Bazaar Release Cycles

status:Current policy, as of 2010-10.
blueprint:<https://blueprints.launchpad.net/bzr/+spec/6m-cycle>

Our users want easy access to bug fixes without other changes to the core product. They also want a Just Works experience across the full Bazaar ecosystem. To deliver the first and enable the second, we’re adopting some standard process patterns: a 6 monthly release cycle and a stable series. These changes will also have other benefits, including better availability of bug fixes in OS distributions, more freedom to remove old code, and less work for in packaging.

See also:

The Process

Bazaar will make a major release every six months, which will be supported at least until the time of the next major release and generally 18 months after the first final release in a series. During this support period, we’ll make incremental releases which fix bugs, but which do not change network or disk formats or command syntax, and which do not require updates to plugins.

We will also run a development series, which will become the next major release. We’ll make a beta release from this every four weeks. The beta releases will be as stable as our current monthly releases and completely suitable for everyday use by users who can tolerate changes from month to month.

Having the stable series isn’t a reason to cut back on QA or to make the trunk or development releases unstable, which would only make our job harder. We keep our trunk in an always-releasable state, and that should continue: any beta release could potentially be supported in the long term, but we identify particular releases that actually will be supported.

The trunk will never be frozen: changes that pass review, other quality checks and that are agreed amongst the developers can always be landed into trunk. The only restrictions will be on branches specifically targeted at a release.

Schedule

2.0.0 --- 2.0.1 -- 2.0.2 -- ...
 \
  +--2.1.0beta1 -- 2.1.0beta2 -- ... -- 2.1.0rc1 -- 2.1.0 -- 2.1.1 -- ...
                                                     \
                                                      \
                                                       +-- 3.0.0beta1 ...

Starting from the date of a major release:

At four-week intervals we make a new beta release. There will be no separate release candidate, but if a serious problem is discovered we may do the next beta ahead of schedule or make a point release. There will be about five or six releases in that series.

In parallel with this, bugs targeted to the previous major release are merged into its branch. We will make bugfix releases from that branch as appropriate to the accumulation of changes, perhaps monthly, perhaps more often if there are serious bugs, perhaps much less often if no new changes have landed.

We will synchronize our major releases with Ubuntu, so that they come out in sufficient time for some testing and margin of error before Ubuntu’s upstream freeze.

Regularity

We value regular releases. We prefer to slip a feature or fix to a later release rather than to make a release late. We will normally only slip a release to fix a critical bug.

Numbering

The number for a six-month cycle is chosen at the start, with an increment to either the first field (3.0.0) or second field (3.1.0) depending on what we expect to be the user impact of the release. We expect releases that culminate in a new disk format or that require changes in how people use the tool will get a new major number. We can change (forward only) if it turns out that we land larger changes than were expected.

We will always use the 3-digit form (major.minor.micro) even when referring to the initial major release. This should help clarify where a patch is intended to land. (eg, “I propose this for 2.0.0” is clear, while “I propose this for 2.0” could mean you want to make the 2.0.0 release, or that you just want to land on the 2.0.x stable release series.)

Terminology

Major releases (2.0.0 or 2.1.0)

The big ones, every six months, intended to ship in distributions and to be used by stability-oriented users.

Release candidate (2.0.0rc1)

A preview of a major release, made one or a few weeks beforehand at the time the release branch is created. There should be few if any changes from the rc to the stable release. We should avoid the confusing phrasing “release candidate 2.0.0rc1 is released”; instead use “available.” Starting with the 2.3 series we don’t plan on making release candidates anymore.

Bugfix releases (2.0.1)

Based on the previous major release or bugfix; contains only bugfixes and perhaps documentation or translation corrections.

Stable series

A major release and its descendant bugfix releases.

Stable release

Either a major release or a bugfix release.

Beta release (3.0.0beta1)

Made from trunk every month, except for the month there’s a major release. Stable and suitable for users who want the latest code and can live with some changes from month to month.

