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Collaborative document editing allows multiple users to edit the same document simultaneously, with changes from all users appearing in real […]

A distributed task queue decouples work from workers: producers enqueue tasks, workers consume and execute them asynchronously. This enables horizontal

A shopping cart stores a user’s intended purchases before checkout. The design must handle: anonymous users (no account), authenticated users

An ad click tracking system records every click on an advertisement, deduplicates fraudulent clicks, aggregates counts for billing, and provides

A ticketing system (like Ticketmaster for concerts, or an airline seat booking system) manages limited inventory under simultaneous high demand.

A ride-sharing platform (Uber, Lyft) matches riders with nearby drivers in real time, handles surge pricing, tracks vehicle positions, and

A key-value store is the simplest form of a database: store a value associated with a key, retrieve the value

A payments system processes financial transactions — charge customers, transfer funds, process refunds. Unlike typical web applications, payments demand ACID

A recommendation system suggests relevant items to users — products to buy, videos to watch, friends to follow. At Netflix,

A social network feed aggregates posts from accounts a user follows, ranked by relevance or recency. At Instagram or Twitter

A file storage system (like Dropbox or Google Drive) stores user files, provides sync across devices, and handles concurrent edits.

A search autocomplete system provides real-time suggestions as users type, ranking completions by popularity or relevance. At Google or Amazon

A video streaming platform (like Netflix or YouTube) must ingest raw video, encode it into multiple quality levels, distribute it

A URL shortener converts a long URL into a short alias (e.g., bit.ly/abc123) that redirects to the original. Building one

A notification system delivers messages to users through one or more channels — push notifications, email, SMS, in-app. At scale