System Design: Design a URL Shortener

In this exercise, you need to design a URL Shortener service like bit.ly or tinyurl. Your solution should address how to create a short version of a long URL, redirect users from the short URL to the original long URL, and handle potential challenges such as rate limiting, managing the database, and scaling.
Key Considerations: How do you ensure that the shortened URL is unique? How will you implement a cache for the redirects to improve performance? What kind of database schema would you use?