LinkedIn System Design Questions

Design a Top-K Ranking Service

Design a system that continuously computes the top K items from a very large event stream: for example the K most-viewed URLs, the K services with the most error logs in the last 5 minutes, the top 100 most active users in a…

Also reported as: Design a Top-K search words service · Design an exception monitoring system with top‑K

Design a Multi-Region Malicious-IP Detection System

Design a service that decides whether a request's source IP (or a URL shared on the platform) is malicious and returns a low-latency decision: allow, challenge, rate-limit or block. Signals come from application and network…

Also reported as: Design a malicious-URL checking service using an isMalicious API

Scale a NoSQL Key–Value Store to One Million QPS

Design a horizontally scalable distributed key-value store like DynamoDB or Cassandra, or scale a single-node store to a stated workload (e.g., 50M keys, 1M requests/sec, 50/50 reads and writes). Support get/put/delete with…

Also reported as: Design a scalable key-value store

Design a CI Job Scheduler with Live Build Output

Design a CI/CD platform like GitHub Actions or Jenkins. Source changes trigger pipelines of dependency-linked tasks (build, test, deploy) that run on a pool of workers with different capabilities. Users see live status and…

Also reported as: Design a CI/CD Task Scheduler

Design a Calendar System

Design a calendar service like Google Calendar. Users create, update and delete events (one-time and recurring), invite attendees who can accept or decline, view day/week/month/agenda views across time zones, share calendars, get…

Also reported as: Design a Global Calendar Service

Design a metrics platform without alerting

Design an internal metrics monitoring platform like Datadog or Prometheus. Agents on hosts and services emit metrics (counters, gauges, histograms with tags); the system ingests millions of data points per second, stores them as…

Also reported as: Design a company-wide monitoring system

Reduce Stale Listings in a Job Marketplace

Design a system that detects and resolves stale job postings in a large job marketplace. A listing may be stale because it was filled, closed on the employer's own site, abandoned, duplicated, or expired. Discuss the signals…

Collect and Query Recent User Activity

Design a system that collects user activity events and answers queries over recent periods, such as the number of actions a user performed in the last minute, hour or day, or activity within a time range and geographic area.…

Design a Ranked Social Feed with Hybrid Fan-Out

Design the home feed of a social network like Instagram or Twitter. Users create posts (text, photos, video), follow other users, and open a personalized feed of recent posts from people they follow, possibly mixed with global or…

Review a Web Application Architecture

You are given an existing architecture or design document (for example Client -> DNS -> Load Balancer -> Application Service -> Database, plus a call to a third-party API) that contains omissions and unsafe assumptions. Review…

Scale a Distributed Randomized Multiset

Having designed a single-machine randomized multiset that supports insert(val), remove(val) and getRandom() (returning each element with probability proportional to its count) in O(1), scale it across many servers while keeping…

Design an in-memory key-value store using maps

Design a low-level, embeddable key-value store library using only in-memory map-like data structures. Support put, get and delete; versioned reads (get the value of a key as of version or timestamp T); snapshots and rollback to…

Design scalable job scheduler and query dashboard

Design a distributed job scheduler that runs user-defined jobs at a specified time (one-time) or on a recurring cron schedule, at a scale of millions of jobs per day. Requirements: jobs fire close to their scheduled time, each…