Cloudflare

7 design problems known to be asked at Cloudflare.

Medium4

Mediumhigh level design distributed systems data structures

Bloom Filters: The Yes/No That Saves a Disk Seek

A tiny bit-array that answers 'definitely not here' or 'maybe here' in constant time — how Cassandra, HBase, and Chrome skip millions of expensive lookups for keys that were never there.

Asked at Google, Amazon, Meta +3

Aug 8, 2026·12 min read

Mediumhigh level design distributed systems networking

CDNs: Move the Bytes to the User, Not the User to the Bytes

A user in Sydney pays ~200ms to reach a Virginia origin before any work happens — physics you can't code around. This is how CDNs cheat the speed of light with edge PoPs, anycast routing, and HTTP caching.

Asked at Cloudflare, Akamai, Fastly +3

Aug 8, 2026·16 min read

Mediumhigh level design distributed systems networking

Load Balancing: One Front Door for a Fleet That Keeps Changing

A single server melts under Black-Friday traffic. Load balancing is how you put ten servers behind one address — and the L4-vs-L7 choice, the routing algorithm, and the health checks are where systems live or die.

Asked at Amazon, Google, Netflix +3

Aug 8, 2026·16 min read

Mediumhigh level design distributed systems networking

Rate Limiting: How a Bucket of Tokens Keeps Your API Alive

One misbehaving client with a retry loop can take down a service faster than any DDoS. Rate limiting is the cheap, boring control that decides who gets served — here's how the five classic algorithms actually work, and how to make one correct across a fleet.

Asked at Stripe, Cloudflare, GitHub +2

Aug 8, 2026·16 min read

Hard3

Hardsystem design interview algorithms caching concurrency

Design a Distributed Rate Limiter

Picking token bucket vs sliding window is the easy part. The real interview is what happens when the limiter runs on fifty machines: where the counter lives, why 'local counters' quietly lets 50× your limit through, and how to enforce one global cap without a round-trip on every request.

Asked at Stripe, Cloudflare, GitHub +2

Aug 9, 2026·17 min read

Hardhigh level design distributed systems consensus

Raft, Driven: Watch a Cluster Elect, Replicate, and Heal

Everyone can recite 'Raft elects a leader by majority.' The hard part is the log — how a follower that missed writes during a partition gets repaired, and the one subtle rule that stops a committed entry from ever being lost. Drive a live 5-node cluster, then take it apart.

Asked at Google, Amazon, Microsoft +4

Aug 9, 2026·20 min read

Hardhigh level design distributed systems databases storage

Storage Engines: LSM-Trees vs B-Trees

Every database makes one foundational choice — how to lay bytes on disk. B-trees update in place; LSM-trees never overwrite, they append and compact. That single decision sets your write throughput, read latency, and space usage. Drive an LSM write path — memtable, flush, compaction — and see the tradeoff for yourself.

Asked at Google, Meta, Amazon +3

Aug 9, 2026·14 min read