Evaluating API Changes

This guideline provides help to developers, core reviewers, and QA engineers on evaluating whether a given API-impacting change is acceptable with respect to the OpenStack governance policy on API stability[1].


As of May 2015 this guideline captures historical stances on evaluating API changes as they were defined on the OpenStack wiki.


The following types of changes are generally considered acceptable:

  • The change is the only way to fix a security bug.
  • Fixing a bug so that a request which resulted in an error response before is now successful.
  • Adding a new response header.
  • Changing an error response code to be more accurate.

The following types of changes are acceptable when conditionally added as a new API extension:

  • Adding a property to a resource representation.
  • Adding an optional property to a resource representation which may be supplied by clients, assuming the API previously would ignore this property.

The following types of changes are generally not considered acceptable:

  • A change such that a request which was successful before now results in an error response (unless the success reported previously was hiding an existing error condition).
  • Changing or removing a property in a resource representation.
  • Changing the semantics of a property in a resource representation which may be supplied by clients.
  • Changing or removing a response header.
  • Changing which response code is returned on success.

You may feel a particular case is special and warrants an incompatible API change. Please consider these responses to commonly used justifications:

“The change is needed to improve API consistency.”

Your desire to improve API consistency is appreciated, but all APIs have warts. Inconsistencies that need breaking changes can be fixed in a new API version. Please find a way to channel your efforts into preparations to fix these consistencies in the next API version.

“It is unlikely that any existing users of the API would be affected.”

It is difficult to predict how people are using the APIs. Developers do the strangest things. As our APIs become more adopted over time, it will only become more futile to attempt to make such predictions. One thing you can do is to help improve our documentation about how our APIs should be used. If we can document in future versions that we do not make guarantees about certain behaviours, that may give us some small additional leeway when it comes to making changes.

“The existing API is not well documented.”

If an API’s behavior isn’t adequately documented, then developers using the API have no choice but to go by what they observe the behavior to be.

“The change does not impact users of OpenStack’s client libraries or command line interfaces.”

We encourage developers to develop against OpenStack REST API. There will be many tools and applications which favor the REST API over our libraries or command line interfaces.


The following examples represent a selection of appropriately implemented API changes based on the guidance provided in this document.

Failing silently

At one point the change password nova API would return success even if the system was unable to do it. This made sense to fix because any client checking the response code of this request surely wants to know if the request failed. Any existing client which this change affects was broken anyway because it was failing to actually change the admin password.

Out of spec features belong in extensions

The config_drive attribute on servers is not part of the 1.1 Compute API spec, so it was moved into an extension which could be disabled.

Adding new header OK; changing response code not so much

Rather than returning “200 OK” for successful volume creation, we should return “201 Created” and include a Location header. It was decided that it is safe to add the Location header, but not change the response code.

Inappropriate extension

Sometimes you come across an extension that makes no sense and is highly unlikely to be used by anyone.

Bugfixes OK

Fixing incorrect counting of hosts in instance usage audit log.

Extension aliases

500 error

Inconsistent handling of volume attach device

Missing Property From XML Representation


[1]: https://wiki.openstack.org/wiki/Governance/Approved/APIStability

Mailing list discussion, “Standardizing status codes in the native API (July 2012)”. http://lists.openstack.org/pipermail/openstack-dev/2012-July/thread.html#132

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