Thursday, July 09, 2009

DVCS(hg) vs SVN workflows

Suppose we work in an international company, remote sites, not so good connection between those sites.
We have the following scenarios:

1. We have to do a rather big refactoring, which will take several weeks.
- With svn: we create a 'feature branch' where we'll do the refactoring. We update daily from trunk, we push to trunk regularly/at the end. The feature branch is allowed to be CI-red. (Due to an svn issue merging back-and-forth became a problem for svn-merge-tracking, and we had to do a tree-diff on 2 local sandboxes)
- With hg: you just create a clone-branch, you work on it, you can merge with<->from trunk regularly,
whithout any merging problems.
- Guerilla tactics: shadow the svn trunk/branch with some hg repositories and to the merges with hg (no tree-diff), at the end push the changes to svn

2. Junior (remote) developer needs help fixing a unit-test.
- With svn: do a branch, svn switch to it, push the code with the broken tests, fix them, merge back to trunk.
- With webex: do a remote session an explain the fix
- With hg: just pull the changes, make a fix, push them back to the remote developer. (hint: hg is more network-friendly then svn)

3. An interface between 2 components is changed radically. Both the user & the implementation must be changed.
- With svn: do a branch, do all the changes, merge to trunk
- With hg: change the interface in a cloned repository, then share this repository between the user & implementer. They can work in parallel, the last one finishing pulls the changes from the other one and does the integration.

4. Junior developer, friday@17.00, before a 3 week holiday, wants to commit his _weekly_ changes. He cannot create a local ci build within 1 hour so he doesn't commit anything.
- He shouldn't integrate friday@17.00. (but he did)
- With svn: create a branch, svn switch, push your changes to the server
- With hg: a colleague could pull the changes an do the integration + push the changes, without the need to go through the server.

5. Always keep the code green.
- With svn: run the CI on the pre-commit hook (TeamCity does it). The code get's in the repository only if the build is green
- With hg: maintain 2 repositories: in the 1st one we push the changes, which are peeked by the CI, validate, and on success pushed to another repository. (or just do a rollback; much easier with hg)

A lot of the hg power comes from the fact that is changeset-based (unlike svn) and merges are really easy. Another big plus point goes to the distributed nature of hg: it enables more flexible workflows; in a centralized vcs all the communication is done through the server.

links: hg, git

2 comments:

AndiZo said...

Another good scenario for hg might be, when QA is to develop their system tests for a new or changed feature.

andiz said...

CRE Express podcast about distributed version conrtol in german available at http://chaosradio.ccc.de/cre130.html