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As we saw in the first half of this book, RPM can make life much easier for the user. With automated installs, upgrades, and erasures, RPM can take a lot of the guesswork out of keeping a computer system up-to-date.
But what about people that sling code for a living? Does RPM have anything to offer them? The answer is yes! One of the best things about RPM is that although it was designed to make life easier for users, it was written by people that would be using it to build many packages. So the design philosophy of RPM has a definite bias toward making life easier for developers. Here are some of the reasons you should consider building packages with RPM:
While many developers might use RPM to package their own software, just as many, if not more, are going to be packaging software that they have not written. Because of this, there are some aspects to RPM's design that are geared toward "third-party" package builders. One such aspect is RPM's use of "pristine" sources.
When a third-party package builder decides to package someone else's software, they often get the software from the Net, normally as a tar file compressed with something like GNU zip. That's probably about the only generalization we can make when talking about software that is eligible for packaging. Once we look inside the tar file, there are a world of possible differences:
The application could be available in pure source form, in pure binary form, or some combination of both.
The application might have been written to be built using make, imake, or a script included with the sources. Or, it might have to be built entirely by hand.
The application might need to be configured prior to use. Maybe it uses GNU configure, a custom configuration script, or one or more files that need to be edited to reflect the target environment.
The application might have been written to reside in specific directories, and those directories do not exist, or are not appropriate on the target system.
The application might not even support the target environment, requiring all manner of changes to port it to the target environment.
We could go on, but you probably get the idea. It's a rare application that comes off the Net ready to package, and the changes required vary widely. What to do?
This is where the concept of pristine sources comes in. RPM has been designed to use the sources as they come from the application's developer, no matter how it has been packaged and configured. The main benefit is that the changes you as a package builder need to make, remain separate from the original sources, in a distinct collection of patches.
This may not sound like much of an advantage, but consider how this would work if a new version of the application came out. If the new version had a few localized bug fixes, it's entirely possible the original patches could be applied, and a new package built, with a single RPM command. Even if the patches didn't apply cleanly, it would at least give an indication as to what might need to be done to get the new version built and packaged.
If your users sometimes customize packages, having pristine sources makes it easier for them, too. They can see what patches you've created and can easily add their own.
Another benefit to using pristine sources is that it makes keeping track of multiple versions of a package simple. Instead of keeping patched sources around, or battling a revision control system, it's only necessary to keep:
The original sources in their tar file.
A copy of the patches you applied to get the application to build.
A file used by RPM to control the package building process.
With these three items, it's possible to easily build the package at any time. Keeping track of multiple versions only entails keeping track of each version of these three components, rather than hundreds or thousands of patched source files.
In fact, it gets better than that. RPM can also build a source package containing these three components. The source package, named using RPM's standard naming convention, keeps everything you need to recreate a specific version of a package, in one uniquely named file. Keeping track of multiple versions of multiple packages is simply a matter of keeping the appropriate source packages around. Everything else can be built from them.