System Design (HLD)URL Shortener

Designing a URL Shortener

LevelIntermediate

Duration90 mins

TopicURL Shortener

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Requirements: Shorten, Redirect, Analytics

The Power of Short URLs

In the age of social media, where every character counts and attention spans are measured in milliseconds, URL shorteners have become critical infrastructure for the internet. Services like Bitly, TinyURL, and Twitter's t.co handle billions of redirects daily, powering everything from marketing campaigns to tweet embeds.

What appears to be a trivially simple service—converting a long URL into a short one—is actually a fascinating system design challenge that touches on nearly every core distributed systems concept: high-availability storage, extreme read scalability, low-latency serving, analytics at scale, and security considerations.

In this module, we'll design a URL shortening service capable of handling 100 million URL creations per day and 1 billion redirects per day—a scale that requires careful architectural thinking from the ground up.

What You Will Learn

By the end of this page, you will understand the complete functional and non-functional requirements for a URL shortener at scale. You'll learn to systematically decompose the problem, identify the three core operations (shorten, redirect, analytics), and establish the constraints that will drive all subsequent design decisions.

Understanding the Problem Domain

Before diving into requirements, let's thoroughly understand what a URL shortener actually does and why it exists. A URL shortener is a service that:

Accepts a long URL (e.g., https://www.example.com/products/category/electronics/laptops?brand=dell&sort=price&page=12)
Generates a short, unique alias (e.g., https://short.url/a7Xk2B)
Redirects visitors from the short URL to the original long URL
Collects analytics about those redirects (who clicked, when, from where)

Why do URL shorteners exist?

Primary Use Cases for URL Shorteners

•Character Limit Constraints — Platforms like Twitter (now X) historically had strict character limits. A 200-character URL in a 280-character tweet leaves little room for content. URL shorteners compress URLs to 15-25 characters.
•Readability and Shareability — Long URLs with query parameters are visually unappealing and hard to share verbally or in print. Short URLs are clean and memorable.
•Tracking and Analytics — Marketing teams need to measure which campaigns, channels, and content drive traffic. URL shorteners provide click tracking without modifying the destination site.
•Link Management — Enterprise users need to update destination URLs without changing what's already been shared. URL shorteners enable this indirection layer.
•Security and Privacy — Services can mask destination URLs (for affiliate links) or validate link safety before redirect.
•Branding — Custom domains (e.g., amzn.to, youtu.be) reinforce brand identity while providing URL shortening.

Scale of Major URL Shorteners

Bitly processes over 10 billion clicks per month. Twitter's t.co wraps every link in every tweet—hundreds of millions of tweets per day. TinyURL has been running since 2002 with billions of redirects. These services demonstrate that URL shortening is serious infrastructure, not a toy project.

Functional Requirements Analysis

Functional requirements define what the system does—the features and capabilities it must provide. For a URL shortener, we identify three core operations plus several supporting features.

Core Operation 1: URL Shortening

The primary function is accepting a long URL and returning a short one. Let's decompose this thoroughly:

URL Shortening Feature Decomposition
Feature Aspect	Requirements	Design Implications
Input Validation	Accept valid HTTP/HTTPS URLs; reject malformed URLs, local addresses, forbidden domains	Need URL parsing, validation logic, blocklist management
Short Code Generation	Generate unique 6-8 character alphanumeric codes	Need encoding scheme, collision handling, uniqueness guarantees
Custom Aliases	Allow users to request specific short codes (premium feature)	Need availability checking, reservation system, alias validation
Expiration Support	Optional TTL (time-to-live) for short URLs	Need expiration tracking, cleanup jobs, expiration metadata storage
User Association	Link short URLs to user accounts (for management)	Need authentication, user-URL mapping, access control
Duplicate Detection	Same long URL → same short code (or new one based on policy)	Need efficient lookup by long URL, policy configuration

Core Operation 2: URL Redirection

The most frequent operation—redirecting users from short URLs to original destinations. This must be extremely fast and highly available:

Redirection Requirements

•HTTP Redirect Response — Return 301 (permanent) or 302 (temporary) redirect with the original URL. Choice affects browser caching and SEO.
•Sub-50ms Latency — Users should perceive instant redirects. Cache hits must be <10ms; cache misses <50ms.
•High Availability — 99.99% uptime minimum. Redirects failing directly impacts all users of those links.
•Graceful Degradation — Show meaningful error pages for expired, deleted, or malicious links.
•Preview Mode — Optional landing page showing destination URL before redirect (security feature).
•Mobile Deep Links — Support redirecting to app-specific URLs on mobile devices.

