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pkg-gate / doc / sat.txt

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It's become clear that our approach to evaluating
packaging dependencies and constraints needs to become
a lot more sophisticated as we've been trying to make
packaging metadata more accurately reflect the way
we build and test packages.

A significant part of the difficulty is dealing with
externally produced packages; if a variety of versions
are available we may need to iteratively test multiple
versions, evaluating their dependencies to find one that
is compatible w/ the constraints that may be active on
the current image.

One method of doing this sort of automated decision making
is to cast the problem as a series of boolean expressions,
and then apply a SAT solver to find a solution.  These notes
describe the results of my experiments w/ the minisat solver 
Stephen posted some time ago....

Notes:
--------

1) The presence of a particular package version is a 
   single boolean variable; True if it's present,
   False if not.

   The problem set handed to the SAT solver is a series
   of clauses; each clause are boolean variables (or
   their negation) or'd together.  All clauses must be
   true for the solution to exist.

   The clauses need to encode the fact that only one version
   of a package may be installed at a time, and also encode
   all different package dependencies and constraints.

2) Each package has some number of versions, inherently ordered.
   Only one version of a package may be installed at a time

   pkg a -> a.1, a.2, a.3, a.4
   pkg b -> b.1, b.2, b.3, b.4

   Thus for "a":

   !a.1 | !a.2
   !a.1 | !a.3
   !a.1 | !a.4
   !a.2 | !a.3
   !a.2 | !a.4
   !a.3 | !a.4

   where !a represents the negation of a.

   This means that for N versions, we have N(N-1)/2 clauses;
   pruning older non-accessible versions will be required to 
   bound memory consumption.

3) Each version of a package may have dependencies on other
   packages, either w/ or w/o a version.  The version specification
   will likely not be fully specified (eg multiple versions
   may satisfy this requirement).

4) dependencies may be of the following types:

   required: fmri specifies minimum acceptable version

   if a.1 requires b.2, b.3 or b.4:
      !a.1 | b.2 | b.3 | b.4
   
   optional: if present, fmri must be at this level or greater
   if a.1 optionally requires b.3:
      !a.1 | !b.1
      !a.1 | !b.2

   incorporate: if present, pkg must match fmri

   if a.1 incorporates b.3:
      !a.1 | !b.1
      !a.1 | !b.2
      !a.1 | !b.4

   exclude: if present, pkg must be less that version in fmri:

   if a.1 excludes b.3,

      !a.1 | !b.3
      !a.1 | !b.4

   All of these are linear in the number of package versions
   either meeting or failing to meet the dependency.

5) To express state, the presence of a package is encoded as a 
   clause.  We compute the matching fmris and then construct
   a clause that matches one of those fmris.  Specifying a single
   version requires that version to be present in the solution;
   we can also specify current version or newer, or any version of
   a package.

6) The SAT solver will find a particular solution to our packaging
   problem, but there is no way of "preferring" newer packages, and
   preventing the introduction of extraneous unneeded packages.
   As a result, external optimization in the form of repeated 
   solution attempts w/ additional constraints is necessary.  
   The following algorithm has been implemented:

   The packaging problem to be solved is expressed as a series of
   boolean constraints, and a solution obtained.  Then, for each
   fmri appearing in the solution vector, all older versions are
   excluded; in other words, if a.3 is part of the solution, then
   subsequent solutions will not contain a.1 or a.2.  Then a single
   vector is added that is the negation of the just found vector,
   and another solution is found.  For example:

   if solution is a.2, b.3, z.10, we add
  
   # first negations of older versions
   !a.1
   !b.1
   !b.2
   !z.1
   !z.2
   ...
   !z.9
   # now negate just found solution
   !a.2 | !b.3 | !z.10

   The latter vector requires that the new solution not contain
   a.2 and b.3 and z.10; since we've excluded older versions we
   will either get a vector that eliminates one of the packages
   as unneeded (if dependencies allow) or one that has newer 
   versions of one of the needed pkgs.

   We repeat the above process until a solution cannot be found; 
   the last found solution must therefore be the most optimal one.

   The above technique may fail to find the overall optimal 
   solution if newer packages have incorporation dependencies
   on earlier versions of their dependencies.  This is expected
   to be rare.  Pruning the solution space to eliminate older
   packages is necessary due to rapid solution space growth if
   there are multiple versions that satisfy dependencies.


7) In order to prevent rapid growth of clause count as the
   number of versions of packages increases, trimming the
   solution space is essential.  I'm currently using the
   following techniques:

   1) install a new package on existing system

   identify any existing installed constraints,
   and trim pkg catalog to eliminate versions
   outside those constraints.  

   trim pkg catalog to exclude all pkg older than
   those already installed

   input to solver is trimmed catalog, and 
   vectors selecting any version of already installed
   pkgs that meet constraints, plus a vector selected
   any version of desired pkg.

   2) upgrade to latest version of all available pkgs

   identify any existing installed constraints,
   and trim pkg catalog to eliminate versions
   OLDER than those constraints.  
  
   trim pkg catalog to exclude all pkg older than
   those already installed

   input to solver is trimmed catalog, and 
   vectors selecting any version of already installed
   pkgs 

   3) upgrade to specified version 

   identify any existing installed constraints,
   and trim pkg catalog to eliminate versions
   OLDER than those constraints.  
  
   trim pkg catalog to exclude all pkg older than
   those already installed

   input to solver is trimmed catalog, and 
   vectors selecting any version of already installed
   pkgs, plus vector(s) selecting desired constraint(s).

8) One of the most difficult aspects of using a SAT solver
   is providing a reasonable error message when no solution
   can be found.


   Some techniques that I'm experimenting with include:

   Explicitly checking for obvious non-starters (pkg
   version earlier than already installed, pkg version that
   violates constraints on system) prior to passing to SAT
   solver.  This is needed to permit trimming in any case.

   Using the pruned catalog to quickly evaluate the effect
   of constraints.   
   
   
Implementation details
-------------------------

combine catalog object w/ list of installed pkgs and proposed
changes:

class pkg_solver(object):
      def __init__(self, catalog, existing_fmris):

      def solve_install(existing_freezes, proposed_fmris):
      	  """tries to find solution that adds specified fmris
	  to existing set; any existing packages containing
	  incorporate dependencies which are at most only depended on
	  by name (no version) are frozen."""

      def solve_reinstall(existing_freezes, proposed_fmris):
          """tries to find solution that replaces existing version
	  with specified version; this one allows stuff to go
	  backwards if specified on command line"""
      
      def solve_uninstall(existing_freezes, proposed_fmris):
          """tries to remove specified package"""

      def solve_update_all(existing_freezes):
          """find most recent version of all packages"""

      solve* routines return a list of tuples (old_version, new_version)
      for each fmri that is changing; new installs have None as old_version,
      removals have None as new_version.  A returned empty list indicates
      that no action is needed.

      A failure to find a solution throws an exception,
      pkg_solver.No_Solution_Found.

 
 
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