glfs/introduction/important/pkgmgt.xml

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<sect1 id="intro-important-pkgmgt">
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<title>Package Management</title>

<para>Package Management is an often requested addition
to the <acronym>LFS</acronym> Book. A Package Manager allows tracking
the installation of files making it easy to remove and upgrade packages.
And before you begin to wonder, NO&mdash;this section does not talk about any
particular package manager, nor does it recommend one. What it provides is
a roundup of the more popular techniques and how they work. The perfect
package manager for you may be among these techniques or may be a combination
of two or more of these techniques. This section briefly mentions
issues that may arise when upgrading packages.</para>

<para>Some reasons why no package manager is mentioned in <acronym>LFS</acronym>
or <acronym>BLFS</acronym>:</para>

<itemizedlist>
<listitem><para>Dealing with package management takes the focus away from
the goals of these books&mdash;teaching how a Linux system is built.</para></listitem>
<listitem><para>There are multiple solutions for package management, each having
its strengths and drawbacks. Including one that satifies all audiences is
difficult.</para></listitem>
</itemizedlist>

<para>There are some hints written on the topic of package management. Visit
the <ulink url="http://www.linuxfromscratch.org/hints/">Hints subproject</ulink>
to find if one of them fits your need.</para>


<sect2>
<title>Upgrade Issues</title>

<para>A Package Manager makes it easy to upgrade to newer versions when they
are released. Generally the instructions in the <acronym>LFS</acronym> and
<acronym>BLFS</acronym> Book can be used to upgrade to the newer versions.
Here are some points that you should be aware of when upgrading
packages, especially on a running system.</para>

<itemizedlist>
<listitem><para>If one of the toolchain package (glibc, gcc,
binutils) needs to be upgraded to a newer minor vesion, it is safer to rebuild
<acronym>LFS</acronym>. Though you <emphasis>may</emphasis> be able to get by
rebuilding all the packages in their dependency order. We do not recommend it.
For example, if glibc-2.2.x needs to be updated to glibc-2.3.x, it is safer
to rebuild. For micro version updates, a simple reinstallation usually works, but
is not guaranteed. For example, upgrading from glibc-2.3.1 to glibc-2.3.2 will not
usually cause any problems.</para></listitem>
<listitem><para>If a package containing a shared library is updated, and if the
name of the library changes, then all the packages dynamically linked to the
library need to be recompiled to link against the newer library. (Note that there
is no corelation between the package version and the name of the library.) For
example, consider a package foo-1.2.3 that installs a shared library with name
<filename>libfoo.so.1</filename>. Say you upgrade the package to a newer version
foo-1.2.4 that installs a shared library with name <filename>libfoo.so.2</filename>.
In this case, all packages that are dynamically linked to <filename>libfoo.so.1</filename>
need to be recompiled to link against <filename>libfoo.so.2</filename>. Note that
you should not remove the previous libraries till the dependent packages are
recompiled.</para></listitem>
<listitem><para>If you are upgrading a running system, be on the lookout for
packages that use <command>cp</command> instead of <command>install</command> to
install files. The latter command is usually safer if the executable or library
is already loaded in memory.</para></listitem>
</itemizedlist>

</sect2>


<sect2>
<title>Package Management Techniques</title>

<para>The following are some common package management techniques. Before
making a decision on a package manager, do a research on the various
techniques, particularly the drawbacks of the particular scheme.</para>

<sect3>
<title>It is all in my head!</title>

<para>Yes, this is a package management technique. Some folks do not find the
need for a package manager because they know the packages intimately and know
what files are installed by each package. Some users also do not need any
package management because they plan on rebuilding the entire system
when a package is changed.</para>

</sect3>

<sect3>
<title>Install in separate directories</title>

<para>This is a simplistic package management that does not need any extra package
to manage the installations. Each package is installed in a separate directory.
For example, package foo-1.1 is installed in <filename>/usr/pkg/foo-1.1</filename>
and a symlink is made from <filename>/usr/pkg/foo</filename> to
<filename>/usr/pkg/foo-1.1</filename>. When installing a new version foo-1.2,
it is installed in <filename>/usr/pkg/foo-1.2</filename> and the previous
symlink is replaced by a symlink to the new vesion.</para>

