~bzr-pqm/bzr/bzr.dev

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Config specs

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A *config spec* is a set of instructions for assembling a source tree

from separately maintained branches.  This is like CVS's "modules"

format.  Config specs are very commonly used in Clearcase, and the

main way of getting a new checkout.

Scenarios:

 * One module is shared by several projects, and it needs to be

   present in a subdirectory of their trees to build.

 * There are different commit or stability rules for different parts

   of a tree.

The simplest form is just a specification of branch sources, and where

they should be assembled into the tree.  Example::

  .              http://foo.net/fooapp

  ./libbaz       http://foo.net/libbaz

Normally the entire branch will be placed at this point, but sometimes

subdirectory should be selected.  Selecting just a single file rather

than subdirectory might be problematic because we wouldn't have

anywhere to put the control files.

Each branch will have a .bzr directory at the top level.  Changes on

each branch can be pushed back independently.

The tool needs to support (and be tested with) nested checkouts.

Commands should normally only apply to a single tree, but there might

be an option to descend into nested trees.

One very important use case is a naive user checking out a tree that

happens to use a configspec.  This is transparent in cvs, but not at

all so in Arch.

The ``svn:externals`` mechanism in Subversion works like this.

One interesting idea from Aaron is that all branches should be like

this: the latest-patch pointer can be generalized to in fact name

several patches that should be assembled in subdirectories.  A

simplest branch names just one, assembled to the base directory.  You

might want to specify the parent branches for each one as well....