Lesson 1: Design a URL Shortener#

Goal: Design a service like TinyURL that takes a long URL and converts it into a short alias (e.g., http://tiny.url/xyz).

Requirements#

Functional#

shorten(long_url) -> short_url
redirect(short_url) -> long_url
Custom aliases (optional).

Non-Functional#

Highly Available: If the service is down, URL redirection stops working.
Low Latency: Redirection must happen in milliseconds.
Read-Heavy: 100:1 read-to-write ratio.

Core Design#

1. Database Choice#

Since we need fast lookups and the data model is simple (Key-Value), a NoSQL Key-Value Store (like DynamoDB or Redis) is ideal.

Key: short_alias
Value: long_url

2. Hashing Algorithm#

How do we generate the alias?

MD5/SHA256: Too long.
Base62 Encoding: Converts a unique ID (from a counter or database ID) into a string of characters [a-z, A-Z, 0-9].

🛠️ Sruja Perspective: Modeling the Flow#

We can use Sruja to model the system components and the user scenario for redirection.

architecture "URL Shortener" {
    requirement R1 functional "Shorten long URL"
    requirement R2 functional "Redirect short URL"
    requirement R3 availability "High availability for redirects"
    requirement R4 latency "Low latency (< 200ms)"

    // Define the system boundary
    system TinyURL "TinyURL Service" {
        container WebServer "API Server" {
            technology "Go"
            scale {
                min 3
                max 20
                metric "cpu > 70%"
            }
        }
        
        datastore DB "UrlStore" {
            technology "DynamoDB"
            description "Stores mapping: short_alias -> long_url"
        }
        
        container Cache "Cache" {
            technology "Redis"
            description "Caches popular redirects"
        }

        WebServer -> Cache "Reads"
        WebServer -> DB "Reads/Writes"
    }

    person User "User"

    // Define the redirection scenario
    scenario RedirectFlow "User clicks a short link" {
        User -> WebServer "GET /xyz"
        WebServer -> Cache "Check cache for 'xyz'"
        Cache -> WebServer "Miss"
        WebServer -> DB "Get long_url for 'xyz'"
        DB -> WebServer "Return 'http://example.com'"
        WebServer -> User "301 Redirect to 'http://example.com'"
    }
}