Popular System Design Interview Question:
How to design a URL shortener?

Here is how to answer with a step-by-step approach: ↓

1. Requirements

Functional requirements:

• A user enters a URL and gets back a short URL

• The short URL redirect to the long URL when accessed

• The lifetime of the short URL can be customized

• The usage of the short URL can be tracked for analytics

Non Functional requirements:

• High availability

• Minimal latency for URL redirection

• each short URL shall be unique

1. Design the Data Model

We create three tables:

• ShortURL (urlId, creationDate, userID, longURL) -> ~= 500 bytes

• User (userId, creationDate, lastUpdate, email) -> ~= 100 bytes

• UsageMetric (metricId, url_id, timestamp, metric_type) -> ~= 200 bytes

UsageMetric has a one to many strong relationship with ShortURL.

User has a one to many weak relationship with ShortURL.

1. Back of the Envelope estimation

• 10M users / create 3 URL per month -> 30M writes and 2B reads per month -> ~= 12 writes, 770 reads per second

1. High level system design

There are two main data flows: request a short URL and access a short URL. From the user device, each request hit the Load Balancer before going to a web server. Without the load balancer, the requests would be unevenly distributed to the servers, causing congestions and failures.

The functionalities for requesting and accessing a short URL are encapsulated by two different set of APIs (write and read APIs) handled by separated services.

To avoid single point of failures, the database can be replicated using a single-leader architecture. The write requests go to the leader, the read requests to the replicas. An in-memory cache is also used to reduce latency. Since the requests do not require any complex joins or strong relationships with data a NoSQL database using urlId as primary key fits the requirements.

1. Analytic Service

A frontend web page sends requests to the analytic service and displays the analytics to the users. If real-time statistics are not required, the service can be implemented by batch jobs running at regular intervals.

h/t: Fernando Franco