Requirements:

bottle
redis

Installation:

$ sudo pip install bottle
$ sudo pip install redis
$ wget http://redis.googlecode.com/files/redis-2.4.17.tar.gz
$ tar xzf redis-2.4.17.tar.gz
$ cd redis-2.4.17
$ make


Running URL Shortener:

$ src/redis-server

$python urlshortener.py

$python test_urlshortener.py


Design trade-offs:

The URL shortener service needs to be very fast with less latency.
THe key-value storage schema of redis comes in handy while delivering 
quick lookup and redirection service to the clients. 
The in-memory persistent storage also helps in reducing the latency of the service.
Redis provides us with mechanism to back-up the in-memory data onto the disk.
Depending on the needed reliability and latency constraints we can back-up the
data onto disks as frequently necessary without causing performance delays.

I have used HTTP 301 redirect instead of the HTTP 302 redirect. 
Using Http 301 informs the client that the website it is requesting for has 
moved permanently to the new address, which in our case would point to 
the long_url. Whereas Http 302 informs the client that the move is temporary. 
Also using 301 redirects will cause the long_url to show up in search engine results 
whereas 302 causes the shortened url to show up in the search engine results.


References:

http://bottlepy.org
https://github.com/andymccurdy/redis-py
http://redis.io