📝 System Design Interview Classics · C4 Architecture · Pro
URL Shortener System Design
The classic interview design for a service like bit.ly: short code generation, fast redirects from a cache, and click analytics processed separately.
C4 Architecture diagrams are part of Pro. Anyone can view this one; generating and editing it needs Pro.
Drawing diagram…
What this diagram shows
- Short codes come from a pre-generated key range, so there are no collisions
- Redirects are served from Redis for speed
- Click analytics go through a queue so redirects stay fast
Prompt used
URL shortener design: users create short links through a web app and API. A key generation service hands out unused 7-character codes from pre-allocated ranges. Links are stored in Cassandra. The redirect service looks up codes in a Redis cache first, falls back to Cassandra, and returns an HTTP 301 redirect. Each click is sent to Kafka and an analytics service aggregates clicks per link into ClickHouse.
Mermaid code
C4Container
title URL Shortener
Person(user, "User", "Creates and shares links")
Person(visitor, "Visitor", "Clicks short links")
System_Boundary(us, "URL Shortener") {
Container(api, "Link API", "Go", "Create and manage links")
Container(kgs, "Key Generation Service", "Go", "Hands out unused codes")
Container(redir, "Redirect Service", "Go", "Looks up code and redirects")
Container(an, "Analytics Service", "Java", "Aggregates clicks")
ContainerDb(cache, "Cache", "Redis", "Hot short codes")
ContainerDb(db, "Link Store", "Cassandra", "Code to long URL")
ContainerQueue(q, "Click Events", "Kafka", "One event per click")
ContainerDb(ch, "Analytics DB", "ClickHouse", "Clicks per link")
}
Rel(user, api, "Creates link", "HTTPS")
Rel(api, kgs, "Gets a code")
Rel(api, db, "Saves link")
Rel(visitor, redir, "Opens short URL", "HTTPS")
Rel(redir, cache, "Reads")
Rel(redir, db, "Reads on cache miss")
Rel(redir, q, "Publishes click")
Rel(an, q, "Consumes")
Rel(an, ch, "Writes counts")Related templates
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