Core Operation 3: Analytics Collection

Every redirect represents a data point. Analytics transform URL shorteners from simple utilities into valuable marketing infrastructure:

Analytics Data Requirements

•Click Counting — Total clicks per short URL, with real-time or near-real-time updates.
•Temporal Distribution — Clicks over time (hourly, daily, weekly aggregations) for trend analysis.
•Geographic Data — Country, region, city derived from IP address geolocation.
•Referrer Information — Which sites/platforms referred the traffic (Twitter, email, direct, etc.).
•Device/Browser Data — Device type, OS, browser from User-Agent parsing.
•Unique vs. Total Clicks — Distinguish unique visitors from repeat clicks (requires session/fingerprint tracking).

Analytics Cannot Block Redirects

Critical design constraint: analytics collection must never delay or block redirects. Users click links expecting instant response. If analytics processing is slow or failing, redirects must still work. This implies asynchronous analytics processing.

Supporting Features and Edge Cases

Beyond core operations, production URL shorteners require numerous supporting features. These often distinguish basic hobby projects from enterprise-grade systems:

Supporting Feature Requirements
Feature Category	Capabilities	Why It Matters
User Management	Registration, authentication, API keys, rate limits per user	Enable personalized experiences, prevent abuse, monetization
Link Management	Edit destination URLs, enable/disable links, bulk operations	Enterprise users need workflow tools, not just URL generation
API Access	REST/GraphQL APIs with authentication, webhooks for events	Programmatic access for integration with marketing tools, CMS systems
Custom Domains	Users bring their own domains (e.g., link.company.com)	Branding, trust, enterprise requirement
QR Code Generation	Generate QR codes for short URLs	Physical-to-digital bridging for marketing materials
Security Features	Phishing detection, malware scanning, spam prevention	Protect users, maintain service reputation, legal compliance

Critical Edge Cases to Handle

Robust system design anticipates edge cases before they become production incidents:

Edge Cases Requiring Special Handling

•Expired Links — What happens when a TTL-limited link expires? 404? Custom message? Redirect to homepage?
•Deleted Links — Should short codes be recycled? (Generally no—prevents hijacking known links)
•Circular Redirects — User shortens a short URL that eventually points back. Must detect and prevent loops.
•Destination Site Down — Should we detect and warn? Cache last-known-good destination?
•Concurrent Custom Alias Requests — Two users request same custom alias simultaneously. Conflict resolution needed.
•URL Changes — Long URL with tracking params changes slightly. Same short code or new one?
•Unicode/IDN URLs — Support for international domain names and Unicode paths.
•Maximum URL Length — Define limits on input URL length (2000 chars? 8000 chars?).

Scope Management in Interviews

In a system design interview, you won't cover all features. List the ones you're aware of, then explicitly scope: 'For this discussion, let's focus on core shortening, redirection, and basic analytics, deferring custom domains and QR codes.' This shows awareness without scope creep.

Non-Functional Requirements

Non-functional requirements define how well the system works—its quality attributes. For a URL shortener, these are particularly critical because the service must be fast, available, and scalable to be useful at all.

Scalability Requirements

Let's establish concrete scale targets that will drive architecture decisions:

Scale Targets for URL Shortener Design
Metric	Target Value	Implication
URL creations per day	100 million (1,157 per second avg, 5,000 peak)	Write-heavy during URL creation, needs efficient storage
Redirects per day	1 billion (11,574 per second avg, 50,000+ peak)	Read-heavy overall; 10:1 read/write ratio is conservative
Total URLs stored	100 billion over 10 years	Massive storage requirements; data retention policies needed
Analytics events per day	1 billion+ (one per redirect)	High-throughput event streaming; aggregate storage
Geographic distribution	Global (6 continents)	Multi-region deployment; data residency considerations

Latency Requirements

URL shorteners are in the critical path for every click. Slow redirects directly impact user experience and can increase bounce rates:

Latency Targets

•URL Creation: <200ms end-to-end (acceptable for write operations)
•Redirect (cache hit): <10ms at server (before network transit)
•Redirect (cache miss): <50ms at server including database lookup
•Redirect P99: <100ms (99th percentile must not exceed this)
•Analytics query: <500ms for standard reports; <5s for complex analytics

Availability and Reliability Requirements

A URL shortener being down means every link using it is broken—potentially millions of links embedded across the internet:

Availability Requirements
Aspect	Requirement	Translation
Redirect availability	99.99% (four nines)	~52 minutes downtime per year
URL creation availability	99.9% (three nines)	~8.7 hours downtime per year
Analytics availability	99.5%	More tolerant; eventual consistency acceptable
Data durability	99.999999999% (eleven nines)	Zero data loss acceptable; use replication
Disaster recovery	RPO: 1 minute, RTO: 5 minutes	Minimal data loss, fast recovery

Read vs. Write Availability Trade-offs

Notice that redirect availability (99.99%) is higher than creation availability (99.9%). This is intentional—reads are far more critical and frequent. During partial outages, we might disable URL creation while maintaining redirects. This matches business priority: existing links must work.

