DefCore: What OpenStack Devs Need to Know

As DefCore has become more established over the past year or so, I’ve started getting more questions from project developers and PTL’s. In most cases, they’re interested to know whether things they’re thinking about doing in their projects are “ok from a DefCore perspective” or would be “ok from an interoperability perspective”. In some cases they’re basically just giving a quick (and appreciated!) heads-up and sometimes they’re generating terrific discussions.

So before we go on, let’s take a quick timeout to give a some kudos to developers and PTL’s that are now more than ever thinking about interoperability early on when making technical decisions–that is fantastic.

In the interest of making DefCore more approachable, I thought I’d try to write up some of the general themes that people are asking about and that OpenStack devs seem to want to know about. So, what does an OpenStack developer or PTL need to know about DefCore with respect to things you want to do in your project?

Three terms you should know.

Consult the Lexicon for more lingo, but here a couple of key terms you should definitely know:

  • Capability: Generally, a feature or thing you can do with the software, usually exposed as an API. For example, listing volumes in Cinder or starting an instance in Nova. Capabilities are defined by tests (more on that later).
  • Criteria: One of the factors DefCore considers when determining whether a given Capability should be required for OpenStack Powered™ products. There are twelve in all which you can find information about here.
  • Guideline: A document published by DefCore and approved by the Board of Directors every six months which details the Capabilities that OpenStack Powered™ products must expose to end users and the sections of upstream code they must use to do so. I’ve heard these referred to as “the DefCore standards” occasionally. You can find the two most recently Board-approved Guidelines as of this writing here and here.

Any Capability DefCore considers must have tests.

When DefCore considers adding new Capabilities to a Guideline, there must be some way for clouds to prove that they expose those Capabilities and do so in an interoperable way. The teams of developers in the OpenStack community who are hacking on the code are in the best position to define what a given feature does and how it should work, so DefCore uses tests that are under the governance of the TC. Today, those tests are mostly the ones that live in Tempest, but we’re also now able to use tests that live in the individual project trees as long as they’re runnable via the Tempest plugin interface (by the recent request projects that have additional tests in-tree).

Key takeaway: how DefCore defines a working Capability is dependant on how you define it in your tests (which must be in Tempest or runnable via the Tempest plugin interface).

Capabilities and their tests should not require admin credentials.

If I’m using one of the many fine public clouds that are built on OpenStack, I’m not going to have admin privileges (many users of private clouds won’t either). Therefore, things that are generally only exposed to admins aren’t going to appear in DefCore Guidelines: they can’t be interoperable if end users can’t actually use them. In many cases, what is exposed to admins vs ordinary users is controlled by policy (e.g. the policy.json file that ships with most projects). In many products the default settings that ship from upstream are used by default, so that often becomes a good indicator: if it’s admin-only on (such as the ability to create provider networks in Neutron) it’s probably not going to be exposed to end users in most clouds and will therefore probably not appear in DefCore Guidelines.

The focus on end-user-accessible capabilities has an important implication for tests. If your tests (in Tempest or in-tree) use admin credentials for features that are generally exposed to end users, we can’t include those tests in DefCore Guidelines. If all tests for a Capability use admin credentials, the Capability effectively has no tests we can use, and therefore the whole Capability won’t make it into a Guideline.

If users can’t run the tests to verify that clouds support the Capabilities in question, having them in the Guideline becomes fairly toothless. It is therefore very important that Capabilities you want to see included in Guidelines both be exposed to end users by default and have tests that don’t require admin credentials.

Key takeaway: DefCore considers only things that are exposed to end users. Write your tests and policy files accordingly.

Adoption is key.

I’ve previously written about how DefCore is a trailing indicator of market acceptance. Basically it boils down to this: if few users using a given Capability and few products are supporting it, it’s very unlikely to appear in DefCore’s Guidelines. Market acceptance in practice often ends up affecting or reflecting a number of the DefCore Criteria. For instance:

  • Widely Deployed: Well, this one’s sort of obvious.
  • Used by Tools: If a feature or API isn’t supported by many clouds, it’s less likely that external SDK’s (jClouds, Fog, etc) and tools are going to add support for it since there’s probably less demand for doing so.
  • Used by Clients: if a feature isn’t supported by OpenStack’s own clients (OpenStackClient, the various python clients, perhaps Horizon) then that’s probably a sign that the feature isn’t fully matured yet and/or fewer people are using it (since there’s no client support).
  • Documented: End users tend to have a harder time using a given feature if it is poorly documented (or not documented at all).
  • Etc, etc, etc (you get the picture)

Key takeaway: if you want to something end up as a required Capability in the DefCore Guidelines, the best thing you can do is convince the broader ecosystem to adopt it first.

Note that you don’t have to just write a feature and hope people like it. Projects like libcloud or jClouds would probably be happy to review patches you submit that add support for a new API. The docs team would probably be happy to review patches to the docs describing how to use your new feature. Planet OpenStack would probably be happy to aggregate your blog post about how great it is and why people should use it.

More than one way of doing things may be ok…sort of.

