# Design a url shortner


A URL shortener is a proxy service that provides a mapping between the short and full representation of a URL. The short URL has the advantage of being small. The service can also provide useful analytics.

The API provides two endpoints:

1. `POST /api/v1/data/shorten` `payload: { url: string }`
2. `GET /api/v1/{shortUrl}`

## Back of the envelope calculations

### Requests

Assume

> 100M writes / day

With ~100K seconds a day

> = ~1K writes / second

Assuming a typical `10:1` read:write ratio

> = ~10K reads / second

We'd need to cache.

### Storage

Having

> 100M records / day

at `100 bytes per record` is

> ~= 1 GB / day ~= 400 GB / year

## Redirects

- `301` — permanent, where the browser caches subsequent requests and reduces load.
- `302` — temporary, where the browser's requests always query the service first, which is useful for analytics.

## Encoding mechanism

- Hashes are too long, and prefixes could result in collisions that need to be handled.
- Using a unique integer that is [base62](https://en.wikipedia.org/wiki/Base62) encoded avoids collisions at the expense of enumeration attacks.

Why base62? With 62 characters `[a-zA-Z0-9]`, a 7-character short code covers `62^7 ~= 3.5T` URLs — plenty of headroom for the storage estimate above.

Happy Hackin'!

### References

- [base62 encoded](https://en.wikipedia.org/wiki/Base62)

* * *

> This article is part of the [system design series](https://blog.danwald.me/series/system-design) where I am summarizing chapters from The System Design Interview: [Volume 1](https://www.amazon.ae/Independently-Published-System-Design-Interview/dp/B08CMF2CQF) / [Volume 2](https://www.amazon.ae/System-Design-Interview-Insiders-Guide/dp/1736049119) amongst other related content