How URL Shortener works

URL Shortener, as text

Turns long links into short ones and redirects fast, with a cache in front of the database.

Clients

Browser

Client · Hypothetical. Someone creating a short link, or clicking one.

Gateways

Load balancer

Gateway · Hypothetical. Spreads requests across the service instances.

Services

Shorten API

Service · Hypothetical. Creates a short code for a long URL.

ID generator

Service · Hypothetical. Hands out unique numbers so two links never share a code.

Redirect service

Service · Hypothetical. Looks up a short code and redirects the browser.

Queues

Click events

Queue · Hypothetical. Records each click for analytics without slowing the redirect.

Caches

Link cache

Cache · Hypothetical. Keeps popular short codes in memory.

Databases

Link database

Database · Hypothetical. The durable record of every short code and its long URL.

Flow: Create a short link

From pasting a long URL to getting a short link back.

1. Browser → Load balancer (Shorten this URL). Someone pastes a long URL and presses Shorten.
2. Load balancer → Shorten API (Shorten this URL). The load balancer sends the request to one of the shorten API instances.
3. Shorten API → ID generator (Next id?). The shorten API asks for the next unique number.
4. ID generator → Shorten API (1048576). It gets a number no one else will get, and encodes it in base62 as a short code like aZ3k9.
5. Shorten API → Link database (Save aZ3k9). It saves the short code together with the long URL.
6. Link database → Shorten API (Saved). The database confirms the link is stored.
7. Shorten API → Load balancer (Short link). The shorten API answers with the new short link.
8. Load balancer → Browser (Short link). The browser shows the short link, ready to copy.

Flow: Follow a short link

From clicking a short link to landing on the long URL.

1. Browser → Load balancer (GET /aZ3k9). Someone clicks a short link.
2. Load balancer → Redirect service (GET /aZ3k9). The load balancer sends the click to one of the redirect service instances.
3. Redirect service → Link cache (Look up aZ3k9). The redirect service checks the cache first.
4. Link cache → Redirect service (Long URL). Most of the time the cache already has it, so no database call is needed.
5. Redirect service → Load balancer (301 redirect). The redirect service answers with a redirect to the long URL. It also drops a click event on the queue for analytics, without waiting for it.
6. Load balancer → Browser (301 redirect). The browser follows the redirect and opens the long page.