Testing

We'll scope the problem to handle only the following use cases

User enters a block of text and gets a randomly generated link
Expiration
Default setting does not expire
Can optionally set a timed expiration
User enters a paste's url and views the contents
User is anonymous
Service tracks analytics of pages
Monthly visit stats
Service deletes expired pastes
Service has high availability
Out of scope

User registers for an account
User verifies email
User logs into a registered account
User edits the document
User can set visibility
User can set the shortlink
Constraints and assumptions

State assumptions

Traffic is not evenly distributed
Following a short link should be fast
Pastes are text only
Page view analytics do not need to be realtime
10 million users
10 million paste writes per month
100 million paste reads per month
10:1 read to write ratio
Calculate usage

Clarify with your interviewer if you should run back-of-the-envelope usage calculations.

Size per paste
1 KB content per paste
shortlink - 7 bytes
expiration_length_in_minutes - 4 bytes
created_at - 5 bytes
paste_path - 255 bytes
total = ~1.27 KB
12.7 GB of new paste content per month
1.27 KB per paste * 10 million pastes per month
~450 GB of new paste content in 3 years
360 million shortlinks in 3 years
Assume most are new pastes instead of updates to existing ones
4 paste writes per second on average
40 read requests per second on average
Handy conversion guide:

2.5 million seconds per month
1 request per second = 2.5 million requests per month
40 requests per second = 100 million requests per month
400 requests per second = 1 billion requests per month
Step 2: Create a high level design

Outline a high level design with all important components.