My team recently decided not to use the “trunk” branch that is typical of most subversion repository layouts. We found that at any given moment there was always a particular branch that functioned in the traditional role that trunk would hold in other repositories. That is, we always have a highest-numbered branch that represents the next release that we are working on. Therefore a merge to trunk is simply superfluous, so we got rid of trunk.
Does anyone else out there do this?
If so, have you noticed any pros/cons?
Even if your team does not do this, does anyone have any thoughts on this layout?
You’re talking about the the Promotional Model – Perforce’s paper highlights the problems with it – communicating the changing roles of code-lines and moving work between branches.
An expansion on my views of the problems listed:
1) Changing policy of code-lines:
Every code line has a policy, whether it’s written down and formalised, or entirely implicit in a developer’s head. It defines e.g:
line
(e.g. do the unit tests have to pass,
regression tests, full system test)
changes
allowed
With the trunk approach, the policies remain fixed, so are easier to write down, which makes them easier to communicate (more important in a larger team).
e.g. Trunk:
Version branch:
Tag branch:
Developer’s private branch:
If you have the promotion model, then you have one policy while in main development, then have to tell everyone when you change policy while preparing for release, then another policy (for the code line) once the line is released.
The promotion model is an easy one to get into, it maps directly onto the non-source control way of working. But it hurts once you start getting large teams.
If you look at the Linux kernel development you can see the strain between the Promotional model and the Trunk model: Linus’ tree is Promotional – it goes through cycles between the merge window, and the rc/stabilisation period. But Linux-next and -mm have sprung up to give a more trunk like line.
Distributed SCM/VCS are somewhat different anyway, the policies don’t have to be distributed to all developers because each develop has his own trees, and pulls changes when he wants.
Open-source projects suffer from the problem that they can’t force people to do the drudge work of stabilising a release, after branching from trunk. Therefore the promotion model is more important as a way of forcing stabilisation efforts, rather than just working on features.
2) Moving work:
A developer might be working on a feature when the policy for the line he’s working on changes to bug-fixes only, he now needs to switch his working copy to a different code-line.
This may be anywhere from easy to extremely difficult depending on the SCM system in use.
This problem doesn’t happen if the developer is working on trunk, as their work is always going to trunk. If the developer is on a private or feature branch then their work will only impinge on trunk (and the release) at all.
If a feature is already checked in, but gets delayed from the version it’s currently in, you have to work out how to remove it. This problem still existing with the trunk model, but might be a little easier to solve – cherry-picking things from trunk for the release.
This is where feature branches help – but they have an integration cost.