Design a URL Shortening Service
Company: DoorDash
Role: Software Engineer
Category: System Design
Difficulty: medium
Interview Round: Technical Screen
Design a service that creates short links for destination URLs and redirects requests for those short links.
### Clarifying Questions
The report supplies the system name without workload details. Clarify link creation rate, redirect traffic, expiration, custom aliases, editability, and abuse controls. For a baseline, choose generated immutable aliases with optional expiration; these are design choices rather than reported requirements.
### What a Strong Answer Covers
Creation and redirect APIs, collision handling, durable mappings, cache behavior, availability, and validation. Explain which consistency requirements apply to newly created, expired, or disabled links.
### Follow-up Questions
- How would you prevent a cached mapping from continuing to serve after a link is disabled?
- What changes if users may edit destinations after creation?
Overview: Design URL creation and redirect flows with durable unique aliases and explicit cache behavior. Address collisions, newly created links, expiration, disablement, and the tradeoffs introduced by editable destinations.