Libsolv-History(3)
==================
:man manual: LIBSOLV
:man source: libsolv

Name
----
libsolv-history - how the libsolv library came into existence

History
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This project was started in May 2007 when the zypp folks decided to switch
to a database to speed up installation. As I am not a big fan of databases,
I (mls) wondered if there would be really some merit of using one for solving,
as package dependencies of all packages have to be read in anyway.

Back in 2002, I researched that using a dictionary approach for storing
dependencies can reduce the packages file to 1/3 of its size. Extending
this idea a bit more, I decided to store all strings and relations
as unique 32-bit numbers. This has three big advantages:

- because of the unification, testing whether two strings are equal is the
  same as testing the equality of two numbers, thus very fast
- much space is saved, as numbers do not take up as much space as strings
  the internal memory representation does not take more space on a
  64-bit system where a pointer is twice the size of a 32-bit number

Thus, the solv format was created, which stores a repository as a string
dictionary, a relation dictionary and then all packages dependencies.
Tests showed that reading and merging multiple solv repositories takes
just some milliseconds.

=== Early solver experiments ===
Having a new repository format was one big step, but the other area
where libzypp needed improvement was the solver. Libzypp's solver was
a port from the Red Carpet solver, which was written to update packages
in an already installed system. Using it for the complete installation
progress brought it to its limits. Also, the added extensions like
support for weak dependencies and patches made it fragile and
unpredictable.

As I was not very pleased with the way the solver worked, I looked at
other solver algorithms. I checked smart, yum and apt, but could not
find a convincing algorithm. My own experiments also were not very
convincing, they worked fine for some problems but failed miserably
for other corner cases.

=== Using SAT for solving ===
SUSE's hack week at the end of June 2007 turned out to be a turning point
for the solver. Googling for solver algorithms, I stumbled over some note
saying that some people are trying to use SAT algorithms to improve
solving on Debian. Looking at the SAT entry in Wikipedia, it was easy
to see that this indeed was the missing piece: SAT algorithms are well
researched and there are quite some open source implementations.
I decided to look at the minisat code, as it is one of the fastest
solvers while consisting of too many lines of code.

Of course, directly using minisat would not work, as a package solver
does not need to find just one correct solution, but it also has to
optimize some metrics, i.e. keep as many packages installed as possible.
Thus, I needed to write my own solver incorporation the ideas and
algorithms used in minisat. This wasn't very hard, and at the end of
the hack week the solver calculated the first right solutions.

=== Selling it to libzypp ===
With those encouraging results, I went to Klaus Kaempf, the system
management architect at SUSE. We spoke about how to convince the
team to make libzypp switch to the new solver. Fortunately, libzypp comes
with a plethora of solver test cases, so we decided to make the solver pass
most of the test cases first. Klaus wrote a "deptestomatic" implementation
to check the test cases. Together with Stephan Kulow, who is responsible for the
openSUSE distribution, we tweaked and extended the solver until most of
the test cases looked good.

Duncan Mac-Vicar Prett, the team lead of the YaST team, also joined
development by creating Ruby bindings for the solver. Later, Klaus
improved the bindings and ported them to some other languages.

=== The attribute store ===
The progress with the repository format and the solver attracted another
hacker to the project: Michael Matz from the compiler team. He started
with improving the repository parsers so that patches and content files
also generate solvables. After that, he concentrated on storing all
of the other metadata of the repositories that are not used for solving,
like the package summaries and descriptions. At the end of October, a first
version of this "attribute store" was checked in. Its design goals were:

- space efficient storage of attributes
- paging/on demand loading of data
- page compression

The first version of the attribute store used a different format for
storing information, we later merged this format with the solv file
format.

=== libzypp integration ===
Integration of the sat-solver into libzypp also started in October 2007 by
Stefan Schubert and Michael Andres from the YaST team. The first
versions supported both the old solver and the new one by using the
old repository read functions and converting the old package data
in-memory into a sat solver pool. Solvers could be switched with
the environment variable ZYPP_SAT_SOLVER. The final decision to
move to the new solver was made in January of 2008, first just by
making the new solver the default one, later by completely throwing out
the old solver code. This had the advantage that the internal solvable
storage could also be done by using the solver pool, something Michael
Matz already played with in a proof of concept implementation showing
some drastic speed gains. The last traces of the old database code
were removed in February.

Author
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Michael Schroeder <mls@suse.de>
