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HLD 101

System Design Interview Course

Twelve concepts, in the order they make sense. Read straight through, they take you from “why does one server stop working” to defending a partition key under pressure. Read out of order, they are a reference to revise from the night before an interview. Either way, every problem on this site assumes these and nothing more.

12 concepts2h 19m totalno prior experience assumedAlready comfortable? Skip to the problems
The track0 / 4· about 30m left
Start — Why one server stops being enough
STAGE 01

Thinking in systems

The vocabulary and the arithmetic. Everything after this assumes you have these.

3 concepts · 22m
01Why one server stops being enoughName the resource that saturates first, and explain why the second machine is never free.7 minused in 9 problems02Back-of-the-envelope mathTurn "design Twitter" into QPS, bytes per day and machine count, out loud, in three minutes.7 minused in 9 problems03Latency numbers, and where the time actually goesBudget a 200ms SLO across network, disk and cache hops without guessing at any of them.8 minused in 8 problems
STAGE 02

Moving requests

How a request finds a machine — and how you stop it having to travel at all.

3 concepts · 37m
04Load balancing and health checkscoming soonChoose L4 or L7 on purpose, and say what happens to in-flight requests when a node dies.12 minused in 7 problems
05Caching: placement, eviction, invalidationcoming soonPick a cache layer deliberately, and answer “what about stale data?” with a policy rather than a shrug.16 minused in 8 problems
06CDNs and the edgecoming soonDecide what belongs at the edge, and name the things that can never go there.9 minused in 5 problems
STAGE 03

Storing state

Where the data lives once there is too much of it for one box — and what that costs you.

3 concepts · 39m
07SQL vs NoSQL, decided properlycoming soonJustify the choice from your access patterns, not from a comparison table of buzzwords.13 minused in 9 problems
08Sharding and partitioningPick a partition key, predict the hot shard before the interviewer does, and plan the resharding.8 minused in 7 problems
09Replication, consistency and quorumscoming soonSay precisely what a client can observe after a write, and what stops being true during a partition.18 minused in 8 problems
STAGE 04

Coordinating work

What happens when the work outlives the request that asked for it.

3 concepts · 41m
10Queues, brokers and delivery guaranteescoming soonMove work off the request path, and defend at-least-once instead of promising exactly-once.15 minused in 6 problems
11Idempotency and deduplicationcoming soonDesign a retry-safe write, and show exactly where the dedup key lives and when it expires.12 minused in 6 problems
12Timeouts, retries and backpressurecoming soonDescribe how the system sheds load on a bad day instead of collapsing under it.14 minused in 7 problems

Finish the track, then start here.

It leans on seven of the twelve ideas above and none of the ones you skipped. The shortest honest first problem.

Design a URL Shortener