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Solving Sudoku With dpkg

Daniel Re:RPM versus APT (190 comments)

So now I have a new question: how do RPM-based distros manage without this feature.

I don't use RPM distributions, so I can't answer that question. In Debian, we've gotten along fine for a long time without a complete resolver [0] (the resolvers most users use are still incomplete) because the problem is so easy. The situations users usually run into can be solved with a greedy algorithm like your initial suggestion; with a few extra heuristics you can cover many other typical scenarios (such as the one I posted above). When users hit a situation that it can't solve, they either add instructions to force a resolution, or they ask for help on a mailing list and someone tells them how to add instructions that force a resolution. :-) I presume that users of RPM distributions do something similar.

The main impetus behind the aptitude resolver was that I wanted to improve the UI by providing user-visible explanations for the resolver's decisions and by allowing users to interactively guide the resolver towards a solution (for instance, by telling it that they don't want it to install a particular package version). I also wanted to add support for installing versions other than the current "candidate" [1]. That I could make it complete at the same time was the icing on the cake. (but some very useful icing for people who have complicated dependency situations!)

Daniel

[0] "complete" here means "able to find any solution to a dependency problem". Actually, aptitude only guarantees that it will find minimal solutions, but that's what you want (if "install A" solves a dependency problem, it's possible that "install A and install B" also does; aptitude might return both of these or only the smaller one -- ideally it would only ever return the smaller one, but I wasn't able to find a technique that would quickly minimize solutions).

[1] An apt term: the candidate version of a package is the version that apt will select to install if it's told to install that package. If versions 1, 2, and 3 of A are available and 2 is the candidate, apt-get install A will install version 2. The standard dependency resolver can't handle situations where a version other than the candidate is needed to resolve a dependency.

more than 4 years ago

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