Back-of-Envelope Capacity Estimation

Rough capacity estimation helps validate whether our design decisions are feasible and identifies the primary resource constraints.

Storage Estimation

Let's calculate how much storage we need for URL mappings:

storage_estimation.txt
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URL Storage Calculation
=======================
 
Data per URL entry:
- Short code:          7 bytes (6-8 chars, let's use 7)
- Long URL:          200 bytes (average, can be up to 2KB)
- Creation timestamp:  8 bytes
- Expiration time:     8 bytes (optional)
- User ID:             8 bytes (optional)
- Click count:         8 bytes
- Flags/metadata:     10 bytes
-----------------------------------
Total per URL:       ~250 bytes
 
URLs created per day:        100,000,000
Bytes per day:               100M × 250 = 25 GB/day
Bytes per year:              25 GB × 365 = 9.125 TB/year
Bytes over 10 years:         ~91 TB for URL mappings alone
 
With indexes and replication (3x):
Total storage (10 years):    ~275 TB + indexes ≈ 400 TB

Bandwidth Estimation

Let's estimate network bandwidth for redirection traffic:

bandwidth_estimation.txt
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Bandwidth Calculation
=====================
 
Redirects per day:           1,000,000,000 (1 billion)
Redirects per second (avg):  1B / 86,400 ≈ 11,574 RPS
Redirects per second (peak): ~50,000 RPS (4-5x average)
 
Request size (incoming):
- HTTP headers:         500 bytes (average)
- Short URL in path:    20 bytes
Total incoming:         ~520 bytes per request
 
Response size (outgoing):
- HTTP 301/302 headers: 300 bytes
- Location header:      200 bytes (destination URL)
- Body (minimal):       100 bytes
Total outgoing:         ~600 bytes per response
 
Incoming bandwidth:
- Average: 11,574 × 520 bytes = 6.0 MB/s
- Peak:    50,000 × 520 bytes = 26 MB/s
 
Outgoing bandwidth:
- Average: 11,574 × 600 bytes = 6.9 MB/s
- Peak:    50,000 × 600 bytes = 30 MB/s
 
Total peak bandwidth: ~60 MB/s (~480 Mbps)
Note: This is modest; network is NOT the bottleneck.

Cache Estimation

Caching is critical for achieving low-latency redirects. Let's estimate cache requirements:

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Cache Sizing Calculation
========================
 
Observation: URL access follows power-law distribution
- 20% of URLs receive 80% of traffic
- Top 1% of URLs receive ~50% of traffic
 
To achieve 90%+ cache hit rate:
- Cache the top 10-20% of URLs by access frequency
 
URLs created per day:    100,000,000
Active URLs (accessed in last 30 days): ~500,000,000
Top 20% active URLs:     100,000,000
 
Memory per cached entry:
- Short code:            7 bytes
- Long URL:            200 bytes
- Metadata (TTL, etc):  50 bytes
- Redis overhead:       50 bytes
Total:                 ~300 bytes
 
Cache size for top 20%:
100M × 300 bytes = 30 GB per region
 
With 5 global regions: 150 GB total cache
(This is very achievable with modern Redis clusters)

Estimation Conclusion

Our estimates show the system is feasible: ~400TB storage over 10 years is manageable with distributed databases; ~60 MB/s bandwidth is modest; 30GB cache per region is achievable with Redis. The primary challenges are managing 50K+ RPS during peaks and achieving sub-50ms latency globally.

Read/Write Ratio Analysis

Understanding the read/write ratio is fundamental to URL shortener design because it profoundly impacts every architectural decision.

The Extreme Read-Heavy Nature

read_write_analysis.txt
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Read/Write Ratio Calculation
=============================
 
Writes (URL creation):      100 million/day = 1,157/second avg
Reads (redirects):        1,000 million/day = 11,574/second avg
 
Read/Write Ratio: 1000M / 100M = 10:1
 
This is actually conservative. In practice:
- Popular links get clicked thousands of times
- Many URLs created by spammers never get clicked
- Marketing campaigns generate millions of clicks per URL
 
Real-world ratio often: 100:1 or higher

Architectural Implications of Read-Heavy Workload

This extreme read-heavy ratio shapes nearly every design decision:

Design Implications of 10:1+ Read/Write Ratio

•Caching is Essential — With 10x+ more reads than writes, aggressive caching provides enormous benefits. We should cache recent/popular URLs in memory.
•Read Replicas Make Sense — Database read replicas can horizontally scale read capacity. Slight staleness is acceptable for redirects.
•Write Performance is Secondary — We can afford slower writes. Synchronous replication, stricter validation, and more complex write paths are acceptable if they simplify reads.
•Eventually Consistent Analytics — Click counts can lag behind reality. Batch updates and stream processing are appropriate for analytics.
•CDN-Friendly — Redirects for the same short code always return the same response (until updated). CDN caching can reduce origin load dramatically.