Development series

The development releases leading up to a stable release.

Bug Work

Bug fixes should normally be done first against the stable branch, reviewed against that branch, and then merged forward to trunk.

It may not always be easy to do this, if fixing the bug requires large changes or the affected code is different in the stable and development branches. If the tradeoff does not seem worthwhile the bug can be fixed only in the development branch, at least in the first instance. If users later want the fix backported we can discuss it.

Developers can merge the release branch into trunk as often as they like, only asking for review if they’re making nontrivial changes or feel review is needed.

Feature and Performance Work

Features can be landed to the development branch at any time, and they’ll be released for testing within a month.

Performance bugs, although important, will generally not be landed in a stable series. Fixing performance bugs well often requires nontrivial code changes or new formats. These are not suitable for a stable series.

Performance bugs that can be fixed with a small safe patch can be considered for the stable series.

Plugins

Plugins that want to cooperate with this should make a series and a branch that matches each bzr stable series, and follow similar rules in making releases from their stable branch. We’d expect that plugins will make a release between the first beta release of a series and the final major release.

Within a stable series, anything that breaks any known plugin is considered an API break and will be avoided. Before making each bugfix release, we’ll test that code against important plugins.

Within a development series, the focus is on helping plugin authors keep up to date by giving clear error messages when an interface is removed. We will no longer focus on letting old plugin code work with new versions of bzrlib, which is an elusive target in Python.

This may mean that in cases where today a plugin would keep running but give warnings, it will now fail altogether with an error.

In return we expect more freedom to change and cleanup bzrlib code without needing to keep old code around, or write extra compatibility shims, or have review turnarounds related to compatibility. Some changes, such as removing module-global variables, that are hard to do now, will be possible to do safely.

Discussion of plugins here includes programs that import and use bzrlib but that aren’t technically plugins. The same approach, though the technical considerations are different, should apply to other extensions such as programs that use bzr through the shell interface.

Data and Network Formats

Any development release should be able to interoperate with the previous stable release, and any stable release should be able to interoperate with the previous stable release. This is a minimum and normally releases will be able to interoperate with all previous releases as at present.

Each major release will have one recommended data format which will be the default. The name of the format will indicate which release series (not specific release) it comes from: ‘2a’ is the first supported format for the 2.0.x series, ‘2b’ the second, etc. We don’t mention the particular release that introduced it so as to avoid problems predicting precisely when it will land.

During a development series we may have a series of experimental formats. We will not leave people stranded if they test these formats, but we also won’t guarantee to keep supporting them in a future release. If something inserted in one development release turns out to be bad it can just be removed in the next.

Hosting Services

The guarantees made above about format and network interoperation mean that hosting services such as Launchpad, Savannah, FedoraHosted, and Sourceforge could choose to run either the stable or beta versions. They might find it useful to run the beta version on their own beta server.

Simultaneous Installation

Some people may want to simultaneously install and use both a stable release and development release.

This can be handled in various ways either at the OS packaging or the Python level. We don’t propose to directly address it in the upstream source. (For example, we will not change the bzrlib library name from one release to the next.)

The issue already exists with people who may want to use for example the previous bzr release and the trunk. There is a related issue that plugins may be compatible with only some of the Bazaar versions people want to use at the same time, and again that is something that can be handled separately.

OS Distributions

OS distributors will be recommended to ship the bzr stable release that fits their schedule, the betas leading up to that release during their own beta period, and the bugfix releases following on from it. They might also choose to offer the beta releases as an alternative package.

Packaging

At present we have three upstream-maintained PPAs containing Ubuntu packages of Bazaar: bzr/daily (snapshots), bzr-beta-ppa/ppa (beta releases) and bzr/ppa (ie stable). Beta contains the monthly beta releases, and the stable PPA contains stable releases and bugfixes to those releases.

