To use this application you need to own a suitable short domain.
There is a demo here which can be used to try the shortener out. It runs on the free tier of App Engine so will run out of resources if used heavily.
There are two reasons for creating your own shortening service.
First, there are circumstances where people have to type in short
URLs. In our case we are building mobile guides and short URLs
are one way for visitors to discover the guides. Traditional short
URLs uses a combination of lower and upper-case letters and digitsL.
This is good for making sure that URLs are short, but it can be a pain
to type digits and capitals on a mobile device. Hence our shortener
uses only the 26 lower-case characters a-z.
Second, when it comes to analytics, it can be useful for more than one short URL map to a given long URL. For instance, there may be multiple physical locations which display the same URL and it may be important to know which one was accessed. In this application, using the REST API, its possible for multiple short URLs mapping to a single long URL.
Using only the lower case latin characters is not as big a drawback as it seems if you own a domain as its likely that the number of URLs you want to encode won't exceed the low millions. You can get 300 million URLs into 6 lower case characters, and its much easier to type 6 lower-case characters than 4 mixed characters on a mobile.
The implementation is written in Java and runs on Google App Engine. A demo is available here. The demo uses App Engine's free tier, so may not be available if heavy use is being made of it.
You can run you own shortener (but, of course, you'll need your own short domain) by installing it from Github
The most interesting aspect of the shortener is the generation of the short URLs when the implementation is distributed. We have chosen to create a generator which eventually iterates over all the long integers. The generation isn't in strict order (its not clear what this would mean in a distributed system) but assuming a fair random number generator every integer should eventually be output.
The implementation uses a sharded generator, with a central distribution entity which provides each shard with a range of numbers. The shards distribute the numbers and get replenished from the central entity when they are exhausted.