Write Path (Optimize for Durability)

•Synchronous replication to multiple zones
•Strong consistency for uniqueness checks
•Validation and sanitization of input URLs
•Write-through cache invalidation
•Asynchronous indexing for analytics

Read Path (Optimize for Latency)

•Multi-layer caching (CDN → Redis → DB)
•Read from nearest replica
•Pre-computed redirects in memory
•Minimal per-request processing
•Asynchronous analytics emission

Key Insight for Interviews

When asked 'Why cache?' in a URL shortener interview, the answer isn't just 'for speed'—it's 'because our read/write ratio exceeds 10:1, caching reduces database load by an order of magnitude while providing sub-10ms latency for the vast majority of requests.' Numbers matter.

System Boundaries and API Design

Defining clear system boundaries and APIs helps us understand exactly what the system does and how external clients interact with it.

Core API Endpoints

Let's define the primary API contract:

api_specification.ts
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// ===============================
// URL Shortener Core API
// ===============================
 
// 1. Create Short URL
// POST /api/v1/shorten
interface CreateShortUrlRequest {
  longUrl: string;              // Required: destination URL
  customAlias?: string;         // Optional: user-requested short code
  expiresAt?: string;           // Optional: ISO timestamp for expiration
  title?: string;               // Optional: descriptive title
  tags?: string[];              // Optional: categorization tags
}
 
interface CreateShortUrlResponse {
  shortCode: string;            // Generated short code (e.g., "a7Xk2B")
  shortUrl: string;             // Full short URL (e.g., "https://short.url/a7Xk2B")
  longUrl: string;              // Original URL (echoed back)
  createdAt: string;            // ISO timestamp
  expiresAt?: string;           // Expiration if set
}
 
// 2. Redirect (Browser-facing, not API)
// GET /{shortCode}
// Response: 301/302 Redirect to longUrl
// Headers: Location: {longUrl}
 
// 3. Get URL Info
// GET /api/v1/urls/{shortCode}
interface GetUrlInfoResponse {
  shortCode: string;
  shortUrl: string;
  longUrl: string;
  createdAt: string;
  expiresAt?: string;
  clicks: number;               // Total click count
  status: "active" | "expired" | "disabled";
}
 
// 4. Get URL Analytics
// GET /api/v1/urls/{shortCode}/analytics
interface GetAnalyticsRequest {
  startDate: string;            // ISO date
  endDate: string;              // ISO date
  granularity: "hour" | "day" | "week" | "month";
}
 
interface GetAnalyticsResponse {
  shortCode: string;
  totalClicks: number;
  uniqueClicks: number;
  clicksByDate: { date: string; clicks: number }[];
  clicksByCountry: { country: string; clicks: number }[];
  clicksByReferrer: { referrer: string; clicks: number }[];
  clicksByDevice: { device: string; clicks: number }[];
}
 
// 5. Update Short URL
// PATCH /api/v1/urls/{shortCode}
interface UpdateUrlRequest {
  longUrl?: string;             // Update destination
  expiresAt?: string;           // Update expiration
  disabled?: boolean;           // Enable/disable
}
 
// 6. Delete Short URL
// DELETE /api/v1/urls/{shortCode}
// Response: 204 No Content

HTTP Status Codes and Error Handling

Precise HTTP status codes communicate system state to clients effectively:

HTTP Status Codes for URL Shortener
Scenario	Status Code	Response
URL created successfully	201 Created	Full ShortUrl response body
Redirect successful	301 or 302	Empty body, Location header
URL not found	404 Not Found	Error message, suggestions
URL expired	410 Gone	Expiration notice
Custom alias taken	409 Conflict	Suggestion for alternatives
Invalid long URL	400 Bad Request	Validation error details
Rate limit exceeded	429 Too Many Requests	Retry-After header
Authentication required	401 Unauthorized	Auth instructions
Server error	500/503	Error ID for support

301 vs 302 Redirect

301 (Permanent) tells browsers to cache the redirect—future visits skip our service entirely. Good for SEO but prevents analytics. 302 (Temporary) makes browsers always hit our service first—essential for click tracking. Most URL shorteners use 302 (or 307) for clicks they want to track, 301 only when explicitly requested.

