Recently I've returned to working on the OpenStack code base after a couple of years of working on related projects, and the sheer scope of the changes to the code, both in size and structure, is a bit overwhelming. I've also had to catch up with the current issues being discussed among the community, as a project that is growing the way OpenStack is will always have pain points. One such discussion that caught my attention concerns the very definition of what OpenStack is. The discussion not only addresses some of the experiences I've had returning to the world of OpenStack development, but it also feels like a continuation of the discussions we had when we were first shaping the project 4 years ago.
Sean Dague wrote an interesting take on this, envisioning OpenStack as a set of layers. The lowest layers provided the basic compute infrastructure, and the higher layers either built on this, or added additional capabilities. While I can see the basis for this division, it struck me as somewhat arbitrary; I mean, are Heat and Trove really that similar in either purpose or architecture?
Yesterday I read Monty Taylor's blog post on this topic, and I think he has much more accurately captured the relationship between the various parts of OpenStack. His post is much more far-reaching than simply describing OpenStack; it's really more concerned with how to help OpenStack move forward, and uses these relationships to make his case.
I think that working with the notion that there is a fundamental base for OpenStack (what Monty and Sean both refer to as 'Layer #1'), and that everything else is an add-on to this base, will have several positive results. First, it simplifies the internals of these projects. As Monty points out, "nova shouldn't need a config file option pointing to where glance is, it should just be able to ask the keystone service catalog". In other words, these Layer #1 projects can assume that they are all present, and don't have to add needless complexity to handle these relationships. Glance and Keystone have to be there, or Nova can't work - period.
Second, it streamlines the testing process considerably. The only tests for changes to any of these Layer #1 projects that need to be run are those for their internal workings, and those for their interfaces. This smaller, more manageable gate should relieve (but not eliminate) some of the testing bottlenecks OpenStack experiences today.
Speaking of interfaces, this is the third, and in my opinion, the most significant benefit of this concept: it forces the interfaces between the various projects to be defined clearly, and makes testing any potential change to these interfaces much cleaner. Let's take the example of Trove: it started as a separate database-as-a-service built on top of Nova, but now is being consumed internally to handle Nova's database needs. With the current "everything is OpenStack" approach, the lines between Nova and Trove might start to blur, and before long Trove and Nova would become too tightly bound to separate. By keeping the interface between the two clean and separate, it allows for the possibility that another DBaaS project may come along that performs better for Nova's needs, and can be adopted without having to rip out any hard assumptions about Trove. Similarly, other DBaaS could be developed that are optimized for other use cases, and these can test their interfaces to assure that they can also be implemented as a drop-in replacement for Trove.
Monty identified yet another benefit: there will no longer be a "race" for teams to get officially designated as part of OpenStack, as this designation would go away. Instead, there would be a known set of interfaces that a project would have to support, and if there is more than one group with an idea for implementing a particular need, they can each develop their code without having to first get "blessed" as somehow the "official" OpenStack solution for this need. Developing to a known interface will allow the community to test these different solutions much more easily, and allow a superior solution arise, instead of having to settle for the solution that got there first.
Finally, I see a side benefit to this kind of simplicity of structure. Someone coming to OpenStack for the first time, the sheer number of projects is daunting. Never mind having to learn all the names and what each project does; it's the feeling that OpenStack is too big to wrap your brain around that can potentially discourage new developers. A clear separation between the heart of OpenStack and the various peripheral projects would eliminate that confusion.
I'm greatly encouraged by these discussions, as it helps clarify the long-term direction of OpenStack. And I'm extremely happy to be part of it all once more.