# yaml-language-server: $schema=../references/diagram.schema.json
title: URL shortener system design
subtitle: Redirects are served from the CDN and a Redis cache in front of PostgreSQL; every click streams through Kafka into an analytics pipeline
direction: down
layout: { spacing: compact }

legend:
  solid: Request or query
  dashed: Asynchronous event or stream

groups:
  analytics: { type: zone, label: Click analytics pipeline }

nodes:
  clients: { icon: generic:users, label: Clients, note: Browsers and apps }
  cdn: { icon: generic:cdn, label: CDN, note: Caches popular redirects }
  lb: { icon: generic:load-balancer, label: Load balancer }
  api: { icon: generic:servers, label: API servers, note: Stateless, badge: x3 }
  cache: { icon: logos:redis, label: Redis, note: Hot short codes }
  db: { icon: logos:postgresql, label: PostgreSQL, note: Sharded by short code }
  ids: { icon: tabler:hash, label: ID generator, note: Base62 key ranges }
  events: { icon: logos:kafka-icon, label: Kafka, note: clicks topic, in: analytics }
  stream: { icon: logos:apache-flink-icon, label: Apache Flink, note: Counts per link, in: analytics }
  olap: { icon: si:clickhouse, label: ClickHouse, note: Click analytics, in: analytics }
  dashboard: { icon: generic:dashboard, label: Dashboards, in: analytics }

edges:
  - clients -> cdn: GET /abc123
  # Creating a link skips the CDN and goes straight to the load balancer.
  - clients -> lb: POST /links
  - cdn -> lb: cache miss
  - lb -> api
  # All three data-store edges are labelled, so Redis, PostgreSQL and the ID generator share one row; Kafka sits in the analytics group.
  - api -> cache: lookup, fill on miss
  - api -> db: read on miss, insert new
  - api -> ids: new link
  - api --> events: click event
  # Redirects the CDN answers never reach the API, so the CDN ships its own logs.
  - cdn --> events: edge logs
  - events --> stream --> olap
  # Written in layout order with the arrow flipped: dashboards query ClickHouse.
  - { from: olap, to: dashboard, arrow: start, label: query }