Requirements Summary and Next Steps

We've completed a thorough requirements analysis for our URL shortener. Let's consolidate the key findings:

Core Operations Summary

Core Operations at a Glance
Operation	Volume	Latency Target	Availability
URL Shortening	100M/day (1.2K/sec)	<200ms	99.9%
URL Redirection	1B/day (12K/sec, 50K peak)	<50ms	99.99%
Analytics Query	~1M/day	<500ms	99.5%

Key Design Drivers

•Extreme Read-Heavy Workload — 10:1+ read/write ratio means optimize aggressively for read latency.
•Short Code Uniqueness — Must guarantee no collisions across billions of URLs.
•Sub-50ms Redirect Latency — Requires multi-layer caching and optimized data paths.
•High Availability for Redirects — 99.99% uptime; redirects must never block on analytics.
•Global Scalability — Must serve users worldwide with consistent performance.
•Flexible Analytics — Collect comprehensive data without impacting core redirect flow.

What's Next

With requirements established, subsequent pages will address:

URL Encoding Strategies — How to generate unique, short codes efficiently
Redirect Latency Optimization — Caching strategies, database selection, global distribution
Analytics Collection — Event streaming, aggregation, and query optimization
Custom Short URLs — Handling user-provided aliases safely
Scaling Reads — Techniques for handling billions of daily redirects

Page Complete

You now have a comprehensive understanding of URL shortener requirements. The three core operations—shorten, redirect, analytics—along with detailed capacity estimates and API design provide the foundation for all subsequent architectural decisions. Next, we'll explore URL encoding strategies for generating unique short codes.

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Loading learning content...

System Design (HLD)URL Shortener

Designing a URL Shortener

LevelIntermediate

Duration90 mins

TopicURL Shortener

1 / 6

Requirements: Shorten, Redirect, Analytics

The Power of Short URLs

What You Will Learn

Understanding the Problem Domain

Before diving into requirements, let's thoroughly understand what a URL shortener actually does and why it exists. A URL shortener is a service that:

Accepts a long URL (e.g., https://www.example.com/products/category/electronics/laptops?brand=dell&sort=price&page=12)
Generates a short, unique alias (e.g., https://short.url/a7Xk2B)
Redirects visitors from the short URL to the original long URL
Collects analytics about those redirects (who clicked, when, from where)

Why do URL shorteners exist?

Primary Use Cases for URL Shorteners

•Character Limit Constraints — Platforms like Twitter (now X) historically had strict character limits. A 200-character URL in a 280-character tweet leaves little room for content. URL shorteners compress URLs to 15-25 characters.
•Readability and Shareability — Long URLs with query parameters are visually unappealing and hard to share verbally or in print. Short URLs are clean and memorable.
•Tracking and Analytics — Marketing teams need to measure which campaigns, channels, and content drive traffic. URL shorteners provide click tracking without modifying the destination site.
•Link Management — Enterprise users need to update destination URLs without changing what's already been shared. URL shorteners enable this indirection layer.
•Security and Privacy — Services can mask destination URLs (for affiliate links) or validate link safety before redirect.
•Branding — Custom domains (e.g., amzn.to, youtu.be) reinforce brand identity while providing URL shortening.

Scale of Major URL Shorteners

Functional Requirements Analysis

Functional requirements define what the system does—the features and capabilities it must provide. For a URL shortener, we identify three core operations plus several supporting features.

Core Operation 1: URL Shortening

The primary function is accepting a long URL and returning a short one. Let's decompose this thoroughly:

URL Shortening Feature Decomposition
Feature Aspect	Requirements	Design Implications
Input Validation	Accept valid HTTP/HTTPS URLs; reject malformed URLs, local addresses, forbidden domains	Need URL parsing, validation logic, blocklist management
Short Code Generation	Generate unique 6-8 character alphanumeric codes	Need encoding scheme, collision handling, uniqueness guarantees
Custom Aliases	Allow users to request specific short codes (premium feature)	Need availability checking, reservation system, alias validation
Expiration Support	Optional TTL (time-to-live) for short URLs	Need expiration tracking, cleanup jobs, expiration metadata storage
User Association	Link short URLs to user accounts (for management)	Need authentication, user-URL mapping, access control
Duplicate Detection	Same long URL → same short code (or new one based on policy)	Need efficient lookup by long URL, policy configuration

Core Operation 2: URL Redirection

The most frequent operation—redirecting users from short URLs to original destinations. This must be extremely fast and highly available:

Redirection Requirements

•HTTP Redirect Response — Return 301 (permanent) or 302 (temporary) redirect with the original URL. Choice affects browser caching and SEO.
•Sub-50ms Latency — Users should perceive instant redirects. Cache hits must be <10ms; cache misses <50ms.
•High Availability — 99.99% uptime minimum. Redirects failing directly impacts all users of those links.
•Graceful Degradation — Show meaningful error pages for expired, deleted, or malicious links.
•Preview Mode — Optional landing page showing destination URL before redirect (security feature).
•Mobile Deep Links — Support redirecting to app-specific URLs on mobile devices.

