Unstable APIs without an incubator

We want to get rid of the incubator, but there is no replacement process for stabilising new APIs.

Problem description

The incubator with its copy-paste approach to code has a harmful side to it - we end up with copied code in a bunch of projects that needs to be updated and can skew - in particular we not uncommonly find that projects have edited their local copies and synchronisation is tricky.

Proposed change

When a new project is started, create the library immediately. Use an API namespace within the library to offer concurrent access to multiple API versions during the early evolution of the library, with graduation involving exposing the API at the top of the library. CI checks will let us evaluate the impact of removing the interim API versions when consumers no longer use them.

Project versions that release while using interim versions will cause us to retain the interim version for some time - in line with normal backward compatibility concerns.

Once projects have finished early evolution and are ready to stabilise their API, they will release a 1.0.0 version a non-namespaced API.

API update policy

Non-breaking changes will proceed as normal. Breaking changes during the early evolution period will create a new API namespace, using a serial number - 1, 2, 3 etc.

Code structure

Each unstable API version will be in a subdirectory of the project along with its tests.

E.g. if a new library fred has had two unstable API versions, the code tree would look like:


Where v2 starts as a copy of v1, and v1 is frozen from that point on.

Using a namespaced version

Code should be imported as follows:

from fred import v1 as fred


Where a library consumes entrypoints as part of its API, while the API is being namespaced, so must the entrypoints, in the same way:




We could just apply semver from the very first commit. Folk have expressed great concern about the impact of development that this would have.

Use library name namespaces

Rather than having multiple versions inside one distribution, we could version the name of the library. E.g. fred0, fred1, fred2 etc until the API stabilises. This pollutes the global namespace though, and will require lots of changes to global-requirements, making it high friction.

Impact on Existing APIs

We’ll want a tool to warn on the use of older-than-latest api versions in oslo projects: much like the automated update-from-incubator scripts do today.

Security impact

It may be slightly easier to keep using old versions of in-incubation code.

Performance Impact

Packages will git a little bigger (multiple copies of similar code), but the number of copies on disk will reduce (since they won’t be copied into each project).

Configuration Impact


Developer Impact

Projects will no longer be able to make local changes to incubated projects and will instead need to work with oslo to get their changes done centrally from the start.

Existing workflows will change, so we need to socialise and communicate effectively about the change.

Testing Impact

No impact.



Written at Dims’ request, no current volunteers to execute.

Primary assignee:

<launchpad-id or None>

Other contributors:

<launchpad-id or None>


Target Milestone for completion:

Work Items

  1. Document this somewhere in olso docs

  2. Write the update / lint thing to check for the latest version being used.

  3. Profit.


N/A - replaces Incubation entirely.

Documentation Impact

None that I know of.


No dependencies, though a mild interaction with the deprecation policy will happen - in that we’ll be releasing things we want to delete as soon as possible. See above for details.



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