Maximum RPM (RPM):An Introduction to Package Management:EarthWeb Inc.-






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It would be nice to be able to get more detailed information on a specific
package. This might consist of anything from the date the package was installed to a list of files it installed on your system.
Being able to access this information in a variety of ways can be helpful, too.
Instead of simply finding out what files a package installed, it might be handy to be able to name a particular file and find out which package installed it.
If this amount of
detail is possible, it should be possible to see if the way a package
is presently installed varies from the way it was originally installed. It's not at all
unusual to make a mistake and delete one file—or 100. Being able to tell if one or
more packages are missing files could greatly simplify the process of getting an ailing
system back on its feet again.
Files containing
configuration information can be a real headache. If it were
possible to pay extra attention to these files and make sure any changes made to them
weren't lost, life would certainly be a lot easier.



1.2. Package Management: How to Do It


Well, all this sounds great: easy install, upgrade, and deletion of packages; getting package
information presented several different ways; making sure packages are installed correctly; and even tracking changes to config files. But how do you do it?


As mentioned earlier, the obvious answer is to let the computer do it. Many groups have
tried to create package management software. There are two basic approaches:



Some package management systems
concentrate on the specific steps required to manipulate a package.
Other package management systems take a different approach, keeping track of the files on the system and manipulating packages by concentrating on the files involved.


Each approach has its good and bad points. In the first method, it's easy to install new
packages, somewhat difficult to remove old ones, and almost impossible to obtain any
meaningful information about installed packages.


The second method makes it easy to obtain information about installed packages and
fairly easy to install and remove packages. The main problem with using this method is that
there may not be a well-defined way to execute any commands required during the installation
or removal process.


In practice, no package management system uses one approach or the other; all are a
mixture of the two. The exact mix and design goals will dictate how well a particular package
management system meets the needs of the people using it. At the time Red Hat Software started
work on its Linux distribution, there were a number of package management systems in use,
each with a different approach to making package management easier.


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1.2.1. The Ancestors of RPM


Because this is a book about the Red Hat Package Manager, a good way to see what RPM is
all about is to look at the package management software that preceded RPM.



1.2.1.1. RPP


RPP was used in the first Red Hat Linux distributions. Many of RPP's features would be
recognizable to anyone who has worked with RPM. Some of these innovative features are



Simple,
one-command installation and uninstallation of packages.
Scripts that
can run before and after installation and uninstallation
of packages.
Package verification.
The files of individual packages can be checked to see that
they haven't been modified since they were installed.
Powerful querying. The database of packages can be queried for information about installed packages, including file lists, descriptions, and more.


Although RPP possessed several of the features that were important enough to continue on
as parts of RPM today, it had some weaknesses, too:



It didn't use
pristine sources. Every program is made up of programming
language statements stored in files. This source code is later translated into a binary
language that the computer can execute. RPP's packages were based on source code that had been modified specifically for RPP. Since the sources were no longer as the program's creator had written them, they were no longer
pristine. This is a bad idea for a number of fairly technical reasons. Not using pristine sources made it difficult for
package developers to keep things straight, particularly if they were building hundreds
of different packages.
It couldn't guarantee that executables were built from packaged sources. The
process of building a package for RPP was such that there was no way to ensure that
the executable programs were built from the source code contained in an RPP
source package. Once again, this was a problem for package builders, especially those
who had large numbers of packages to build.
It had no
support for multiple architectures. As people started using RPP, it
became obvious that the package managers that were unable to simplify the process of building packages for more than one architecture, or type of computer, were going to be
at a disadvantage. This was a problem, particularly for Red Hat Software, because it was starting to look at the possibility of creating Linux distributions for other
architectures, such as the Digital Alpha.


Even with these problems, RPP was one of the things that made the first Red Hat Linux
distributions unique. Its ability to simplify the process of installing software was a real boon to
many of Red Hat's customers, particularly those with little experience in Linux.


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1.2.1.2. PMS


While Red Hat Software was busy with RPP, another group of Linux devotees was hard
at work with its package management system. Known as PMS, its development, lead by Rik
Faith, attacked the problem of package management from a slightly different viewpoint.


Like RPP, PMS was used to package a Linux distribution. This distribution was known as
the BOGUS distribution, and all the software in it was built from original unmodified sources.
Any changes that were required were patched in during the process of building the software. This
is the concept of pristine sources and is PMS's most important contribution to RPM. The
importance of pristine sources cannot be overstated. It allows the packager to quickly release
new versions of software and to immediately see what changes were made to the software.


The chief disadvantages of PMS were weak querying ability, no package verification, no
multiple architecture support, and poor database design.



1.2.1.3. PM


Later, Rik Faith and Doug Hoffman, working under contract for Red Hat Software,
produced PM. The design combined all the important features of RPP and PM,
including one-command installation and uninstallation,
scripts run before and after installation and uninstallation,
package verification, advanced querying, and pristine sources. However, it retained RPP's and
PMS's chief disadvantages: weak database design and no support for multiple architectures.


PM was very close to a viable package management system, but it wasn't quite ready for
prime time. It was never used in a commercially available product.



1.2.1.4. RPM Version 1


With two major forays into package management behind them, Marc Ewing and Erik
Troan went to work on a third attempt. This one would be called the
Red Hat Package Manager,
or RPM.


Although it built on the experiences of PM, PMS, and RPP, RPM was quite different
under the hood. RPM was written in the Perl programming language for fast development.
Version 1 of RPM focused on addressing the flaws of its ancestors. In some cases, the flaws were
eliminated, while in others the problems remained.


Some of the successes of RPM version 1 were



Automatic handling
of configuration files. The contents of config files are
often changed from what they were in the original package, making it
hard for a package manager to know how a particular config file should be handled during
installations, upgrades, and erasures. PM made an attempt at config file handling, but in RPM
it was further improved. In many respects, this feature is the key to RPM's power
and flexibility.



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