Core Operation 3: Analytics Collection

Every redirect represents a data point. Analytics transform URL shorteners from simple utilities into valuable marketing infrastructure:

Analytics Data Requirements

•Click Counting — Total clicks per short URL, with real-time or near-real-time updates.
•Temporal Distribution — Clicks over time (hourly, daily, weekly aggregations) for trend analysis.
•Geographic Data — Country, region, city derived from IP address geolocation.
•Referrer Information — Which sites/platforms referred the traffic (Twitter, email, direct, etc.).
•Device/Browser Data — Device type, OS, browser from User-Agent parsing.
•Unique vs. Total Clicks — Distinguish unique visitors from repeat clicks (requires session/fingerprint tracking).

Analytics Cannot Block Redirects

Supporting Features and Edge Cases

Beyond core operations, production URL shorteners require numerous supporting features. These often distinguish basic hobby projects from enterprise-grade systems:

Supporting Feature Requirements
Feature Category	Capabilities	Why It Matters
User Management	Registration, authentication, API keys, rate limits per user	Enable personalized experiences, prevent abuse, monetization
Link Management	Edit destination URLs, enable/disable links, bulk operations	Enterprise users need workflow tools, not just URL generation
API Access	REST/GraphQL APIs with authentication, webhooks for events	Programmatic access for integration with marketing tools, CMS systems
Custom Domains	Users bring their own domains (e.g., link.company.com)	Branding, trust, enterprise requirement
QR Code Generation	Generate QR codes for short URLs	Physical-to-digital bridging for marketing materials
Security Features	Phishing detection, malware scanning, spam prevention	Protect users, maintain service reputation, legal compliance

Critical Edge Cases to Handle

Robust system design anticipates edge cases before they become production incidents:

Edge Cases Requiring Special Handling

•Expired Links — What happens when a TTL-limited link expires? 404? Custom message? Redirect to homepage?
•Deleted Links — Should short codes be recycled? (Generally no—prevents hijacking known links)
•Circular Redirects — User shortens a short URL that eventually points back. Must detect and prevent loops.
•Destination Site Down — Should we detect and warn? Cache last-known-good destination?
•Concurrent Custom Alias Requests — Two users request same custom alias simultaneously. Conflict resolution needed.
•URL Changes — Long URL with tracking params changes slightly. Same short code or new one?
•Unicode/IDN URLs — Support for international domain names and Unicode paths.
•Maximum URL Length — Define limits on input URL length (2000 chars? 8000 chars?).

Scope Management in Interviews

Non-Functional Requirements

Scalability Requirements

Let's establish concrete scale targets that will drive architecture decisions:

Scale Targets for URL Shortener Design
Metric	Target Value	Implication
URL creations per day	100 million (1,157 per second avg, 5,000 peak)	Write-heavy during URL creation, needs efficient storage
Redirects per day	1 billion (11,574 per second avg, 50,000+ peak)	Read-heavy overall; 10:1 read/write ratio is conservative
Total URLs stored	100 billion over 10 years	Massive storage requirements; data retention policies needed
Analytics events per day	1 billion+ (one per redirect)	High-throughput event streaming; aggregate storage
Geographic distribution	Global (6 continents)	Multi-region deployment; data residency considerations

Latency Requirements

URL shorteners are in the critical path for every click. Slow redirects directly impact user experience and can increase bounce rates:

Latency Targets

•URL Creation: <200ms end-to-end (acceptable for write operations)
•Redirect (cache hit): <10ms at server (before network transit)
•Redirect (cache miss): <50ms at server including database lookup
•Redirect P99: <100ms (99th percentile must not exceed this)
•Analytics query: <500ms for standard reports; <5s for complex analytics

Availability and Reliability Requirements

A URL shortener being down means every link using it is broken—potentially millions of links embedded across the internet:

Availability Requirements
Aspect	Requirement	Translation
Redirect availability	99.99% (four nines)	~52 minutes downtime per year
URL creation availability	99.9% (three nines)	~8.7 hours downtime per year
Analytics availability	99.5%	More tolerant; eventual consistency acceptable
Data durability	99.999999999% (eleven nines)	Zero data loss acceptable; use replication
Disaster recovery	RPO: 1 minute, RTO: 5 minutes	Minimal data loss, fast recovery

Read vs. Write Availability Trade-offs

Back-of-Envelope Capacity Estimation

Rough capacity estimation helps validate whether our design decisions are feasible and identifies the primary resource constraints.

