RPM Weak Dependencies and the install media compose process

[Stephen Gallagher]

(Please keep the conversation on the devel list; I'm CCing it the rel
-eng list to make sure all the relevant people see the initial message)

This past week, the Fedora Packaging Committee approved the use of
"weak dependencies" in Fedora. What this means is that RPM packages can
now have three levels of dependency-resolution: Requires, Recommends
and Suggests.
 * Requires: the requested package cannot function without this
additional package installed
 * Recommends: the requested package can function in some minimal
capacity without this additional package installed, but the majority of
installations will want it for full productivity. These are usually
core plugins for the primary package. DNF defaults to installing
Recommends: dependencies automatically.
 * Suggests: the requested package can easily function without this
additional package. This module may provide some less-common
functionality that a user might want. DNF defaults to *not* installing
Suggests: packages automatically.

Traditionally, we have only supported "Requires" dependencies and thus
the creation of install media (Live and otherwise) has been relatively
straightforward: we create a kickstart file that is fed into the
compose process containing a list of packages and groups that we want
installed onto the target system and the compose process automatically
pulls in all of the dependencies. However, with the advent of weak
dependencies, we have new questions that need answering about how this
compose process should work. (We also need to investigate what exactly
happens with the tools we have today - some of which still use yum, not
DNF - when weak dependencies are added to the mix).

From my perspective, there are three ways that we could choose to go:

1) Follow the default DNF behavior: Requires: and Recommends: packages
are included on the install media (and therefore also installed
together onto the target system)

2) Include *all* dependencies - Requires, Recommends and Suggests - on
the install media. The installer would still follow DNF defaults, so
the target system would get only the Requires and Recommends packages
unless the Suggests: packages are explicitly selected (which will also
require the creation of additional comps.xml changes to include the
Suggests packages)

3) Include only Requires: dependencies by default and require spin
-kickstarts owners to explicitly add any Recommends or Suggests
packages that they also want to include. Packages added explicitly will
be installed as described in 2) (requiring additional comps.xml changes
to include Suggests stuff)


Note that at the moment, DNF itself does not have a configuration
option to tweak the default install behavior, so 'dnf install'
effectively treats Recommends the same way as Requires (but 'dnf
remove' will treat them differently, of course). I discussed this with
the DNF developers this morning and they hope to have a configuration
option and/or command-line argument available to change this behavior
before Beta Freeze, so we should still be able to ship F23 with any of
the above options.

I think the best time to make these decisions is now, well in advance
of the Alpha Freeze so we have time to make adjustments as needed.
Thank you for reading to the end, I know the above has been a wall-o'
-text.
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