Some folks have been surprised to learn that there’s actually no rule barring DefCore Guidelines from including multiple ways to do something. There are a couple of scenarios in which this happens:

  1. API iteration. Sometimes it’s simply a matter of more than one version of an API being available: for example, in the Liberty release one could generate a token by talking to the Keystone v2 API (which was designated as SUPPORTED) or the Keystone v3 API (which was marked CURRENT). Two API’s usually have separate tests, and all versions of the API might not be available in all the releases covered by a single Guideline. DefCore considers them separate Capabilities. Clearly, this can be a bit of a conundrum: projects may want long support periods for older API’s because they want to give end users ample time to migrate, or simply don’t ever want to break users. API microversioning has become a popular way of helping ease introduction of new API’s in recent years.

  2. Same output, very different mechanics. In some cases, there are Capabilities in which the end result is the same but the mechanics and use cases are very, very different. For example: I can create an image by snapshotting a running compute instance. At the end of that operation, what I end up with is a bootable image. I could also create a bootable image by uploading an image file to Glance. The use cases here are obviously quite different though: in one case I’m creating an image from an instance I already have running in the cloud, and in the other I’m uploading one from outside the cloud. So it’s quite reasonable to think that end users might want to have both Capabilities exposed to them. Additionally, here again we have separate API’s with separate tests. This scenario generally isn’t as much of a problem as long as it’s well understood and well documented.

  3. Really pretty much the same. In a few cases, there really is more than one way to do a thing without substantial differences in the use cases. For example, I can list images available to me via the Glance API or via the Nova image API (which is basically a proxy to Glance v1). Both give me a listing of images, and assuming both are available I could probably use either one for most use cases.

  4. Some mixture of 1-3. Occasionally we run into cases where there’s some combination of the above scenarios in play. For example, there are plenty of clouds where two versions of the Glance API and the Nova image API are all exposed to users, so there are at least 3 ways to list images.

It’s important to note that when there’s more than one way to do something it’s sometimes harder for any of them to make their way into DefCore Guidelines. That boils down to adoption and inertia: if there are several ways to do the same thing then different SDK’s, tools, and end users often pick different ways of doing things, and no one (or more) way becomes clearly well adopted. Once SDK’s, tools, and end users have settled on a method of doing something, there’s a real cost to them to change to something else (and if it isn’t broken, why change?). Therefore they tend to stick with the option they’ve chosen unless they’re given a really good reason to switch (like, for example, a given API being officially deprecated).

In some cases we may find that more than one method of doing something does actually become very well adopted, supported by tools and clients, etc and therefore score high enough to be included in a Guideline. But in practice, it’s often a tougher hill to climb.

Key takeaway: be careful when introducing multiple ways to do things. It’s not strictly disallowed, but creating fragmentation might make it harder for capabilities to become well adopted and therefore become interoperable.

As an aside, there are a few things projects can do to help nudge acceptance of one method or another when there are multiple ways to do something:

  • If your project really doesn’t intend to maintain an older API any more, it may be useful to start the deprecation process sooner rather than later as a clear signal to users, SDK’s, and tools that it’s time to move (this also signals DefCore that the old API shouldn’t score points for the Future Direction criteria, too).
  • If you have a feature that you think is primarily oriented at internal use (e.g. one OpenStack service talking to another) and shouldn’t be exposed to end users, you should clearly document it as such (and, if it makes sense to do so, restrict it’s use via default settings in the policy files).

Keep in mind there’s a long tail.

DefCore Guidelines cover three OpenStack releases. For example, the most recently Board-approved DefCore Guideline covers Juno, Kilo, and Liberty. The previous Guideline covered Icehouse, Juno, and Kilo. When a product asks for an OpenStack Powered™ logo and license agreement from the OpenStack Foundation, they can choose to do so under either of the two most recently Board-approved Guidelines. Which Guideline the product used is noted in the MarketPlace.

Because a Guideline applies to three releases, a given Capability must have been supported in at least all three of those upstream releases. If you introduce a new feature today, it’s going to take some time before it’s realistically possible to get it into the DefCore Guidelines. And bear in mind that it’s not just time: once the Capability has been around for a while, it still needs to meet all the other DefCore Criteria before it makes it’s way into the Guidelines.

Key takeaway: be patient and promote adoption. The Criteria we’re using are documented, so if you’re motivated to help a given capability achieve those criteria (maybe by contributing support for it to popular tools/SDK’s, or by beefing up documentation, or by generally evalgelizing a feature or API), go for it!

There are some useful documents in the DefCore Git repo.

Much of the documentation about how and when DefCore does things and how it makes it’s decisions is actually documented. You’re always free to come ask questions (see below), but if you’re the sort that would like a little bedtime reading, you might start with things like:

  • The current process doc which explains timelines, key actors, etc.
  • The Criteria doc which explains factors that DefCore looks at when determining whether to include a Capability in it’s Guideline documents.
  • The Foundation’s Interop page which has information about the OpenStack Powered™ program and what’s included in it today. Among other things, you’ll find links to the two most recent Board-approved Guidelines and contact information.

Oh, and if by chance you’re a fan of Doctor Who you might also want to peruse this presentation that I did a while back. It’s a little dated now, but you’ll at least be mildly entertained.

Questions? Comments? Drop by #openstack-defcore on IRC or drop the DefCore Committee a line at

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