Storage Estimation

Let's calculate how much storage we need for URL mappings:

storage_estimation.txt
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URL Storage Calculation
=======================
 
Data per URL entry:
- Short code:          7 bytes (6-8 chars, let's use 7)
- Long URL:          200 bytes (average, can be up to 2KB)
- Creation timestamp:  8 bytes
- Expiration time:     8 bytes (optional)
- User ID:             8 bytes (optional)
- Click count:         8 bytes
- Flags/metadata:     10 bytes
-----------------------------------
Total per URL:       ~250 bytes
 
URLs created per day:        100,000,000
Bytes per day:               100M × 250 = 25 GB/day
Bytes per year:              25 GB × 365 = 9.125 TB/year
Bytes over 10 years:         ~91 TB for URL mappings alone
 
With indexes and replication (3x):
Total storage (10 years):    ~275 TB + indexes ≈ 400 TB

Bandwidth Estimation

Let's estimate network bandwidth for redirection traffic:

bandwidth_estimation.txt
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Bandwidth Calculation
=====================
 
Redirects per day:           1,000,000,000 (1 billion)
Redirects per second (avg):  1B / 86,400 ≈ 11,574 RPS
Redirects per second (peak): ~50,000 RPS (4-5x average)
 
Request size (incoming):
- HTTP headers:         500 bytes (average)
- Short URL in path:    20 bytes
Total incoming:         ~520 bytes per request
 
Response size (outgoing):
- HTTP 301/302 headers: 300 bytes
- Location header:      200 bytes (destination URL)
- Body (minimal):       100 bytes
Total outgoing:         ~600 bytes per response
 
Incoming bandwidth:
- Average: 11,574 × 520 bytes = 6.0 MB/s
- Peak:    50,000 × 520 bytes = 26 MB/s
 
Outgoing bandwidth:
- Average: 11,574 × 600 bytes = 6.9 MB/s
- Peak:    50,000 × 600 bytes = 30 MB/s
 
Total peak bandwidth: ~60 MB/s (~480 Mbps)
Note: This is modest; network is NOT the bottleneck.

Cache Estimation

Caching is critical for achieving low-latency redirects. Let's estimate cache requirements:

cache_estimation.txt
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Cache Sizing Calculation
========================
 
Observation: URL access follows power-law distribution
- 20% of URLs receive 80% of traffic
- Top 1% of URLs receive ~50% of traffic
 
To achieve 90%+ cache hit rate:
- Cache the top 10-20% of URLs by access frequency
 
URLs created per day:    100,000,000
Active URLs (accessed in last 30 days): ~500,000,000
Top 20% active URLs:     100,000,000
 
Memory per cached entry:
- Short code:            7 bytes
- Long URL:            200 bytes
- Metadata (TTL, etc):  50 bytes
- Redis overhead:       50 bytes
Total:                 ~300 bytes
 
Cache size for top 20%:
100M × 300 bytes = 30 GB per region
 
With 5 global regions: 150 GB total cache
(This is very achievable with modern Redis clusters)

Estimation Conclusion

Read/Write Ratio Analysis

Understanding the read/write ratio is fundamental to URL shortener design because it profoundly impacts every architectural decision.

The Extreme Read-Heavy Nature

read_write_analysis.txt
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Read/Write Ratio Calculation
=============================
 
Writes (URL creation):      100 million/day = 1,157/second avg
Reads (redirects):        1,000 million/day = 11,574/second avg
 
Read/Write Ratio: 1000M / 100M = 10:1
 
This is actually conservative. In practice:
- Popular links get clicked thousands of times
- Many URLs created by spammers never get clicked
- Marketing campaigns generate millions of clicks per URL
 
Real-world ratio often: 100:1 or higher

Architectural Implications of Read-Heavy Workload

This extreme read-heavy ratio shapes nearly every design decision:

Design Implications of 10:1+ Read/Write Ratio

•Caching is Essential — With 10x+ more reads than writes, aggressive caching provides enormous benefits. We should cache recent/popular URLs in memory.
•Read Replicas Make Sense — Database read replicas can horizontally scale read capacity. Slight staleness is acceptable for redirects.
•Write Performance is Secondary — We can afford slower writes. Synchronous replication, stricter validation, and more complex write paths are acceptable if they simplify reads.
•Eventually Consistent Analytics — Click counts can lag behind reality. Batch updates and stream processing are appropriate for analytics.
•CDN-Friendly — Redirects for the same short code always return the same response (until updated). CDN caching can reduce origin load dramatically.

