Reference Manual: RPM's Philosophy

Resolves: rpm-software-management#2897

docs/CMakeLists.txt

@@ -44,6 +44,7 @@ install(FILES
 	manual/lua.md
 	manual/macros.md
 	manual/more_dependencies.md
+	manual/philosophy.md
 	manual/plugins.md
 	manual/queryformat.md
 	manual/relocatable.md

docs/manual/index.md

@@ -5,6 +5,7 @@ title: rpm.org - RPM Reference Manual
 # RPM Reference Manual
 
 * [About this manual](about.md)
+* [RPM's Philosophy](philosophy.md)
 
 ## Package Management
 

docs/manual/philosophy.md

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+---
+layout: default
+title: rpm.org - RPM's Philosophy
+---
+# RPM's Philosophy
+
+RPM is a general purpose software manager. It has
+two major parts: rpmbuild that combines upstream sources with a meta
+data and build instructions in a "spec file" and additional files and
+patches. It allows to bundle all this up into a source package and
+build the software into a binary package. The second part - the rpm
+utility - can then install, update or remove the packages after
+checking for consistency.
+
+RPM differs from special purpose package managers - like those
+targeting a specific programming language - in trying to not make
+assumptions about how software looks like or gets packaged. Packaging
+software can be messy and RPM accommodates for that.
+
+It still offers help to create a POSIX like operation system. Macros
+parameterize locations to make the build instructions more generic and
+more easily transferable between distributions. Nevertheless RPM
+packages are targeting a specific distribution (and release thereof). They are
+typically not suited to be installed or even built elsewhere without
+tweaking. RPM acknowledges that different distributions have
+different goals, resulting in different design decisions. The
+specifics of the packages often reflect these.
+
+RPM as a upstream project still tries to keep distributions from
+diverging unnecessarily but is aware that these differences are the
+reason distributions exist in the first place.
+
+## RPM in the Software handling stack
+
+RPM sees itself as filling specific layers in the software handling
+stack. It relies on build systems like make and the upstream provided
+build scripts to orchestrate the actual build and passes the acquiring
+and selection of the "right" packages up to updaters like yum, dnf,
+zypper, etc.
+
+The stack typically looks like this:
+
+ * Upstream build scripts using tools like maker, cmake, ant, ...
+ * rpmbuild for running those via a Spec file
+ * Build systems for installing build dependencies and keeping track of build artifacts
+ * Package repositories offering binary packages
+ * Installers/Updaters selecting and downloading packages
+ * rpm checking for consistency and installing/updating the packages
+
+As such RPM is only a part in a larger set of tools and some users may
+never actually interact with RPM directly.
+
+## Design goals
+
+* General purpose software management
+* Building from pristine sources
+* Unattended operation
+  * for both building
+  * for installing/updating/deleting packages
+* Reproducibility of builds and installs
+* Verifiability of packages and installed software
+
+### Rolling out (security) updates quickly.
+
+Getting updates installed quickly is one of the main design
+goals. Many features and following design decisions are supporting
+this goal.
+
+### Packaging dependencies separately
+
+Libraries should be packaged separately and binaries should link to
+the version provided by system packages. Static linking is
+discouraged. This limits the number of packages that need updates or
+re-builds in case of a vulnerability.
+
+### Unattended operation
+
+Package installation and update is unattended and must not require
+user interaction. This allows automatically installing security - and
+other - updates.
+
+Building the packages also runs unattended. This prevents user input
+from changing the output but also makes rebuilding packages - e.g. for
+changed dependencies - possible at large scale.
+
+### Clear update path
+
+Each package name creates an update path where packages with the same
+name and a newer version (technically Epoch-Version-Release) are an
+update for the packages with lower version. Packages are not allowed
+to make assumptions on what intermediate packages were installed or
+not.
+
+### Bundle Sources, Changes and Build instructions together
+
+Source packages bundle upstream sources, patches and build
+instructions into one entity. If changes need to be made everything
+needed is available, including a list of packages needed to run the
+build.
+
+#### Separate Upstream Source from Patches
+
+Source packages are supposed to contain the unaltered upstream sources
+in a way that their checksum can be checked. All changes can be done
+by applying patches or running build scripts. This makes it easy to
+understand what the packager did to the software. This is important to
+be able to figure out where issues arise from and to see which issues
+are fixed in the package - even if the upstream version is still
+vulnerable.
+
+#### Support back porting fixes
+
+A distribution often has a different approach to fixing software from an
+upstream project. Upstream projects are often content with fixing
+their latest release(s). Distribution often support a particular
+version of each software and need those to be fixed. The Sources and
+Patches approach makes handling this a lot easier (although it is
+lately being extended with the use of version control like git). 
+
+### Allow 3rd party packages
+
+Although RPM is designed to package a whole distribution it explicitly
+supports 3rd parties to also provide packages without becoming part of
+the distribution and their processes.
+
+Rpmbuild can be run locally without the use of a big build
+system. Dependencies are in large part automatic and target libraries
+inside the packages instead of relying on conventions like package
+names. This way 3rd party packages can be checked for compatibility
+beyond them just claiming that they target the distribution in question.
+
+For proprietary softerware nosource packages allow to not ship the
+sources but only the build instructions to keep proprietary source
+private. They still can be used to generate binary packages in
+combination with the sources.
+
+### Handling all system files
+
+RPM takes ownership of all system files and their life cycle. While
+packages can copy files in their scriptlets at installation time this
+is strongly discouraged. Even files that are not actually shipped can
+be added to the package as %ghost files to allow RPM to handle
+them. All system files and directories should be owned by a package.
+
+Scriptlets should be used sparingly. Most use cases - like updating
+caches and indexes - can be dealt with by using a central
+filetrigger. Although these files may get altered they still should be
+owned by a package.
+
+### Verify files
+
+RPM keeps checksums of all files it maintains. Packages have checksums
+and can and should be signed. Files that need to change on disk - like
+config files, databases or indexes - can be marked and are treated
+differently when being changed. While unchange config files may be
+replaced changed ones will be retained or backed up - depending on the
+way they have been packaged.
+
+## Levels of control
+
+Most things in RPM can be configured by macros. This allows different
+actors to set them and overwrite what was set previously.
+
+Most basic are the defaults delivered by RPM upstream. Distributions
+are able to overwrite them by patching or providing new macro
+files. Packages can also ship macro files for other packages to use as
+BuildRequires. Finally most build related macros can also be
+overwritten in the Spec file - giving the packager the last word.
+
+There are a few command line options to allow users to influence how
+packages are installed but their use is discouraged for the most part
+and packages should be installed as they are.