System Design Questions

Core Requirements:

• Generate a unique short alias for a given URL.
• Redirect users when they access the short URL.

Architecture Components:

1. API Servers: To handle incoming read/write requests.
2. Database: A NoSQL database (like Cassandra or MongoDB) to store the mapping of short_url -> long_url.
3. Key Generation Service (KGS): A standalone service that pre-generates random 6-character strings and stores them in a DB. When a new request comes in, KGS provides a pre-generated key.
4. Caching: Redis or Memcached to store frequently accessed URLs.

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Scenario: DB becomes slow.

Fix: Vertical (Increase RAM/CPU - easy but limited) vs Horizontal (Add more machines - hard but infinite).

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Scenario: Network partition occurs. Choice: Consistency (CP - e.g., Zookeeper) or Availability (AP - e.g., Cassandra).

Scenario: In a distributed cache, 1 server goes down.

Fix: Use Ring topology so that only 1/N keys are re-assigned, and the entire cache is not reset.

Scenario: Server capacities are different.

Fix: Round Robin will fail. Use Least Connections or Weighted Round Robin.

Scenario: Need to stream video fast to global users.

Fix: Cache content on edge servers. Push CDN (upload) vs Pull CDN (on-demand).