Development policies

Out Of Tree Support

While nova has many entrypoints and other places in the code that allow for wiring in out of tree code, upstream doesn’t actively make any guarantees about these extensibility points; we don’t support them, make any guarantees about compatibility, stability, etc.

Furthermore, hooks and extension points in the code impede efforts in Nova to support interoperability between OpenStack clouds. Therefore an effort is being made to systematically deprecate and remove hooks, extension points, and classloading of managers and other services.

Public Contractual APIs

Although nova has many internal APIs, they are not all public contractual APIs. Below is a link of our public contractual APIs:

Anything not in this list is considered private, not to be used outside of nova, and should not be considered stable.


Follow the guidelines set in:

The canonical source for REST API behavior is the code not documentation. Documentation is manually generated after the code by folks looking at the code and writing up what they think it does, and it is very easy to get this wrong.

This policy is in place to prevent us from making backwards incompatible changes to REST APIs.

Patches and Reviews

Merging a patch requires a non-trivial amount of reviewer resources. As a patch author, you should try to offset the reviewer resources spent on your patch by reviewing other patches. If no one does this, the review team (cores and otherwise) become spread too thin.

For review guidelines see: Code Review Guide for Nova

Reverts for Retrospective Vetos

Sometimes our simple “2 +2s” approval policy will result in errors. These errors might be a bug that was missed, or equally importantly, it might be that other cores feel that there is a need for more discussion on the implementation of a given piece of code.

Rather than an enforced time-based solution - for example, a patch couldn’t be merged until it has been up for review for 3 days - we have chosen an honor-based system where core reviewers would not approve potentially contentious patches until the proposal had been sufficiently socialized and everyone had a chance to raise any concerns.

Recognising that mistakes can happen, we also have a policy where contentious patches which were quickly approved should be reverted so that the discussion around the proposal can continue as if the patch had never been merged in the first place. In such a situation, the procedure is:

  1. The commit to be reverted must not have been released.

  2. The core team member who has a -2 worthy objection should propose a revert, stating the specific concerns that they feel need addressing.

  3. Any subsequent patches depending on the to-be-reverted patch may need to be reverted also.

  4. Other core team members should quickly approve the revert. No detailed debate should be needed at this point. A -2 vote on a revert is strongly discouraged, because it effectively blocks the right of cores approving the revert from -2 voting on the original patch.

  5. The original patch submitter should re-submit the change, with a reference to the original patch and the revert.

  6. The original reviewers of the patch should restore their votes and attempt to summarize their previous reasons for their votes.

  7. The patch should not be re-approved until the concerns of the people proposing the revert are worked through. A mailing list discussion or design spec might be the best way to achieve this.

Metrics Gathering

Nova currently has a monitor plugin to gather CPU metrics on compute nodes. This feeds into the MetricsFilter and MetricsWeigher in the scheduler. The CPU metrics monitor is only implemented for the libvirt compute driver. External projects like Ceilometer and Watcher consume these metrics.

Over time people have tried to add new monitor plugins for things like memory bandwidth. There have also been attempts to expose these monitors over CLI, the REST API, and notifications.

At the Newton midcycle it was decided that Nova does a poor job as a metrics gathering tool, especially as it’s incomplete, not tested, and there are numerous other tools available to get this information as their primary function.

Therefore, there is a freeze on adding new metrics monitoring plugins which also includes exposing existing monitored metrics outside of Nova, like with the nova-manage CLI, the REST API, or the notification bus. Long-term, metrics gathering will likely be deprecated within Nova. Since there is not yet a clear replacement, the deprecation is open-ended, but serves as a signal that new deployments should not rely on the metrics that Nova gathers and should instead focus their efforts on alternative solutions for placement.

Continuous Delivery Mentality

Nova generally tries to subscribe to a philosophy of anything we merge today can be in production today, and people can continuously deliver Nova.

In practice this means we should not merge code that will not work until some later change is merged, because that later change may never come, or not come in the same release cycle, or may be substantially different from what was originally intended. For example, if patch A uses code that is not available until patch D later in the series.

The strategy for dealing with this in particularly long and complicated series of changes is to start from the “bottom” with code that is no-op until it is “turned on” at the top of the stack, generally with some feature flag, policy rule, API microversion, etc. So in the example above, the code from patch D should come before patch A even if nothing is using it yet, but things will build on it. Realistically this means if you are working on a feature that touches most of the Nova “stack”, i.e. compute driver/service through to API, you will work on the compute driver/service code first, then conductor and/or scheduler, and finally the API. An extreme example of this can be found by reading the code review guide for the cross-cell resize feature.

Even if this philosophy is not the reality of how the vast majority of OpenStack deployments consume Nova, it is a development philosophy to try and avoid merging broken code.