Ensuring API Interoperability

The OpenStack mission includes the following goal for OpenStack clouds: “interoperable between deployments”. In the context of HTTP APIs this means that client code that is written for cloud A should also work on cloud B and any other cloud. Because cloud A and cloud B may be running different releases of the OpenStack services at any given point in time, and they may upgrade their services at different points in time, this has important implications for how API developers add features, fix bugs, and remove functionality from the service APIs.

If a service wants to ensure (and they should) that client code is always interoperable with multiple different OpenStack clouds then:

  • The changes must not violate compatibility nor stability, both of which are defined in more detail below.

  • Changes in resources and request and response bodies and headers are not allowed without signalling a version boundary, except in a very small number of cases (see below).


    APIs which have different behaviors and representations based on the availability of different drivers present significant challenges for interoperability. Where possible such differences should be avoided. Where that’s not possible, interoperability should be considered when cloud A and cloud B have the same sets of drivers available.

  • Version discovery and selection should default to a baseline; making use of new features and changes in an API should be opt-in.

  • When functionality is fully removed, this must result in the baseline being raised. Ideally this should result in the equivalent of a major version update because stability (backwards compatibility) has been violated. When a service uses microversions, “major version update” often means raising the minimum available version.

  • If functionality is going to be removed its removal should follow standard OpenStack deprecation procedures.

  • A service that wants to deprecate some functionality but maintain stability is welcome to do so by documenting the functionality as deprecated but keeping the functionality present in the API.

The gist here is that any service with a published API that also needs to make changes to that API (to fix or evolve it) needs to have a mechanism for versioning. This document does not address versioning, however the only mechanism in active use in OpenStack that has been demonstrated to work for the goals described here are microversions.

The rest of this document will describe the rules that a service MUST follow if it wishes to ensure API interoperability. It makes two assumptions that are important for understanding the compromises this document makes:

  • Change in APIs is an inevitable consequence of evolving projects with a diversity of contributors. In the unlikely event that such change is not a consideration then these guidelines need not apply.

  • The overarching goal of the API Special Interest Group is to encourage consistency in the APIs of all the services. The proposed solutions for enabling compatibility and stability are guidance towards achieving consistency. The assertions about when a change will violate interoperability are true independent of any given solution.

Any service which does not support versioning and wishes to achieve interoperability SHOULD do two things:

  • Add support for versioning.

  • Strive to follow these guidelines in a best-faith manner and where not possible consider changes with regard to reasonably expected behavior in client code.


A project which does not have a robust system for managing version boundaries but must make changes to their API is by definition not adhering to these guidelines for interoperability. The project itself must make the decisions on how to evolve their API. If this leads to conflicts with other projects within the OpenStack ecosystem, the role of the API-SIG is solely to help clarify the guidelines and provide advice on how to minimize the impact of changes. The Technical Committee is the body which provides adjudication and mediation when consensus cannot be reached.

Whether a service follows the guidelines or not, any service should always strive to minimize API changes and version increases as that exacerbates the need for users to “keep up” with the changes.

A service which is new and under development will obviously be undergoing a great deal of change in its early days. During this time versioning is not an indicator or tool for stability. However, once there has been a release or a dependency created with another service, stability and compatibility become critical if interoperability is desired.


For the sake of these guidelines the terms compatibility and stability are given fairly narrow definitions:


An API is compatible when client code written for that service on cloud A will work without changes with the same service on cloud B.


An API is stable when client code written at time X will continue to work without changes at future time Y.

These definitions assume that client code is written to follow the standards of HTTP.

When there is doubt in how these definitions should be applied to a situation consideration should be given first to the perspective and needs of the end users of the API (that is, those individuals who wish to use the same code against multiple clouds), second to the deployers and admins of the clouds, and third to the developers of the service.

Evaluating API Changes

There are two types of change which do not require a version change:

  • The change is required to fix a security bug that is so severe that it requires backporting to all supported releases of the service.

    This case should be rare. For many less severe security situations the right answer is to treat the problem as a bug, make the fix, make a new version and release, and suggest people upgrade. The OpenStack Vulnerability Management Team has the expertise to determine the true severity of a security bug.

  • A bug in the API service which results in a client getting a response with a status code in the 500-599 range being fixed to return an informative error response in the 400-499 range (when the request was erroneous but fixable) or responding with success (when the request was properly formed, but the server had broken handling).

The following changes do require a version change:

  • Adding a new URL.

    This may seem backwards-compatible, as old clients would never use the new URL, but it breaks interoperability between clouds presenting the same version of the API but using different code.

  • Changing the response status code from one form of client error to another (e.g., 403 to 400) or one form of success to another (e.g., 201 to 204).

    There continues to be debate on this topic with regard to changing success codes. A robust client could effectively ride through changes in success if it treated anything from 200 to 299 as success. This requires a different standard of client than these guidelines assume. Because there is already a great deal of client code out in the OpenStack ecosystem, enforcing a client-side standard such as the tolerant reader concept, is not possible.

  • Adding or removing a request or response header.

  • Changing the value of a response header which would change how the response should be processed by the client. For example changing the value of the Content-Type header to add a new media-type.

  • Adding or removing a property in a resource representation in either a request or a response.

  • Changing the semantics or type of an existing property in a resource representation (request or response).

  • Changing the set of values allowed in a resource property, while maintaining its type.

    For example if a property once accepted “foo”, “bar”, or “baz” and “zoom” was added as a legitimate value, that would require a version. If “foo” was removed that too would require a version. Both addition and removal are relevant here because we want two different clouds at the same API version (but with potentially different code releases) to behave the same. To get that, even apparently backwards compatible changes require a version change.

The following changes are possible if a version change is made but due consideration should be given to the impact this will have on existing users. At some point the user will want access to new functionality that is in higher versions or the minimum version of the service will be raised beyond the version where the change happens. The compensating changes in client code will be significant when any change is made, but especially so for these.

  • 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).

  • Removing a URL.


In many cases it will feel like a change is special and violation of these guidelines is warranted. Please consider the following scenarios:

“The change is needed to improve API consistency.”

Your desire to improve API consistency is appreciated and desired, but all APIs have warts. Inconsistencies that need breaking changes could be fixed in a new API version but it isn’t always necessary. Another option is to add a new URL with the different behavior. Consider all the options, finding a way to channel your efforts into improving the overall experience of using the API.

“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. An exception to this rule is when the functionality being changed is something that was literally non-functional.

“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. A change that will violate those observations is a change that requires a version.

“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.

New or Experimental Services and Versioning

As stated above, a brand new service should not commit to stability (as defined here) too early in its development. Only once some form of stability (in the standard English sense) has been reached is it worth considering.

A project which has an existing stable service that wants to experiment with new functionality that it may choose to never stabilize should publish that experimental service at a unique endpoint in the service catalog, separate from the existing service.