System design templates

Social news feed - fan-out on write or on read

Fan-out on write against fan-out on read, and when each loses.

System design · core

Media feed - the feed serves URLs, the edge serves bytes

direct upload, rendition pipeline, and who actually pays for the pixels

System design · core

The professional graph - degrees, and why it is not a join

in-memory adjacency, bidirectional BFS, and an offline people-you-may-know

System design · deep

Comment trees - one range scan, and four writes per vote

materialized paths, ranked subtrees, and the write amplification of a vote

System design · deep

The social graph - one edge, written twice

adjacency in both directions, follow against friend, and counters that drift

System design · core

Home timeline - materialise it, then keep it honest

fan-out on write, the celebrity exception, and the hydrate hop

System design · core

Chat and messaging - one socket, many deliveries

Delivery guarantees, presence and ordering across devices.

System design · core

Transactional email - the receiver decides

queues, backoff, bounces and complaints - and SPF, DKIM and DMARC as parts of the architecture rather than a checklist

System design · core

Presence - the most expensive cheap feature

heartbeats, TTL, and why one green dot costs more events per day than every message in the product

System design · starter

Push notifications - the token outlives the install

APNs and FCM, token lifecycle, batching, and why a dead token beats a failed send to the top of the incident list

System design · core

Team channels - the unread count is the hard part

channel fan-out against DM, threads that do not bump the room, and the badge nobody wants to store

System design · core

Encrypted delivery - one tick, two ticks, blue

sent, delivered, read - and what a server that cannot read the message is allowed to claim

System design · deep

Image CDN - resize on the fly, or store every size

Cache keys, on-demand transforms, and the tail that never hits cache

System design · starter

Live streaming - latency against the buffer that saves you

Ingest, segmenting, and the real cost of every second of latency removed

System design · deep

Streaming playback - the bytes were already next to you

Manifest, adaptive bitrate, edge appliances and pre-positioned content

System design · deep

Transcoding pipeline - 480 tasks, one file

Job queue, worker pools, codecs and what a failed segment costs

System design · core

Video upload - one file in, seven encodes out

Resumable upload, the transcode fan-out, and why it says processing

System design · core

Checkout - holding stock you have not sold yet

reserve, charge, commit, unwind

System design · core

Auction bidding - the clock is part of the system

ordering, proxy bids, soft close

System design · deep

Food delivery - three parties and a promise

state machine, offers, ETA

System design · core

Hotel booking - selling a range, on purpose, slightly twice

nights, holds, overbooking, cancellation

System design · core

Seat sales - fifty thousand people, one hundred seats

queue, hold, expire, issue

System design · deep

URL shortener - one write, a million reads

One write, millions of reads. The warm-up question.

System design · starter

Distributed cache - the ring, and what a node leaving costs

consistent hashing, hot keys, and the miss storm

System design · core

Key-value store - quorums, and where the consistency knob sits

partition, replicate, and choose how much to wait for

System design · deep

Rate limiter - token bucket, and where the counter lives

allow, deny, and the shared counter in between

System design · core

Unique ID generator - 64 bits, and two facts nobody can check

time, machine, sequence - and the clock going backwards

System design · starter

Web crawler - a frontier, and the trap of crawling forever

fetch politely, dedup honestly, never loop

System design · core

Double-entry ledger - the balance is not a column

Teaches why money is stored as immutable movements and read as a derived sum, and what it costs to never update a total.

System design · core

Fraud detection - a decision that must fit in 120 milliseconds

Teaches how a risk decision is split between a synchronous call with a hard deadline and an asynchronous review that is allowed to be right.

System design · deep

Payment gateway - what a timeout means when the card may already be charged

Teaches how a payment system stays correct when its authoritative dependency can fail in a way that tells you nothing.

System design · core

Matching engine - price-time priority, and why the clock cannot decide who was first

Teaches why an exchange assigns order with a sequence number instead of measuring it with a clock, and what a single-threaded book buys in return.

System design · deep

Wallet transfers - the state machine, the hold that expires, and the reversal that cannot

Teaches how money moves between two accounts in one system without ever being in both places or neither, and what it costs to make every step undoable.

System design · starter

Ad serving - an auction inside a page load

Why a system with eighty milliseconds to decide must spend money it cannot yet count.

System design · deep

Analytics pipeline - the lag between an event and the answer

Why a click takes ten seconds to reach one dashboard and until tomorrow to reach the one people trust.

System design · core

Log aggregation - the index is bigger than the data

Why searching your logs costs thirteen times more than keeping them, and what you can no longer ask once you sample.

System design · core

Metrics and monitoring - cardinality is the thing that kills you

Why one label with unbounded values takes down the system that was supposed to explain the outage.

System design · core

Recommendation engine - retrieve a thousand, rank twenty

Why the feed is computed in a hundred milliseconds by a model that is a day out of date.

System design · deep

Fleet tracking - 21,600 writes per vehicle per day

Teaches how a continuous position firehose is split into a tiny live picture and an enormous history, and how retention rather than hardware decides what it costs.

System design · core

Geofencing - many points against many polygons

Teaches how a covering index reduces every-point-against-every-fence to a handful of tests, and why a jittery GPS makes the crossing itself the harder problem.

System design · deep

Maps routing - precompute, then watch it go stale

Teaches how a shortest path over a continental road graph is bought with precomputation, and what live traffic does to work that was done last night.

System design · deep

Proximity search - the index is the design

Teaches how to size a geospatial index against the query it serves, and why the nearest answer is usually in the cell next door.

System design · starter

Ride matching - the nearest car is the wrong car

Teaches how a geospatial index turns supply and demand into a dispatch decision, and why that decision is an assignment over a batch rather than a nearest-neighbour lookup.

System design · core

Calendar scheduling - a time that suits five people

Why a meeting time is computed from rules and guesses rather than read from a table, and what that costs.

System design · core

Code review - the comment is on a line that moved

What it takes to keep a conversation attached to code that is rewritten underneath it while the conversation is still going.

System design · core

Collaborative editor - two carets in one paragraph

How two people type into one document at the same time without either of them losing a character.

System design · deep

Version control - the commit graph is a DAG

Why history is a graph rather than a list, and what that costs at merge, at fetch and at garbage collection.

System design · deep

Whiteboard sync - the cursors cost more than the drawing

Why a shared canvas is mostly a broadcast problem, and what happens to a shape that two people grab at once.

System design · core

E-commerce orders

Orders, line items and the price that must not change after checkout.

Database schema · core

Multi tenant SaaS

Tenant isolation, and every join that could leak across it.

Database schema · deep