<para>The environment variables such as those
mentioned in <xref linkend="intro-important-beyond"/> need to be expanded to
include <filename>/usr/pkg/foo</filename>. For more than a few packages,
this scheme becomes unmanageable.</para>

</sect3>

<sect3>
<title>Symlink style package management</title>

<para>This is a variation of the previous package management technique. Each package
is installed similar to the previous scheme. But instead of making the symlink,
each file is symlinked into <filename>/usr</filename> hierarchy. This removes the need
to expand the environment variables. Though the symlinks can be created by the user,
to automate the creation, many package managers have been written on this approach.
A few of the popular ones are Stow, Epkg, Graft, and Depot.</para>

<para>The installation needs to be faked, so that the package thinks that it is
installed in <filename class="directory">/usr</filename> though in reality it is installed in
<filename class="directory">/usr/pkg</filename> hierarchy. Installing in this manner is not usually a trivial
task. For example, consider that you are installing a package libfoo-1.1. The following
instructions may not install the package properly:</para>

<screen><userinput><command>./configure --prefix=/usr/pkg/libfoo/1.1 
make
make install</command></userinput></screen>

<para>The installation will work, but the dependent packages may not link to
libfoo as you would expect. If you compile a package that links against libfoo,
you may notice that it is linked to <filename>/usr/pkg/libfoo/1.1/lib/libfoo.so.1</filename>
instead of <filename>/usr/lib/libfoo.so.1</filename> as you would expect. The correct
approach is to use <envar>DESTDIR</envar> strategy to fake installation of the package.
This approach works as follows:</para>

<screen><userinput><command>./configure --prefix=/usr
make
make DESTDIR=/usr/pkg/libfoo/1.1 install</command></userinput></screen>

<para>Most of the packages do support this approach, but there are some which do not.
For the non-compliant packages, you may either need to manually install the package,
or you may find that it is easier to install some problematic packages into
<filename>/opt</filename>.</para>

</sect3>

<sect3>
<title>Timestamp based</title>

<para>In this technique, a file is timestamped before the installation of the package.
After the installation, a simple use of the <command>find</command> command with the
appropriate options can generate a log of all the files installed after the timestamp
file was created. A package manager written with this approach is install-log.</para>

<para>Though this scheme has the advantage of being simple, it has two drawbacks.
If during installation, the files are installed with any timestamp other than the
current time, those files will not be tracked by the package manager. Also, this
scheme can only be used when one package is installed at a time. The logs are not
reliable if two packages are being installed on two different consoles.</para>

</sect3>

<sect3>
<title>LD_PRELOAD based</title>

<para>In this approach, a library is preloaded before installation. During
installation, this library tracks the packages that are being installed by
attaching itself to various executables such as <command>cp</command>,
<command>install</command>, <command>mv</command> and tracking the system
calls that modify the filesystem. For this approach to work, all the executables
need to be dymanically linked without the suid or sgid bit. Preloading the
library may cause some unwanted side-effects during installation. Therefore,
do perform some tests to ensure that the package manager does not break
anything and logs all the appropriate files.</para>

</sect3>

<sect3>
<title>Creating Package Archives</title>

<para>In this scheme, the package installation is faked into a separate
tree as described in the Symlink style package management. After the
installation, a package archive is created using the installed files.
This archive is then used to install the package either on the local
machine or can even be used to install the package on other machines.</para>

<para>This approach is used by most of the package managers found in the
commercial distributions. Examples of package managers that follow this
approach are RPM, pkg-utils, Debian's apt, and Gentoo's Portage system.</para>

</sect3>

<sect3>
<title>User Based Management</title>

<para>This scheme, that is unique to <acronym>LFS</acronym>, was
devised by Matthias Benkmann, and is available from the <ulink
url="http://www.linuxfromscratch.org/hints/">Hints Project</ulink>.
In this scheme, each package is installed as a separate user into
the standard locations. Files belonging to a package are easily
identified by checking the user id. The features and shortcomings
of this approach are too complex to describe in this section. For
the details please see the hint at <ulink
url="http://www.linuxfromscratch.org/hints/downloads/files/more_control_and_pkg_man.txt"/>.</para>

</sect3>

</sect2>


</sect1>