61. Design a Collaborative Editor like Google Docs
A hundred people typing in one document, every copy converging, nothing acknowledged lost: one owner per document, an op log with snapshots, and fenced failover.
ClassicMedium
Designs
Pick a system. Work through the problem. Compare your approach.
Classic designs 115
Ordered by design numberCompany tags are community-reported. Counts on cards show how many people reported that design.
62. Design a Flash Sale System
Sell 10,000 units to a million people at noon with a lottery waiting room, a Redis gate before an Aurora ledger and holds that expire.
ClassicMedium63. Design Real-Time Updates for Millions of Clients
Every goal on 25 million open apps within two seconds: SSE or WebSockets, pub/sub to the right server, resume by seq, hot topics, reconnect storms and push.
ClassicMedium64. Design for Contention: Thousands Racing for the Same Item
Design for contention through a limited sneaker drop: row locks, conditional writes, expiring holds, fencing tokens, sagas and one writer per item.
ClassicMedium65. Design Multi-Step Processes: Sagas, Orchestration and Durable Workflows
An order checkout across payment, inventory and shipping, run as a saga.
ClassicMedium66. Design for Read Scale: A Product Page Read a Million Times a Second
Serve a product page a million times a second: a cheap query, replicas, Redis and the edge, with hot keys, stampedes and invalidation handled.
ClassicMedium67. Design for Write Scale: A Million Writes a Second
A live TV vote at a million writes a second: spread by key, buffer in Kafka, aggregate the hot counter in two stages, shed what can wait.
ClassicMedium68. Design Large File Uploads and Downloads
Files up to 50 GB up and down without a byte through the API: presigned URLs, resumable multipart, S3 events, scanning and CloudFront.
ClassicMedium69. Design Long-Running Tasks: Async Jobs with Progress and Results
Accept in milliseconds, work in the background: leases and heartbeats, retries and dead letters, progress by SSE and webhooks, fairness across tenants.
ClassicMedium70. Design an In-Memory Data Store like Redis
An in-memory data store like Redis, from one event loop to Redis Cluster to a durable platform.
ClassicMedium71. Design a Search Engine like Elasticsearch
Search a billion documents in 200 ms: inverted indexes, analysers, Lucene segments, shards and replicas, query then fetch with BM25, fed by CDC.
ClassicMedium72. Design a Distributed Log like Kafka
A partitioned, replicated append-only log: a million messages a second, order per key, nothing acknowledged ever lost, a week of replayable history.
ClassicMedium73. Design a Durable Workflow Engine like Temporal
Workflows as code that survive any crash: event histories and replay, activities retried under timeouts, durable timers, sharded history.
ClassicMedium74. Design a Wide-Column Store like Cassandra
A leaderless wide-column store: a token ring with virtual nodes, consistency tuned per query, an LSM write path and repair that keeps replicas converged.
ClassicMedium75. Design a Key-Value Store like DynamoDB
A key-value store like DynamoDB, from one durable node to Paxos-replicated partitions to global tables run for thousands of tenants.
ClassicMedium76. Design a Relational Database like PostgreSQL
A relational database like PostgreSQL: WAL and MVCC on one server, quorum replication and failover, then a sharded fleet.
ClassicMedium77. Design a Stream Processor like Flink
A stateful stream processor: dataflow graphs, keyed state in RocksDB, event time and watermarks, windows, barrier checkpoints, exactly-once into Kafka.
ClassicMedium78. Design a Coordination Service like ZooKeeper
A coordination service like ZooKeeper: znodes, sessions and watches, a ZAB quorum of five with observers, and the recipes for elections and locks.
ClassicMedium79. Design Object Storage like Amazon S3
An S3-inspired object store: bytes before metadata, a partitioned namespace with cache coherence, then repair, heat, lifecycle and safe change. Three interview boards with explicit assumptions and AWS source boundaries.
ClassicMedium80. Design a Message Queue like Amazon SQS
Build a queue around visibility leases and explicit acknowledgement, then add replicated partitions, FIFO groups, tenant fairness and bounded redrive: three self-contained interview boards.
ClassicMedium81. Design a Proximity Search Service
Radius, k-nearest and map-box search over places that rarely move and drivers that ping every few seconds: R-trees, geohash, H3 and S2 cells, Redis GEO and OpenSearch. One-hour boards for junior, senior and staff, with the theory.
ClassicMedium82. Design Counting at Scale with Probabilistic Data Structures
Unique visitors, top pages, seen URLs and p99 latency over 10 B events a day with HyperLogLog, Count-Min, Bloom filters and t-digests.
ClassicMedium83. Design a Vector Database
Nearest-neighbour search over a billion embeddings: HNSW, IVF-PQ and disk graphs, filters, segments, sharding, and hybrid search with re-ranking.
ClassicMedium84. Design a Time-Series Database
The database under a metrics platform: a WAL and head block, compressed chunks, a label index, rollups, compaction and blocks in S3.
ClassicMedium85. Design a Change Data Capture Pipeline
Every committed change in Aurora and DynamoDB, read from the log and delivered in order per row to search, caches, Redshift and an S3 lake.
ClassicMedium86. Design an IP and URL Denylist
Deny requests by caller address, range or URL at a global edge, with lists from governments, threat feeds and detection reaching 300 proxies in seconds.
ClassicMedium87. Design a Hotel Booking System like Airbnb
Nights as counted inventory, holds that never double-book, search that knows availability, and payments that reconcile.
ClassicMedium88. Design a Reviews and Ratings System
Dish reviews only from customers who ordered them, votes counted from a change stream, reviews ranked by the Wilson bound, payouts made exactly once.
ClassicMedium89. Design a Donations Website for a Charity Event
Ten charities, three days, $100 M: charge through a third-party processor exactly once, queue the broadcast spikes, keep live totals on sharded counters. One-hour boards for junior, senior and staff, with the theory behind them.
ClassicMedium90. Design a Book Seller Aggregator Platform
Buy the cheapest offer under a maximum price from 1,000 sellers: cached offers, coalesced quotes, a hold-order-capture saga, open requests.
ClassicMedium