Technology
17 min read
An educational design for searching a billion web pages: a polite, distributed crawler with a URL frontier and robots.txt, deduplication, building an inverted index with batch processing, document-partitioned shards and replicas, scatter-gather query serving with result caching and tail-latency control, ranking from relevance and link signals, freshness tiers, failures and safety.
Technology
16 min read
Design an online store from product pages to delivery: a catalogue and search served from caches and event-fed indexes, carts in a key-value store, inventory reservations with expiry, a checkout saga across inventory and payment, oversell prevention, and a flash sale that survives a thousandfold spike with a waiting room, bucketed stock and isolation from normal shopping.
Technology
17 min read
Design a food delivery platform where every order coordinates three parties in real time: restaurant discovery and menus as cached read models, an idempotent checkout, an order state machine driven by a saga with timeouts and compensations, dispatching delivery partners timed to food readiness, batching, live tracking, notifications, dinner-peak load, failures and the trade-offs between them.
Technology
18 min read
Design the core of a ride-hailing app: ingesting hundreds of thousands of driver location updates per second, a geospatial index of available drivers, matching riders to drivers with offers and timeouts, never assigning one driver twice, a trip state machine, live tracking for riders, city-based partitioning, payments, failures, privacy and fraud.
Technology
15 min read
Design an on-demand video platform: resumable uploads straight to object storage, a parallel transcoding pipeline that splits videos into chunks and renditions, adaptive bitrate streaming with HLS-style segments and manifests, CDN delivery at terabits per second, storage tiering, view counting, failures, content safety and the metrics that matter to viewers.
Technology
19 min read
Design the payment platform behind checkout and refunds, where correctness matters more than speed: payment intents and attempts, an explicit state machine, idempotency at every hop, asynchronous confirmation through verified webhooks, a double-entry ledger with a runnable example, reconciliation against provider reports, refunds, provider failover, fraud and PCI scope, and an audit trail.
Technology
16 min read
Design ticketing for concerts and matches where thousands of people want the same seats at the same moment: seat inventory modelling, holds with expiry, atomic all-or-nothing seat claims, idempotent payment and confirmation, what happens when payment succeeds after a hold expires, virtual waiting rooms for big on-sales, hot inventory, bots, failures and the invariant that a seat is never sold twice.
Technology
17 min read
Design a service that runs millions of one-time and recurring jobs at the right time, at least once, across many machines: time-bucketed storage, dispatchers that own partitions through leases, atomic claims that prevent double dispatch, queues and workers with heartbeats, retries and dead letters, misfire policies after outages, top-of-the-hour spikes, time zones, and safe HTTP callbacks.
Technology
14 min read
Design the suggestions that appear as users type in a search box: requirements and estimates per keystroke, why querying the database per keystroke fails, an offline pipeline that turns search logs into prefix-to-top-K tables or tries, in-memory serving, client debouncing and CDN caching, trending updates, sharding and replication, privacy filtering and failures.
Technology
16 min read
Design a home feed that shows posts from accounts a user follows: estimates for posting, reading and fan-out, fan-out on read versus fan-out on write, the hybrid model that handles celebrity accounts, precomputed feed storage, hydration, ranking with a chronological fallback, cursor pagination, deletions and privacy, and what changes at social-media scale.
Technology
18 min read
Design one-to-one and group chat for tens of millions of concurrent users: WebSocket connection gateways, a session registry for routing messages to the right server, per-conversation sequence numbers for ordering, acknowledgements and deduplication, offline delivery with push and sync, multi-device cursors, presence, partitioned message storage, failures and security.
Technology
17 min read
Design a notification platform that other services use to reach users by push, SMS, email and in-app inbox: requirements and estimates, a 202-accepted API with idempotency keys, preferences and templates, priority queues per channel, provider rate limits and failover, retries and dead-letter queues, deduplication, delivery tracking, campaign fan-out, failures, security and observability.
Technology
15 min read
Design an in-memory cache service shared by many applications: requirements and estimates, the get/set/delete API, a single-node design with TTLs and LRU eviction, then partitioning with consistent hashing or hash slots, client-side versus proxy routing, replication and failover, rebalancing, hot keys, cache stampedes, and what consistency a cache can and cannot promise.
Technology
19 min read
Design a rate limiter that enforces limits across many API servers: requirements, estimates, where the limiter sits, token bucket, leaky bucket, fixed window, sliding log and sliding window counter algorithms with a runnable comparison, atomic Redis implementations, local plus global hybrids, failure behaviour (fail open or fail closed), multi-region limits and response headers.
Technology
20 min read
The classic first system design problem, worked end to end with the course's method: requirements, estimates, APIs, data model, a simple baseline, short-code generation and collisions, 301 versus 302 redirects, caching a read-heavy path, expiry, analytics off the critical path, sharding, failures, abuse prevention, and the Pastebin variant with object storage.
Technology
14 min read
A practical, repeatable way to run a 45-minute system design interview: what interviewers typically evaluate, a 21-item checklist grouped into seven phases, a time plan, phrases for clarifying, estimating and stating trade-offs, how to handle hints and pushback, how SDE-1 and SDE-2 expectations differ, and a compact worked example.
Technology
16 min read
The security decisions that shape an architecture: trust boundaries and least privilege, authentication with passwords, MFA and identity providers, sessions versus tokens and JWTs, object-level authorisation, service-to-service identity, encryption in transit and at rest, secrets management, input validation and abuse prevention, SSRF, data privacy, audit logs and payment data.
Technology
14 min read
When services communicate by publishing facts instead of calling each other: commands versus events, event notification versus event-carried state transfer, designing and versioning events, ordering, replay, and the two patterns often paired with events: CQRS read models and event sourcing, with honest guidance on when each is worth its complexity.
