Requirements

Functional Requirements:

• Upload & store text/code — a user can paste or write content and the system stores it.
• Generate a unique shareable URL — each paste gets a short, unique link (e.g., pastebin.com/abc123) the user can share.
• Retrieve paste by URL — anyone with the link can view the content.
• Expiration / TTL — pastes live for a set period (e.g., 24h, 1 week, or a custom time) then auto-delete.

Non-Functional Requirements:

• High availability — this is a link-sharing service; people open pastes hours or days later. If the site is down, those links are dead on arrival. Availability beats consistency here (worst case: someone sees a stale paste, not an error).
• Low read latency — Pastebin is heavily read-heavy (think 10:1 read-to-write ratio). The "open a link" path is the hottest path in the system, so reads should be fast and cheap. This will drive your caching strategy later.
• Scalability — assume millions of users. The system should grow horizontally as paste creation and traffic spike (e.g., Reddit Hug-of-Death on a popular paste).

Capacity Estimation

DAU - 5000000

Pastes per day - 1000000

Read to write ratio - 10:1

RPS(write) = 1000000 / 86400 = 12

RPS(read) = 120

Avg paste size = 10 KB

Storage per day = 1000000 * 10 = 10 GB/day

Storage over 5 year = 10 * 365 * 5 = 18 TB

Bandwidth = 120 * 10 KB = 1200

API Design

POST /pastes

{

"content": "...",

"expiration": "24h"

}

-> 201 Created

{

"key": "abc123",

"url": " https://pastebin.com/abc123",

"expire_at": "..."

}

GET /pastes/abc123

-> 200 Ok

{

"content": "...",

"created_at": "...",

}

-> 404 Not Found

{

"error": "Paste expired or does not exist"

}

High-Level Design

CDN - the client sends a request to the CDN to find cacheable content

Load Balancer - if the required content is not cached, the request is sent to a load balancer to distribute the load.

API Gateway - accepts incoming requests from clients and redirects them to the required microservices

Expiration Service - a microservice for checking and monitoring content for elapsed time; if the availability time is exceeded, a message is returned stating that the content is no longer available

Content Service - microservice for working with content that creates new texts by link and returns existing content by short link

Short URL Service - microservice for working with short URLs that creates new short links and returns the full URL for a short link.

Redis - a database for storing the mapping of Short URLs to full URLs

PostgreSQL - database for storing texts

Database Design

Define the data model. Identify the main entities, their attributes, and relationships. Consider the choice of database type (SQL vs NoSQL) and justify your decision based on access patterns...

Detailed Component Design

Deep dive into 2-3 key components. Explain how they work, how they scale, discuss tradeoffs, capacity, and any relevant algorithms or data structures.