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So yeah, it's pretty awesome. But don't just take our word for it. Try a practice problem!
Design YouTube
Design a video platform where users upload videos and a global audience streams them. The hard parts are the transcode pipeline and delivery economics — serving video is an egress-bound problem measured in exabytes.
- Upload a video → it is transcoded and becomes watchable.
- Watch a video with ABR → smooth across connections, with seeking supported.
Design a News Feed
Design the home feed for a social product — a ranked, near-real-time stream of posts from followed accounts. The hard parts are fan-out and ranking, and they are coupled.
- Post text plus optional media references; the post becomes retrievable and eligible for followers' feeds.
- Read a ranked, paginated home timeline of recent posts from followed accounts.
- Follow / unfollow an account, which changes what appears in the follower's feed.
- Record engagement — likes, reshares, views — as signals the ranker uses as features. Recording the event is in scope; the high-throughput engagement store that counts those events is a separate problem, treated here as an existing service the ranker reads.
Design Uber / Nearby Drivers
Design the core of a ride-hailing service — drivers stream location, riders find and match a nearby driver. The twist is that it is write-heavy; the location index takes a firehose of updates, far more than the rider-request reads.
- Update location — updateLocation(driverid, lat, lng), very high frequency.
- Find nearby drivers — given a rider's location, return available drivers within a radius.
- Match — assign one driver to a request and prevent double-assignment.
- Driver availability — online / offline / on-trip state.
Design Instagram
Design a photo-sharing service — upload images, follow accounts, and see a timeline of recent posts. The read path is a News Feed variant; the new problem is the media write path — upload, transcode, durable blob storage, and CDN delivery.
- Upload a photo with caption and location; it becomes visible to followers once processed.
- View a timeline — recent posts from followed accounts, paginated.
- View a profile — a user's own grid of posts.
- Follow / unfollow an account, which changes what appears in the follower's feed.
Design Typeahead / Autocomplete
Design the suggestion service behind a search box — as a user types, return the few most-relevant completions of their prefix faster than the next keystroke. The hard part is ranking a huge corpus inside a per-keystroke time budget.
- Suggest. Given a prefix, return its top-K completions (default K = 10) ranked by
- Rank by popularity. Ordering reflects how often each completion is searched, refreshed
- Ingest signals. Take in the search and click events that popularity is computed from.
Design a Distributed Rate Limiter
Distributed rate limiter system design for API requests, including atomic counters, token buckets, sliding windows, multi-region coordination, and failure behavior.
- Decide allow or deny synchronously for each (key, action) pair.
- Configure limits per key, varying by API tier, endpoint, and tenant.
- Enforce multiple windows per key. A key often has a per-second and a per-day limit; deny on the tighter binding constraint.
- Surface state. On allow, return remaining quota and reset time; on deny, return enough for the gateway to send a 429 with Retry-After.
Design YouTube
Design a video platform where users upload videos and a global audience streams them. The hard parts are the transcode pipeline and delivery economics — serving video is an egress-bound problem measured in exabytes.
- Upload a video → it is transcoded and becomes watchable.
- Watch a video with ABR → smooth across connections, with seeking supported.
Design a News Feed
Design the home feed for a social product — a ranked, near-real-time stream of posts from followed accounts. The hard parts are fan-out and ranking, and they are coupled.
- Post text plus optional media references; the post becomes retrievable and eligible for followers' feeds.
- Read a ranked, paginated home timeline of recent posts from followed accounts.
- Follow / unfollow an account, which changes what appears in the follower's feed.
- Record engagement — likes, reshares, views — as signals the ranker uses as features. Recording the event is in scope; the high-throughput engagement store that counts those events is a separate problem, treated here as an existing service the ranker reads.
Design Uber / Nearby Drivers
Design the core of a ride-hailing service — drivers stream location, riders find and match a nearby driver. The twist is that it is write-heavy; the location index takes a firehose of updates, far more than the rider-request reads.
- Update location — updateLocation(driverid, lat, lng), very high frequency.
- Find nearby drivers — given a rider's location, return available drivers within a radius.
- Match — assign one driver to a request and prevent double-assignment.
- Driver availability — online / offline / on-trip state.
Design Instagram
Design a photo-sharing service — upload images, follow accounts, and see a timeline of recent posts. The read path is a News Feed variant; the new problem is the media write path — upload, transcode, durable blob storage, and CDN delivery.
- Upload a photo with caption and location; it becomes visible to followers once processed.
- View a timeline — recent posts from followed accounts, paginated.
- View a profile — a user's own grid of posts.
- Follow / unfollow an account, which changes what appears in the follower's feed.
Design Typeahead / Autocomplete
Design the suggestion service behind a search box — as a user types, return the few most-relevant completions of their prefix faster than the next keystroke. The hard part is ranking a huge corpus inside a per-keystroke time budget.
- Suggest. Given a prefix, return its top-K completions (default K = 10) ranked by
- Rank by popularity. Ordering reflects how often each completion is searched, refreshed
- Ingest signals. Take in the search and click events that popularity is computed from.
Design a Distributed Rate Limiter
Distributed rate limiter system design for API requests, including atomic counters, token buckets, sliding windows, multi-region coordination, and failure behavior.
- Decide allow or deny synchronously for each (key, action) pair.
- Configure limits per key, varying by API tier, endpoint, and tenant.
- Enforce multiple windows per key. A key often has a per-second and a per-day limit; deny on the tighter binding constraint.
- Surface state. On allow, return remaining quota and reset time; on deny, return enough for the gateway to send a 429 with Retry-After.