design a short url

use cases: 
1.Convert   convert_to_62based(md5(url)+random())
2.Redirect 
3.Delete? 
4.Premium/self defined key
5.UI for convert or API? 
6.URL expire? 
7. Show ads before redirect?
8.scale and performance 
100 requests/seconds, 10 convert, 90 redirect
1year new added  365*24*3600*10= 315M entries
each item 500bytes disk. that's 160G bytes. 5year - 1T?
9. more details of implementation - key-value store: Cassandra (https://en.wikipedia.org/wiki/Apache_Cassandra),  mySQL or plain text file, memory cache, etc


reference:
http://www.hiredintech.com/system-design/

https://developers.google.com/url-shortener/v1/getting_started