Write Path (Optimize for Durability)

•Synchronous replication to multiple zones
•Strong consistency for uniqueness checks
•Validation and sanitization of input URLs
•Write-through cache invalidation
•Asynchronous indexing for analytics

Read Path (Optimize for Latency)

•Multi-layer caching (CDN → Redis → DB)
•Read from nearest replica
•Pre-computed redirects in memory
•Minimal per-request processing
•Asynchronous analytics emission

Key Insight for Interviews

System Boundaries and API Design

Defining clear system boundaries and APIs helps us understand exactly what the system does and how external clients interact with it.

Core API Endpoints

Let's define the primary API contract:

api_specification.ts
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// ===============================
// URL Shortener Core API
// ===============================
 
// 1. Create Short URL
// POST /api/v1/shorten
interface CreateShortUrlRequest {
  longUrl: string;              // Required: destination URL
  customAlias?: string;         // Optional: user-requested short code
  expiresAt?: string;           // Optional: ISO timestamp for expiration
  title?: string;               // Optional: descriptive title
  tags?: string[];              // Optional: categorization tags
}
 
interface CreateShortUrlResponse {
  shortCode: string;            // Generated short code (e.g., "a7Xk2B")
  shortUrl: string;             // Full short URL (e.g., "https://short.url/a7Xk2B")
  longUrl: string;              // Original URL (echoed back)
  createdAt: string;            // ISO timestamp
  expiresAt?: string;           // Expiration if set
}
 
// 2. Redirect (Browser-facing, not API)
// GET /{shortCode}
// Response: 301/302 Redirect to longUrl
// Headers: Location: {longUrl}
 
// 3. Get URL Info
// GET /api/v1/urls/{shortCode}
interface GetUrlInfoResponse {
  shortCode: string;
  shortUrl: string;
  longUrl: string;
  createdAt: string;
  expiresAt?: string;
  clicks: number;               // Total click count
  status: "active" | "expired" | "disabled";
}
 
// 4. Get URL Analytics
// GET /api/v1/urls/{shortCode}/analytics
interface GetAnalyticsRequest {
  startDate: string;            // ISO date
  endDate: string;              // ISO date
  granularity: "hour" | "day" | "week" | "month";
}
 
interface GetAnalyticsResponse {
  shortCode: string;
  totalClicks: number;
  uniqueClicks: number;
  clicksByDate: { date: string; clicks: number }[];
  clicksByCountry: { country: string; clicks: number }[];
  clicksByReferrer: { referrer: string; clicks: number }[];
  clicksByDevice: { device: string; clicks: number }[];
}
 
// 5. Update Short URL
// PATCH /api/v1/urls/{shortCode}
interface UpdateUrlRequest {
  longUrl?: string;             // Update destination
  expiresAt?: string;           // Update expiration
  disabled?: boolean;           // Enable/disable
}
 
// 6. Delete Short URL
// DELETE /api/v1/urls/{shortCode}
// Response: 204 No Content

HTTP Status Codes and Error Handling

Precise HTTP status codes communicate system state to clients effectively:

HTTP Status Codes for URL Shortener
Scenario	Status Code	Response
URL created successfully	201 Created	Full ShortUrl response body
Redirect successful	301 or 302	Empty body, Location header
URL not found	404 Not Found	Error message, suggestions
URL expired	410 Gone	Expiration notice
Custom alias taken	409 Conflict	Suggestion for alternatives
Invalid long URL	400 Bad Request	Validation error details
Rate limit exceeded	429 Too Many Requests	Retry-After header
Authentication required	401 Unauthorized	Auth instructions
Server error	500/503	Error ID for support

301 vs 302 Redirect

Requirements Summary and Next Steps

We've completed a thorough requirements analysis for our URL shortener. Let's consolidate the key findings:

Core Operations Summary

Core Operations at a Glance
Operation	Volume	Latency Target	Availability
URL Shortening	100M/day (1.2K/sec)	<200ms	99.9%
URL Redirection	1B/day (12K/sec, 50K peak)	<50ms	99.99%
Analytics Query	~1M/day	<500ms	99.5%

Key Design Drivers

•Extreme Read-Heavy Workload — 10:1+ read/write ratio means optimize aggressively for read latency.
•Short Code Uniqueness — Must guarantee no collisions across billions of URLs.
•Sub-50ms Redirect Latency — Requires multi-layer caching and optimized data paths.
•High Availability for Redirects — 99.99% uptime; redirects must never block on analytics.
•Global Scalability — Must serve users worldwide with consistent performance.
•Flexible Analytics — Collect comprehensive data without impacting core redirect flow.

What's Next

With requirements established, subsequent pages will address:

URL Encoding Strategies — How to generate unique, short codes efficiently
Redirect Latency Optimization — Caching strategies, database selection, global distribution
Analytics Collection — Event streaming, aggregation, and query optimization
Custom Short URLs — Handling user-provided aliases safely
Scaling Reads — Techniques for handling billions of daily redirects

Page Complete

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