4.1 Inputs and Outputs
Resolution is the step that turns "install this" into "do these things, in this order". It runs entirely on index data, before anything is fetched.
4.1.1 Inputs #
- The goals: the operations the operator asked for. Install a package, upgrade one, downgrade one, or remove one.
- The installed set: what is on the system now, from the package database, with each package's version, architecture, originating repository, and that repository's priority.
- The available set: every package in every configured repository's index, each annotated with the repository it came from and that repository's priority.
The resolver's working model is keyed by the pair (name, root). The same package name installed in two roots is two independent entries, possibly at different versions.
4.1.2 Outputs #
Resolution produces either a plan or a rejection.
A plan is an ordered list of operations. It is partially ordered so that for every install, everything it depends on is already installed or is scheduled earlier; removals are ordered in reverse, so that a dependent is removed before the thing it depended on.
Alongside the plan, resolution emits two other things:
- Authorizations — elevated actions the plan implies, each of which
the operator confirms on its own terms before the plan is applied.
A downgrade, a foreign
replaces, a low-trust provider filling a role. - Notices — informational statements that never block. The substitution notice of §4.2 is one.
A rejection names which condition failed and which packages or constraints were involved.
4.1.3 Determinism #
The resolver is a pure function of its inputs: the same goals, installed set, and available set produce the same plan, every time. That is what makes a dry run trustworthy — the plan shown is the plan that would be applied.
Determinism holds with respect to the inputs as given, including the order candidates appear in. Where the selection rules of §4.3 leave two candidates genuinely tied, the one enumerated first wins, so a re-sorted index can change the outcome.
4.1.4 Index-only #
Resolution never downloads a package. Satisfaction checks and candidate selection use only what the index carries, which is why the index carries a package's relationships at all. Fetching is deferred until after the plan is computed and confirmed.