On August 7, 2006, Apple previewed Mac OS X 10.5 Leopard at the WWDC. As usual, some new features received a lot of excitement whereas some received criticism. Time Machine, Apple's automated backup solution, received acclaim from many people because, as usual, Apple makes the process seem so easy and of course even manages to make it fun with the eye candy and intuitive interface. Some people yawned at the app, claiming that it doesn't deserve any attention because it is just a glorified backup app, which has been available in Windows for years.
Summing it up as a glorified backup app is an obscene oversimplification. As a developer, I immediately recognized in Time Machine the likelihood that it is running on top of a revision control system, and that is what really fascinates me about it, and incidentally that is also what I feel many people misunderstand about it, leading them to discount it. I'm not alone in making this connection; really, any developer who uses and understands source code repositories probably had the thought cross his or her mind. One of the major advantages mentioned in discussions about revision control is the fact that you can always retrieve your file as it existed at any point in the past when it was committed to the repository.
And it's not a stretch to see Apple borrowing revision control software to implement Time Machine. Subversion is a very popular revision control project in the opensource community. Sitting on top of FreeBSD, Apple has incorporated many other opensource projects in Mac OS X. For instance, OpenGL, Apache, SSH, FTP, CUPS, and Samba come to mind, just to name a few. Why not add Subversion to the list of excellent technologies to incorporate into an excellent OS?
If Time Machine is running on top of Subversion or a similar revision control system, that would address the concerns that some people have--people who might not understand how revision control systems work. Some people have expressed a concern that a backup drive would not be able to support Time Machine for very long before filling up. For instance, if you have a 1GB file (perhaps a video clip) and make a very small change to it, will Time Machine record that change by making another copy of the entire file? If Time Machine made its backups as full copies of the changed files, you can imagine how quickly a backup volume would fill up. But that simply isn't how most revision control software works. Most revision control software, including Subversion, uses Delta Compression to calculate only the changes made to a file, and then saves only those changes. Thus, a file can be committed to a repository a dozen times, and yet the amount of drive space taken up would only amount to the changes made, plus perhaps a very slight amount of overhead in the repository for tracking each change.
So that covers feasibility. With some software like Subversion, Time Machine could provide a backup of your entire volume and actually support it for a decent period of time thanks to efficient delta compression. What about the logistics of finding the changed files and saving those changes? This would be simple even with Subversion in its present state. Out of the box, Subversion provides the ability to search for files that have changed since the last "commit", or the last time changes were saved. Time Machine would just have to ask Subversion to report all the files that have changed, then run the commit. This could be scheduled to execute at a given time, say, midnight. In the event that the computer was off at the scheduled time, the steps could be executed immediately upon startup.
In the past, I've thought how nice it would be if all the files on my computer could go under revision control just like my source code when I'm working on an application. It would be just Apple's style to make that possible not just for tech geeks who use Subversion, but to make it possible for anyone, even the guy who doesn't care a lick about understanding revision control.