Lately the OpenStack Board of Directors and Technical Committee has placed a lot of emphasis on making OpenStack clouds from various providers "interoperable". This is a very positive development, after years of different deployments adding various extensions and modifications to the upstream OpenStack code, which had made it hard to define just what it means to offer an "OpenStack Cloud". So the Interop project (formerly known as DefCore) has been working for the past few years to create a series of objective tests that cloud deployers can run to verify that their cloud meets these interoperability standards.
As a member of the OpenStack API Working Group, though, I've had to think a lot about what interop means for an API. I'll sum up my thoughts, and then try to explain why.
API Interoperability requires that all identical API calls return identical results when made to the same API version on all OpenStack clouds.
This may seem obvious enough, but it has implications that go beyond our current API guidelines. For example, we currently don't recommend a version increase for changes that add things, such as an additional header or a new URL. After all, no one using the current version will be hurt by this, since they aren't expecting those new things, and so their code cannot break. But this only considers the effect on a single cloud; when we factor in interoperability, things look very different.
Let's consider the case where we have two OpenStack-based clouds, both running version 42 of an API. Cloud A is running the released version of the code, while Cloud B is tracking upstream master, which has recently added a new URL (which in the past we've said is OK). If we called that new URL on Cloud A, it will return a 404, since that URL had not been defined in the released version of the code. On Cloud B, however, since it is defined on the current code, it will return anything except a 404. So we have two clouds claiming to be running the same version of OpenStack, but making identical calls to them has very different results.
Note that when I say "identical" results, I mean structural things, such as response code, format of any body content, and response headers. I don't mean that it will list the same resources, since it is expected that you can create different resources at will.
I'm sure this will be discussed further at next week's PTG.