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Alternative Dependencies

An alternative dependency can be satisfied by any one of several packages. The resolver tries each option in order and picks the first that exists and satisfies any version constraint.

Explicit alternatives (Debian / Ubuntu)

Debian and Ubuntu use the pipe (|) syntax in dependency declarations:

Depends: debconf (>= 0.5) | cdebconf

This means "debconf at version 0.5 or higher, OR cdebconf." The resolver tries debconf first. If it exists and satisfies the version constraint, it's selected. If not, cdebconf is tried.

Multiple alternatives can be chained:

Depends: awk | mawk | gawk | original-awk

Implicit alternatives (Arch / Alpine / RPM)

Arch, Alpine, and RPM don't have alternative syntax in their dependency declarations — each dependency names exactly one package. Alternatives arise implicitly through virtual packages: when multiple packages provide the same virtual name, they form an alternative group.

For example, if packages A, B, and C all provide libGL.so.1, a dependency on libGL.so.1 is effectively an alternative between A, B, and C. The resolver picks the first provider (alphabetically sorted) that satisfies any version constraint.

Resolution order

For each alternative (whether explicit or implicit):

  1. Get all candidates — the real package name, or all providers of the virtual
  2. For each candidate, check the version constraint against the provides version (for virtual deps) or the package version (for direct deps)
  3. Accept the first candidate that satisfies

If no candidate of any alternative satisfies the constraint, the dependency is unresolvable:

  • For required dependencies (Depends, Pre-Depends) — resolution fails with an error
  • For optional dependencies (Recommends, Suggests) — silently skipped
  • Appendix / Dependencies / BFS Traversal — Depth-first search (DFS) would also visit every reachable package, but BFS has two properties flatroot relies on:
    appendix dependencies resolver
  • Appendix / Dependencies / Install-If Triggers — Install-if is a conditional installation mechanism specific to Alpine Linux. A package can declare trigger conditions that cause it to be automatically installed when all...
    appendix dependencies resolver
  • Appendix / Dependencies / Rich Dependencies — Rich dependencies are boolean expressions used in RPM-based distributions (CentOS, Fedora, AlmaLinux, Rocky, openSUSE) to encode conditional, alternative, and conjunctive...
    appendix dependencies resolver