Design a system to take user-provided URLs and transform them to a shortened URLs that redirect back to original.
Questions:
1. How many URLs shall we keep?
2. Read > Write ?
Solution:
1. URL encoding:
URL --> (a...zA..Z0..9-_) 65^n URLs.
hash: md5 etc.
How to resolve collision?
2. Cache:
KEY-VALUE
LFU or LRU: LFU is better, Frequency
Multi Cache Servers
ZooKeeper
3. Reliability:
4. Rate limit
Block user when requests more than 10/min or 100/hour or 1000/day ...
one bucket every one minute
Memcache
key: 1400_op_username
value: counter
原文：http://www.cnblogs.com/litao-tech/p/4199507.html