How to design a Url shortener

The Framework proposed in this space (Alex Xu) is applied to propose a design : Getting started - a framework to propose...

Introduction

URL shortening is used to create shorter aliases for long URLs. We call these shortened aliases “short links.” Users are redirected to the original URL when they hit these short links. Short links save a lot of space when displayed, printed, messaged, or tweeted. Additionally, users are less likely to mistype shorter URLs.

//Shorten long URL through TinyURL
from https://www.educative.io/courses/grokking-the-system-design-interview/m2ygV4E81AR
to https://tinyurl.com/rxcsyr3r

Step 1 - Understand the problem and establish design scope

How it works ?	URL https://www.blablablabla.com/q=chatsystem&c=logged&cl=long is the original URL and we create un alias with shorter length: https://tinyurl.com/y7keocwj
Traffic volume ?	100 Million URLs per day
How long is the shortened url ?	As short as possible.
Characters in the shortened URL ?	Combination of numbers and characters (a-z, A-Z).
Can it be deleted or updated ?	CANNOT be deleted or updated.

Back-of-the-envelope calculation

For Traffic

Write operations: 100 million URLs are generated per day
Write operation per second: 100 million / 24 / 3600 sec = 1160 operation per sec.
Read operation: Assuming 1160 * 10 = 11600 read per sec.
Assuming the URL shortener service runs for 10 years, this means we must support 100 Million * 365 * 10 = 365 billion records.
Assuming average length is 100

For Storage

Storage requirement over 10 years : 365 billion * 100 bytes = 36.5 TB.

On this page.

Step 2 - High-level design

API endpoints

A URL shortener primary needs 2 API endpoints :

URL Shortening : to create a new short url, a client sends a POST request which contains one parameter: the original long URL.
- POST api/v1/data/shorten : return short url.
URL redirecting : to redirect a short URL to the corresponding long URL, a client sends a GET request.
- GET api/v1/shortUrl : return long url for http redirection.

URL redirecting

301 redirect Shows that the request URL is permanently moved to the long URL. So, the browser caches the response, and subsequent requests for the same URL will not be sent to the URL shortening service. Instead, requests are redirected to the long URL server directly.

URL shortening

How do we encode the actual URL ? www.tinyurl.com/{hashValue}.

Requirements :

Each longURL must be hashed to one hashValue.
Each hashValue can be mapped back to the longURL.

Step 3 - Design deep dive

Data Model

Everything is stored in a hash table, but in the real world systems, it’s limited and expensive. A better option is to store <shortURL, longURL> mapping in a relational DB.

URL
PK	id
	shortURL
	longURL

URL Shortening

It’s expensive to query the DB to check if shortURL exists for every request. We can cache URLs that are frequently accessed.

Scenario 1: URL is expired or user do not have permission

Scenario 2: URL not found and find original URL

Scenario 3: URL found and return error or redirect to original url

URL Redirecting

As there are more reads than writes, <shortURL, longURL> mapping is stored in a cache to improve performance. However, Load Balancer play a role in the URL redirecting.