The Oslo Incubator

Problem description

One source of code for Oslo libraries is code in an existing project that is identified as useful to another project. The existing implementation may be tightly coupled to the application, and need to be modified to make it more general. This spec describes how the “incubator” repository can be used to evolve the module’s API to eventually make it suitable for release as a library.

Proposed Policy

The process of developing a new Oslo API usually begins by taking code which is common to some OpenStack projects and moving it into the oslo-incubator repository. New APIs live and evolve in the incubator until they have matured to meet the graduation criteria described below under “API Principles”.

Using the incubator helps increase developer performance by making it possible to quickly change the API of a module, while postponing adoption of that API in the consuming application until it is convenient for the application review team.

Consider incubated code as “statically linked” to an application, while a graduated modules are “shared libraries”.

Syncing Code from Incubator

APIs which are incubating can be copied into individual openstack projects from oslo-incubator using the script provided (which may be invoked through the convenient An openstack-common.conf configuration file in the project describes which modules to copy and where they should be copied to.

Usually the API maintainer or those making significant changes to an API take responsibility for syncing that specific module into the projects which use it by doing:

$> cd ../
$> git clone git://
$> cd oslo-incubator
$> ./ --nodeps --base nova --dest-dir ../nova --modules jsonutils,gettextutils

Alternatively, it can make sense for someone to batch sync more minor changes into a project together. To sync all code for a specific project, you can do:

$> ./ ../nova
Copying the config,exception,extensions,utils,wsgi modules under the nova module in ../nova

In this latter case, the script uses the openstack-common.conf config file to determine which modules to copy. The format of that file is:

$> cd ../nova
$> cat openstack-common.conf

# The list of modules to copy from oslo-incubator

# The base module to hold the copy of openstack.common

For master oslo-incubator sync requests, we tend to sync whole modules or even all the modules that a project uses (unless there are specific obstacles to doing so). This keeps the project up to date with the version of the modules tested together in the incubator.


Developers making major changes to incubating APIs in oslo-incubator must be prepared to work with the Oslo liaisons to update the copies in the projects which have previously imported the code.

The tests for incubated code should not be synced into consuming projects.

Stable Branches

For stable branches, the process is a bit different. For those branches, we don’t generally want to introduce changes that are not related to specific issues in a project. So in case of backports, we tend to do per-patch consideration when synchronizing from incubator.

Backporting for stable branches is a bit complicated process. When reviewing synchronization requests for those branches, we should not only check that the code is present in all consequent branches of the appropriate project (f.e. for N-2, in both N and N-1), but also that the patches being synchronized were successfully backported to corresponding stable branches of oslo-incubator. So the usual way of oslo-incubator bug fix is (in case of Neutron):

  1. oslo-incubator (master)
  2. neutron (master)
  3. oslo-incubator (stable/icehouse)
  4. neutron (stable/icehouse).

For N-2, it’s even more complicated, introducing more elements in the backporting chain.

Developer Impact

Projects which are using such incubating APIs must avoid ever modifying their copies of the code. All changes should be made in oslo-incubator itself and copied into the project.

All changes related to code synchronization from oslo-incubator, such as updating the use of APIs that have evolved, must be ‘atomic’ with the sync. For example, when syncing a new version of an incubator module that uses a library containing a newly graduated module, it may be necessary to update the rest of the application to use that same library, too. The Oslo team does not support versions of incubated modules with local changes, other than the import statements modified automatically by

API Principles

APIs included in Oslo should reflect a rough consensus across the project on the requirements and design for that use case. New OpenStack projects should be able to use an Oslo API safe in the knowledge that, by doing so, the project is being a good OpenStack citizen and building upon established best practice.

To that end, we keep a number of principles in mind when designing and evolving Oslo APIs:

  1. The API should be generally useful and a “good fit” - e.g. it shouldn’t encode any assumptions specific to the project it originated from, it should follow a style consistent with other Oslo APIs and should fit generally in a theme like error handling, configuration options, time and date, notifications, WSGI, etc.
  2. The API should already be in use by a number of OpenStack projects
  3. There should be a commitment to adopt the API in all other OpenStack projects (where appropriate) and there should be no known major blockers to that adoption
  4. The API should represents the “rough consensus” across OpenStack projects
  5. There should be no other API in OpenStack competing for this “rough consensus”
  6. It should be possible for the API to evolve while continuing to maintain backwards compatibility with older versions for a reasonable period - e.g. compatibility with an API deprecated in release N may only be removed in release N+2


Code in the incubator is expected to move out to its own repository to be packaged as a standalone library or project. When that process starts, the MAINTAINERS file should be updated so the status of the module(s) is “Graduating”. After this all changes should be proposed first to the new library repository, and then back-ported to the incubator if necessary. While the module is in the Graduating state, all bug fixes and features will need to be back-ported to the incubator and maintained in both repositories.

After the first release of the new library, the module(s) should be removed from the master branch of the incubator. During this phase, only critical bug fixes will be allowed to be back-ported to the prior stable branches. New features and minor bugs should be fixed in the released library only, and effort should be spent focusing on having downstream projects consume the library.

Testing Impact

Tests for incubated code should be managed in the test suite in the oslo-incubator repository. Tests should be organized in a way to make them easy to graduate along with the production code when the library is graduating.

The tests for incubated code should not be synced into consuming projects.

Specialist Maintainers

While incubating, all APIs should have at least one specialist API maintainer. The responsibility of these maintainers and the list of maintainers for each incubating API is documented in the MAINTAINERS file in oslo-incubator.

The maintainer’s +1 vote on code related to their specialty is considered as a +2 for approval purposes.


We assume the application from which the incubated code is copied will use the incubated version as part of the evolution process. One other application should be identified for integration as well, to provide more input into whether the API is sufficiently generalized. The number of applications using the incubated module should be minimized until the library graduates.

Because the graduated module is removed from the master branch of the incubator when the library is created, consuming applications should move to adopt the library as quickly as possible. We expect adoption to be complete by the end of the first full cycle after the new library is created.


Each new incubated module should have a full life-cycle plan worked out before incubation begins. See the default spec template in oslo-specs for details.

Anticipated API Stabilization

Incubation shouldn’t be seen as a long term option for any API – it is merely a stepping stone to inclusion into a published Oslo library.



Not all Oslo modules need to be incubated. Ideas for new libraries or library components may lead to creating a library “from scratch”. This approach is suitable when the purpose of the module is understood up front, and there is no existing code coupled to the rest of an application’s code base.

Examples of Oslo libraries that used this approach successfully include cliff, stevedore, and taskflow.


Existing libraries, possibly created outside of the OpenStack project, can be “adopted” by the Oslo team, who then takes over maintenance duties.

Examples of adopted Oslo libraries include tooz and PyCADF.

Incubating in Place

Not all application modules are tightly coupled to the other parts of the application in which they live. In these cases, it may be faster to evolve the API in place in the application’s repository, and graduate directly from there to a new library.

oslo.versionedobjects is an example of an Oslo library that incubated in place (in the nova source repository).


Revision History

Release Name Description
Essex Incubator created as openstack-common repository
Folsom “Managed copy and paste” concepts developed
Juno Decide to delete graduating modules from the incubator after one full cycle.
Kilo Change policy to delete graduating modules immediately after the library is released.


This work is licensed under a Creative Commons Attribution 3.0 Unported License.