Re: [boost] Independent Libraries Versioning proposal

From: Edward Diener (eldiener_at_[hidden])
Date: 2007-06-07 11:05:29

Gennadiy Rozental wrote:
> Hi,
> 
> Following David A. recommendation I put up a wiki page that presents initial 
> version of my proposal for boost development environment, testing and 
> release procedures.
> 
> http://svn.boost.org/trac/boost/wiki/IndependentLibraryVersioning
> 
> It's still missing one key piece - version selection algorithm. I just don't 
> have time to complete it now and won't until weekend. But it should be 
> complete enough to explain all the key points (and some more). I shamelessly 
> stole couple statements from Beman's objective section. Well, we do have 
> similar objectives ;)
> 
> Any comments are welcome.

I like the idea of allowing individual libraries and their dependencies 
being released between full Boost versions when deemed necessary. 
However, from an end user's point of view, allowing a version of some 
library to be released with an earlier dependency version than a version 
of another library with a later dependency version, where the first 
library no longer works with the later dependency version, will be a 
nightmare. Because of that I believe that any individual library needs 
to be co-ordinated with its dependencies so that changes in a dependency 
library, which may be released on its own or with another library, does 
not break the original individual library.

When I say "break" the original individual library I am not speaking 
about a bug which may occur in a dependency but rather a change to the 
public ( or protected ) programming interface of the dependent, which I 
will call, perhaps erroneously, the ABI.

The reason I think such a situation will be a nightmare to end-users is 
because an end-user may then have more than one version of the same 
dependency library within the structure of using a general Boost 
release, and pointing to a different set of header files, with possibly 
the equivalent libraries files for non-header-only libraries, for each 
different use of the libraries, will create great logistical 
difficulties to the smooth use of Boost.

As an example, suppose I use library A and library B, both of which have 
library C as a dependency. Library A uses version 1 of library C and 
library B uses version 2 of library C. Library A is broken, ABI-wise, 
with version 2 of library C. Now I want to use both library A and 
library B in my program ( or module ), often from within the same source 
file compilation. How in the world do I have library A pointing to its 
own version of library C's header files while library B points to its 
own version of library C's header files ?

This can be complicated further if library C has lib files which either 
statically are added to my module or are linked to dynamically through a 
shared library. This situation just increases the nightmare of trying to 
use both the libraries.

As I suggested in another post on this thread, I think it is certainly 
possible for an individual library to be released between full Boost 
releases, but only if the Boost developer of that library makes sure 
that his library is always co-ordinated ABI-wise with any of its 
dependencies. Likewise if a dependency of a library is going to be 
released itself, it is imperative that any libraries for which that 
library is a dependency, and for which the change to the dependency will 
create an ABI problem, also be released with an update at the same time 
which fixes the ABI problem.

It is nice to think that any Boost library should be able to be released 
at any time to work with any version of a dependency, but the reality of 
such a plan will often lead to chaos from the end-user's point of view. 
I am not saying this can not be done for unique individual situations, 
since I have done it myself using the Boost regex++ library, but it 
needs very careful consideration so as not to create a nightmare for the 
end-user.