 1 trillion mappings over 5 years. Address how you would handle hot keys, distributed ID generation, and data consistency across multiple geographic regions.

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🚗 Ride-Sharing & Location

• Design a Nearby Friends Feature  
    Design a real-time system for a social media application that allows users to see their friends on a map when they are within a certain proximity (e.g., 5 miles). The system must handle 10 million daily active users, support frequent location updates, and maintain strict privacy boundaries ensuring only mutual friends can see each other. Focus on how you would manage high-write throughput of GPS data and the spatial querying logic required to match friends efficiently at scale.
• Global Ride-Sharing Platform Design  
    Design a high-scale, global ride-sharing system similar to Uber or Lyft. The system must support real-time driver tracking, efficient rider-to-driver matching using geospatial indexing, and dynamic surge pricing. Key challenges include handling over 1 million concurrent drivers sending location updates every few seconds, managing complex trip lifecycles with strong consistency, and ensuring system resilience across multiple geographic regions. Discuss your choice of spatial indexing (e.g., Quadtree, Geohash, S2), how you handle peak traffic 'hotspots' in dense urban areas, and how you ensure idempotent payment processing and reliable trip state transitions.

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🏨 Booking & Reservations

• Scalable Hotel Reservation System  
    Design a high-scale hotel reservation and search system similar to Booking.com. The system must support millions of hotels and room types while providing sub-second search latency and guaranteeing zero double-bookings. Address how you would handle massive read traffic for hotel searches, maintain strict inventory consistency during peak booking periods, and ensure the system remains resilient if external payment providers fail.
• Cinema Ticket Booking System Design  
    Design a high-scale cinema ticket booking platform capable of handling millions of users. The system must support movie discovery, real-time seat availability, and a secure booking process. A critical constraint is preventing double-booking during high-concurrency events (e.g., blockbuster releases). Address how you would handle temporary seat holds, payment consistency, and system reliability under bursty traffic patterns.
• Hotel Reservation System  
    Design a global hotel reservation system similar to Booking.com or Expedia. The system must support hotel discovery via search (location, dates, room types) and a high-integrity booking flow. A critical requirement is to ensure no two customers can book the same room for overlapping dates (zero overbooking). Consider how the system handles 10 million daily active users, global distribution, and high-concurrency scenarios during peak holiday seasons. Discuss your choice of consistency models, database selection, and how you maintain search performance while ensuring booking accuracy.
• Hotel Reservation System  
    Design a high-scale hotel reservation system similar to Booking.com or Expedia. The system must support global hotel search with real-time availability filters, handle high-concurrency booking transactions without overbooking, and manage a massive inventory of room-nights across different time zones. Focus on the data consistency model for reservations, the search indexing strategy, and how to handle the high read-to-write ratio typical of travel platforms. Provide estimations for a system supporting 20 million daily active users and discuss the trade-offs between availability and consistency in the booking flow.
• Hotel Reservation System Design  
    Design a global hotel reservation system like Booking.com or Expedia. The system should support hotel discovery (search by location/date), real-time availability management, and a secure booking process. Key challenges include handling high-concurrency bookings for limited inventory without double-booking, ensuring low-latency search across millions of records, and maintaining data consistency across the reservation and payment lifecycles. Discuss how you would handle 10 million daily users and the trade-offs between consistency and availability in a distributed environment.

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🤖 AI & ML Infrastructure

• Scalable AI Agent Orchestration System  
    Design an enterprise-grade agentic AI platform capable of performing multi-step tasks using external tools and long-term memory. The system must support asynchronous task execution, human-in-the-loop approvals for sensitive actions, and provide a reliable way to recover state if components fail. Consider scalability for thousands of concurrent agents and security constraints regarding code execution and data privacy.
• Mortgage Agent Lead and Document Management System  
    Design a secure, highly-available mortgage agent platform that facilitates lead management, document collection, and integration with third-party financial services. The system must handle high-volume document uploads (PDFs/Images), ensure strict PII data protection (GLBA compliance), and manage asynchronous workflows like credit report fetching and loan status updates. Focus on document lifecycle management, external API integration reliability, and a robust security architecture for financial data.
• Secure Quantitative Backtesting Platform  
    Design an internal system for quantitative researchers to upload and execute trading strategy code against multi-terabyte historical market datasets. The system must support parallel execution, ensure strict resource isolation and security for untrusted code, and provide reproducible performance reports. Detail the data access strategy for high-throughput historical data, the sandboxing mechanism, and the job orchestration architecture to handle long-running compute tasks.
• Scalable Ads Click Aggregation System  
    Design a high-throughput system to aggregate ad clicks for real-time advertiser dashboards. The system must handle a peak of 1 million clicks per second, provide sub-minute latency for reports, and ensure exactly-once processing for accurate billing. Address challenges such as late-arriving data, hot-key distributions for viral ads, and high-availability storage for time-series aggregates.
• High-Performance Ad Serving & Tracking System  
    Design a scalable ad serving platform capable of handling 10,000 requests per second with sub-100ms latency. The system must support advertiser campaign management, targeted ad retrieval based on keywords and location, real-time budget enforcement to prevent significant overspending, and a robust tracking mechanism for billions of impressions and clicks. Discuss how you would handle the trade-offs between low-latency ad delivery and eventual consistency in billing/budgeting, especially under peak load conditions.
• Scalable Cloud File Synchronization System  
    Design a globally distributed cloud storage and file synchronization service similar to Dropbox. The system must support millions of users, handle file versioning, and efficiently synchronize changes across multiple devices (Desktop, Mobile, Web). Key challenges include optimizing bandwidth for large file updates, ensuring strong consistency for metadata, and handling high-concurrency synchronization notifications. Discuss your strategy for block-level deduplication, data durability, and conflict resolution in a distributed environment.
• Online Chess Platform Design  
    Design a highly scalable online chess platform supporting millions of daily active users. The system must handle real-time move synchronization with sub-100ms latency, maintain authoritative game clocks, and support a fair matchmaking system based on player skill (Elo/Glicko). Detail how you would handle high concurrency during peak hours, ensure game state persistence in the event of server failures, and process post-game rating updates and historical archiving without impacting the live gameplay experience.

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