URL Shortener — Detailed

flowchart TB
  subgraph Client[Clients]
    BR([Browser])
    APP([Mobile / API consumer])
  end

  subgraph Edge
    DNS[DNS<br/>tinyurl.com]
    CDN[CDN<br/>redirect-edge cache]
    LB[L7 LB]
    WAF[WAF / Rate Limit]
  end

  subgraph App[App Tier]
    GW[API Gateway]
    WRITE[Shorten Service<br/>POST /shorten]
    READ[Redirect Service<br/>GET /:code]
    ANL[Analytics Service]
  end

  subgraph ID[ID Generation]
    SNOW[Snowflake / Counter<br/>chunked from ZK/etcd]
    B62[Base62 encode<br/>7 chars = 62^7 ≈ 3.5T]
    CUST[Custom alias check<br/>collision check]
  end

  subgraph Data[Data Layer]
    RDS[(Redis hot cache<br/>code -> url<br/>LRU + TTL)]
    BF[Bloom filter<br/>existing codes]
    KV[(Sharded KV<br/>Cassandra / DynamoDB<br/>by hash code)]
    META[(Postgres<br/>users, custom domains, plans)]
    CLICKS[[(Click events<br/>Kafka -> ClickHouse)]]
  end

  subgraph Async
    Q[[Kafka click topic]]
    AGG[[Stream aggregator<br/>Flink / Spark]]
    DASH[Dashboard]
  end

  subgraph Obs[Observability]
    M[Metrics: redirect QPS,<br/>p99 latency]
    L[Logs]
    T[Traces]
  end

  BR --> DNS --> CDN --> LB --> WAF --> GW
  APP --> DNS
  GW --> WRITE
  GW --> READ
  WRITE --> CUST
  WRITE --> SNOW --> B62
  B62 --> KV
  WRITE --> META
  READ --> RDS
  RDS -. miss .-> KV
  KV -. fill .-> RDS
  READ --> BF
  READ -.fire&forget.-> Q
  Q --> AGG --> CLICKS --> DASH
  App -.metrics.-> M
  App -.logs.-> L
  App -.traces.-> T

    classDef client fill:#dbeafe,stroke:#1e40af,stroke-width:1px,color:#0f172a;
    classDef edge fill:#cffafe,stroke:#0e7490,stroke-width:1px,color:#0f172a;
    classDef service fill:#fef3c7,stroke:#92400e,stroke-width:1px,color:#0f172a;
    classDef datastore fill:#fee2e2,stroke:#991b1b,stroke-width:1px,color:#0f172a;
    classDef cache fill:#fed7aa,stroke:#9a3412,stroke-width:1px,color:#0f172a;
    classDef queue fill:#ede9fe,stroke:#5b21b6,stroke-width:1px,color:#0f172a;
    classDef compute fill:#d1fae5,stroke:#065f46,stroke-width:1px,color:#0f172a;
    classDef storage fill:#e5e7eb,stroke:#374151,stroke-width:1px,color:#0f172a;
    classDef external fill:#fce7f3,stroke:#9d174d,stroke-width:1px,color:#0f172a;
    classDef obs fill:#f3e8ff,stroke:#6b21a8,stroke-width:1px,color:#0f172a;
    class BR,APP client;
    class DNS,CDN,LB,WAF,GW edge;
    class WRITE,READ,ANL,B62,CUST,BF service;
    class SNOW,KV,META,CLICKS datastore;
    class RDS cache;
    class Q,AGG queue;
    class DASH,M,L,T obs;

Key design choices

• ID strategy: pre-generate counter blocks from ZooKeeper/etcd, encode base62, 6–8 chars.
• Alternative: hash(longUrl) → base62 → check collision (deterministic, dedupes).
• Storage: Cassandra/DynamoDB keyed by code, 2 replicas across AZs.
• Redirect: 301 (cacheable, browser caches forever) vs 302 (every hit logged).
• Analytics: never block redirect on logging — async to Kafka.

API

POST /shorten { url, custom_alias?, ttl? } -> { code, short_url }
GET  /:code              -> 301/302 to long url
GET  /api/v1/:code/stats -> { clicks, geo, referers, time series }

Schema

url(code PK, long_url, owner_id, created_at, expires_at, custom bool)
clicks(code, ts, ip_hash, ua, geo, referer)  -- ClickHouse / BQ
users(id, plan, api_key_hash)

Glossary & fundamentals

Concepts referenced in this design. Each row links to its canonical page; the tag column shows whether it is a high-level (HLD) or low-level (LLD) concept.

Tag	Concept	What it is	Page
HLD	Load balancer / GSLB	L4/L7 traffic distribution and failover	load-balancer
HLD	CDN	edge caching for static assets	cdn
HLD	API gateway / BFF	single ingress, auth, rate limit, routing	api-gateway
HLD	Pub/Sub & message brokers	topics, consumer groups, delivery semantics	pub-sub-pattern
HLD	Leader/follower replication	sync/semi-sync/async replication, failover	replication-leader-follower
HLD	Probabilistic data structures	Bloom, HLL, Count-Min, MinHash, t-digest	probabilistic-data-structures
HLD	Observability	metrics, logs, traces, SLOs	observability
HLD	Batch & stream processing	Lambda vs Kappa, watermarks, windows	batch-stream-processing
LLD	Behavioural patterns	Strategy, Observer, State, Command, Chain	behavioral-patterns