Over the past couple of days, I've implemented a private URI shortener service for myself, which I have named "Rshrtnr". The derivation of the name is left as an exercise for the reader.
My main motivation for writing it was a criticism of public URI shortening services that I have been seeing in blogs for a long time: if the service has some downtime or suddenly disappears, all of the links that you have created with it are useless. With my approach, I regain some control of where my shortened links point to, and if the service has downtime and/or disappears, I have more options for restoring it.
The code itself is written in Python. SLOCCount says that the core module runs at around 100 SLOC. Most of my time, however, was taken up by working around problems relating to my webhost's Python installation. The supported Python version is 2.4.x, which is ridiculously old (for reference, Gentoo was the last major Linux distribution to switch from Python 2.4 to 2.5, around July 2008). Additionally, for some reason, if I attempt to change the sys.path variable (i.e., the "include path") to use locally installed modules (I am on a shared host), the entire script breaks with zero logged messages anywhere. It runs fine via the command line, but in FastCGI mode, the strangeness occurs.
The two third-party modules that I used were Paste and mysql-python. I store the URIs and their associated aliases in a simple SQL table, and I use Paste for various WSGI/HTTP-related utilities. I "manually" handle routing via parsing the PATH_INFO environment variable.
There are two ways to specify an alias: either explicitly send a custom one as a query parameter with the URI, or let the app make a random one for you. With the latter behavior, it hashes the URI to generate an eight character "unique" alias. Since there are (in theory) 64^8 possibilities, I don't think I'll run out of aliases any time soon, especially since custom aliases can be anywhere from 1 to 15 characters long.
In my opinion, the most interesting feature is that adding URIs requires one to send an OpenPGP-encoded query string, which needs a public key recognized by the app for the operation to succeed. To write this, I simply parsed the output from sending the OpenPGP message to the gpg binary.
Finally, mod_rewrite magic is used to prettify the shortened URIs. Nothing too exciting about that part.
I had thought about hosting a version of Rshrtnr on Google App Engine, but a key component is missing - OpenPGP support.
If anyone wants me to release it, please comment below. There's currently a bunch of webhost-specific things that I would need to abstract out before I release the code to the general public, and unless someone gives me a very good reason, it will be licensed under the AGPL version 3.