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The new APT 3.0 solver
APT 2.9.3 introduces the first iteration of the new solver codenamed solver3, and now available with the –solver 3.0 option. The new solver works fundamentally different from the old one. How does it work? Solver3 is a fully backtracking dependency solving algorithm that defers choices to as late as possible. It starts with an empty set of packages, then adds the manually installed packages, and then installs packages automatically as necessary to satisfy the dependencies.
This mostly comes in handy in use cases involving Debian experimental or the Ubuntu proposed pockets, where you want to install a package from there, but try to satisfy from the normal release as much as possible. The implication graph building allows us to implement an apt why command, that while not as nicely detailed as aptitude, at least tells you the exact reason why a package is installed. A lot of that complexity you have normally is not there because the manually installed packages and resulting unit propagation (single-solution Depends/Reverse-Depends for Conflicts) already ground us fairly far in what changes we can actually make.
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