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Message #11094
Nova subsystem branches and feature branches
Hey,
We discussed this during the "baking area for features" design summit
session. I found that discussion fairly frustrating because there were
so many of us involved and we all were either wanting to discuss
slightly different things or had a slightly different understanding of
what we were discussing. So, here's my attempt to put some more
structure on the discussion.
tl;dr - subsystem branches are managed by trusted domain experts and
feature branches are just temporary rebasing branches on personal github
forks. We've got a tonne of work to do figuring out how this would all
work. We should probably pick a single subsystem and start with that.
...
Firstly, problem definition:
- Nova is big, complex and has a fairly massive rate of churn. While
the nova-core team is big, there isn't enough careful review going
on by experts in particular areas and there's a consistently large
backlog of reviews.
- Developers working on features are very keen to have their work
land somewhere and this leads to half-finished features being
merged onto master rather than developers collaborating to get a
feature to a level of completeness and polish before merging into
master.
Some assumptions about the solution:
- There should be a small number of domain experts who can approve
changes to each of major subsystems. This will encourage
specialization and give more clear lines of responsibility.
- There should be a small number of project dictators who have final
approval on merge proposals, but who are not expected to review
every patch in great detail. This is good because we need someone
with an overall view of the project who can make sure efforts in
the various subsystems are coordinated, without that someone being
massively overloaded.
- New features should be developed on a branch and brought to a level
of completeness before being merged into master. This is good
because we don't want half-baked stuff in master but also because
it encourages developers to break their features into stages where
each stage of the work can be brought to completion and merged
before moving on to the next stage.
- In essence, we're assuming some variation of the kernel distributed
development model.
(FWIW, my instinct is to avoid the kernel model on projects. Mostly
because it's extremely complex and massive overkill for most
projects. Looking at the kernel history with gitk is enough to send
anyone screaming for the hills. However, Nova seems to be big
enough that we're experiencing the same pressures that drove the
kernel to adopt their model)
Ok, what are "subsystem branches" and how would they work?
- Subsystem branches would have a small number of maintainers who can
approve a change. These would be domain experts providing strong
oversight over a particular area.
(In gerrit, this is a branch with a small team or single person who
can +1 approve a review)
- Project dictators don't need to do detailed reviews of merge
proposals from subsystem maintainers. The dictator's role is mostly
just to sign off on the merge proposal. However, the dictator can
comment in the proposal on things which could have been done better
and the subsystem maintainer should take note of these comments and
perhaps retroactively fix them up. Ultimately, though, the dictator
can have exercise a veto if the merge proposal is unacceptable or
if the subsystem maintainer is consistently making the same
mistakes.
- It would be up to the project dictators to help drive patches
through the right subsystem branches - e.g. they might object if
one subsystem maintainer merged a patch that inappropriately cut
into another subsystem or they might refuse to merge a given patch
into the main branch unless it went through the appropriate
subsystem branch.
(In gerrit, this would mean a small team or single person who can
+1 approve merge proposals on master. They would -1 proposals
submitted against master which should have been submitted against a
subsystem branch.)
- Subsystem branches might not necessarily be blessed centrally. It
might be a case that anyone can create such a branch and, over
time, establish trust with the project dictators. Subsystem
branches would come and go. This is the mechanism by which
subsystem maintainership is transferred between people over time.
(In gerrit, this means people need to easily be able to create
their own branches)
(What's more difficult to imagine in gerrit is how a new, potential
subsystem maintainer comes along, starts hoovering up patches into
her branch and submitting them in batches. Where does she hoover
them up from and how does she say "I've merged this into my branch,
don't merge it via another branch")
- Bisectability remains important. Subsystem maintainers don't merge
broken commits into their subsystem branch and the project
dictators can enforce this using their veto. It is not good enough
for subsystem maintainers to consistently merge broken commits into
their branch, fix it up with a later commit and include both
commits their merge proposals.
(I don't think we'd use Jenkins to enforce this, but subsystem
maintainers might use it as a tool to help them catch issues. So,
the full set of gating tests would only gate merges into master but
subsystem branches might choose to gate merges into their branch on
the unit tests. Subsystem maintainers might also use Smokestack to
pre-gate merge proposals to the subsystem branch)
- Subsystem branches would not rebase unless the project dictator
outright rejects a merge request from the subsystem branch (i.e.
"I'm not merging commit abcdef0! Fix it and rebase!"). This means
the subsystem maintainer will need to regularly (probably only when
there are conflicts to be dealt with) merge master back into the
subsystem branch.
- Plausible subsystem branches are e.g.:
- OpenStack APIs
- EC2 API
- virt
- libvirt driver
- xenapi driver
- vmware driver
- networking
- volumes
- scheduler
- RPC
Deciding which areas make sense as a subsystem branch is
non-trivial.
Should there be a "DB" subsystem? Probably not, because that would
mean every new feature needs to come through this branch or,
alternatively, the DB maintainer would need to accept DB schema
additions without the context of how it's being used higher up the
stack.
Ok, so why does it make sense to have an "OpenStack APIs"
subsystem? Don't all features affect that branch too? Well, maybe,
but the APIs really do need strong oversight. Perhaps we can be
confident that we can add e.g. a new scheduler feature through the
scheduler branch and then later merge any API additions through the
APIs branch.
And how about feature branches?
- Feature branches are relatively short-lived (i.e. weeks or months
rather than years) branches for a specific feature. They are a
mechanism for developers to work on a patch series in the open until
the feature is complete enough to be merged into a subsystem branch
or master.
(I'm not sure gerrit is right for this. Why not just do it in
folk's github forks? I think all people are looking for is for
people to be more aware of feature branches. How about if you put
details of your feature branch in the blueprint for the feature?)
(If not using gerrit, can developers configure Jenkins to CI their
branch? Or is Smokestack the right tool?)
- Feature branches rebase, do not contain merge commits and each
commit on the branch is functional, bisectable and self-contained.
- When a feature branch is ready to be merged into a subsystem
branch, the patch series is submitted for review. The subsystem
maintainer will likely require changes to individual patches and
these changes would be made on the feature branch and squashed back
into the appropriate individual patch.
(Ideally gerrit's "topic review" feature will get upstream and
we'll use that. This would mean that a patch series could be
proposed for review as a single logical unit while still keeping
individual patches as separate commits)
- Because feature branches rebase, active day-to-day collaboration
with others is difficult. You certainly can't have multiple people
rebasing the same branch, that way lies madness.
There are ways to have multiple people work actively on the same
rebasing branch e.g.
http://blogs.gnome.org/markmc/2011/02/26/git-rebasing-cont/
but, ultimately, feature branches are going to be owned by a single
person who might incorporate patches from others.
(Incorporating the work of others, rebasing and squashing means a
patch might have multiple contributors but only one author listed in
git. That makes CLA enforcement impossible, but we should drop the
CLA in favour of the kernel-like Signed-off-by: tag. See this
discussion: https://lists.launchpad.net/openstack/msg06544.html )
- One option for longer-lived, active collaboration is for a subsystem
maintainer to create a feature branch and review the work as it is
ongoing. The idea being that the subsystem maintainer commits to
not requiring the feature branch to be rebased before it is merged
into the subsystem branch.
Cheers,
Mark.
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