Weak Dependencies Policy
Weak dependencies are basically variants of the
Requires: tag and
are matched against (virtual)
Provides: and package names using
Epoch-Version-Release range comparisons, just like regular
Requires:. They come in two strengths: "weak" and "hint" and two
directions "forward" (analogous to
Requires:) and "backwards"
(which has no analog in the previous dependency system).
Weak dependencies allow smaller minimal installations while keeping the default installation feature rich. They also allow packages to specify preferences for specific providers while maintaining the flexibility of virtual provides, for example, preferring ruby vs jruby or community-mysql vs mariadb.
Weak dependencies are by default treated similarly to regular
Requires:. Matching packages are added to the dnf transaction. If
adding the package would lead to an error dnf will by default ignore the
dependency. This allows users to exclude packages that would be added by weak
dependencies or remove them later. (Future versions of dnf might also allow to
switch weak deps off on the command line.)
Weak dependencies may only be used in a package if the package still functions without the dependency present. It is acceptable, however, to create packages that have very limited functionality without adding any of its weak requirements. Weak dependencies should be used where possible to minimize the installation for reasonable use cases, especially for building virtual machines or containers that have a single purpose only and do not require the full feature set of the package.
Typical use cases for weak dependencies are:
- Documentation viewers if missing them is handled gracefully
- Plug-ins or add-ons
- Support for file formats
- Support for protocols
Hints are by default ignored by dnf. They may be used by GUI tools to offer add-on packages that are not installed by default but might be useful in combination with the installed packages. The requirements of the main use cases of a package should not merely be referenced by hints but included by strong or weak dependencies.
dnf (or more precisely libsolv) will use weak dependencies and hints to decide which package to use if there is a choice between multiple equally valid packages. In these cases packages that are pointed at by dependencies from installed or to be installed packages are preferred. Note, that this does not alter the normal rules of dependency resolution. For example, weak dependencies cannot enforce a older version of a package to be chosen.
If there are multiple (typically virtual) providers for a dependency the
requiring package may add a Suggests: to provide a hint to the dependency
resolver as to which poption is preferred.
Enhances: should only
be used for the rare occasion when the main package and other providers agree
that adding the hint to the required package is for some reason the cleaner
Real life example
Package mariadb: Provides: mysql
Package community-mysql: Provides: mysql
If you want to prefer mariadb over community-mysql -> add
mariadb to Package A.
Forward vs Backward Dependencies
Forward dependencies are, as Requires, evaluated for packages that are being installed. The best of the matching (fulfilling) packages are also installed. For reverse dependencies the packages containing the dependency are installed if a matching package is getting installed also.
In general forward dependencies should be used. Add the dependency to the package getting the other package added to the system.
Reverse dependencies are mainly designed for 3rd party vendors who can attach their plug-ins/add-ons/extensions to distribution or other 3rd party packages. Within Fedora the control over which packages a package requires should stay with the package maintainer. There are, however, cases when it is easier for the requiring package not needing to care about all add-ons. In this cases reverse dependencies may be used with the agreement of the package maintainer of the targeted package.
Note, that EPEL or other third party repositories may have (and are encouraged to have) a different policy.