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Chapter 14. Adding Dependency Information to a Package
Since the very first version of RPM hit the streets, one of the side effects of RPM's ease of use was that it made it easier for people to break things. Since RPM made it so simple to erase packages, it became common for people to joyfully erase packages until something broke.
Usually this only bit people once, but even once was too much of a hassle if it could be prevented. With this in mind, the RPM developers gave RPM the ability to:
Build packages that contain information on the capabilities they require.
Build packages that contain information on the capabilities they provide.
Store this "provides" and "requires" information in the RPM database.
In addition, they made sure RPM was able to display dependency information, as well as to warn users if they were attempting to do something that would break a package's dependency requirements.
With these features in place, it became more difficult for someone to unknowingly erase a package and wreak havoc on their system.
An Overview of Dependencies
We've already alluded to the underlying concept for RPM's dependency processing. It is based on two key factors:
Packages advertise what capabilities they provide.
Packages advertise what capabilities they require.
By simply checking these two types of information, many possible problems can be avoided. For example, if a package requires a capability that is not provided by any already-installed package, that package cannot be installed and expected to work properly.
On the other hand, if a package is to be erased, but its capabilities are required by other installed packages, then it cannot be erased without causing other packages to fail.
As you might imagine, it's not quite that simple. But adding dependency information can be easy. In fact, in most cases, it's automatic!