Some platforms with relatively less active packagers may choose to ship only the stable releases. This is probably better than having them only intermittently or slowly ship the monthly releases.

Binary installers should use a version number like ‘2.0.0-1’ or ‘2.0.0beta1-1’ so that the last component just reflects the packaging version, and can be incremented if a new installer is made with no upstream source changes.

Code Freeze vs Announcement

We will separate the code freeze for a particular release from its actual announcement, allowing a window of approximately one week for plugins to be released and binary installers to be built. On the date the announcement is published, people will be able to easily install it.

Weekly Metronome Mail

Every week the release manager should send a mail to the Bazaar list covering these points (as appropriate):

  • Early communication about changing dependencies or defaults
  • Reminder re lifecycle and where we’re up to right now, in particular the dates for the next release and/or candidate.
  • Summary of recent successes and pending work.
  • Reminder re release objectives
  • Reminder re things needing attention, e.g. bug triage, reviews, testing of certain things, etc.

Questions

Do users actually want this?
Apparently yes, because it’s often requested and often raised as a problem.
Would this confuse users?
It shouldn’t, because it’s a fairly standard scheme.
Won’t it take more time to fix bugs in multiple places?
It shouldn’t, because we’ll only do this when the stable bugfix seems economical. When we fix bugs today in both trunk and release branches it normally does not take much more time.
What about bzr in Ubuntu LTS, with a five-year support life?
Most bugs are either fixed within six months, or not fixed at all, or not very important, or fixed as part of a large rework of the code that would be too large to backport. However, if there are fixes that are especially desired in an old release and feasible to do, we can do them without making a general commitment.
Will anyone test the beta releases?
Probably yes, our most active users will run them, but if people would really rather not test them, forcing them is not helpful.
Isn’t this a step backwards to a slower, less-agile process?
No, our trunk stays releasable, and we ship every month. We’re just cutting out things that hold us back (continuous rather than episodic API stability; RCs every month) and giving users what they demand.
How about calling the monthly releases “milestone” or “next” not “beta”?
Those words are less scary but they also have less clear meanings.

Expected Benefits

If this plan works, we’ll expect to see the following changes. If they don’t occur, we’ll think again:

  • We see a distribution curve of users and bug reports across nightly, monthly and stable releases, indicating that each has value.
  • API changes are easier or safer to make during beta periods, without being held back by fears of compatibility or
  • The stable releases are actually stable and don’t introduce regressions or break plugins.
  • Many bugs are fixed in stable branches, without developers feeling this is a waste of time.
  • Distributions ship the stable releases in their stable releases and the bugfix releases in their bugfix releases.
  • Plugin authors follow this policy, making their own bugfix releases.
  • Users like it.

After doing this for the 2.0 cycle (September 2009 through to early 2010), it seems to be going well.

Reviewing for the Stable Branch

These are guidelines and can be interpreted case-by-case.

  • All changes to the stable branch should fix a bug, even if you would not normally file a bug for the change. The bug description should if at all possible explain how to manually verify the bug in a way that will fail before and pass after the change. (These are requirements for the SRU process.)
  • The change should be reasonably small and conservative.
  • Remember that the patch will be read during the SRU process and so keeping the patch small is useful even beyond keeping the logical changes small. Avoid doing mechanical bulk changes on the stable branch.
  • Use particular care for things that may behave differently across platforms, encodings or locales. It’s harder to thoroughly test these things before a release.
  • Generally speaking, just cleaning things up is not a sufficient reason to make changes to the stable branch. It has to actually fix a bug.
  • Changes to the stable branch should include tests as usual.
  • Don’t change or remove existing APIs that might be used by plugins, even if they are underscore-prefixed. Adding APIs that are also being added to the trunk branch may make sense.
  • Keeping consistency with trunk is useful, but less important than keeping the stable branch stable.
  • (more items welcome)

References

  1. List thread “[rfc] six-month stable release cycles“, July 2009.