You heard wrong.

There’s a meme out there, helpfully nudged along by Google, that Kubernetes is Borg “done right” and the successor. It’s even mentioned in this article. Neither of those things are true. Not even remotely. Please pass along to everyone to stop repeating the meme, because it distracts from Kubernetes’ true purpose, which is to lock people into GKE and force competitors to ship a Kubernetes runtime to compete. It’s partly the OpenStack playbook: if everybody runs the same platform, competition inherently drifts toward other aspects of the businesses, such as customer support. Seriously, I’m the only one who noticed the timing of Kubernetes and Google Cloud? Really? But Google shipped it, so now it has an ecosystem, and zealots who force entire teams onto it for zero upside and nonzero overhead, while the system that actually looks like Borg, the far superior to k8s HashiCorp Nomad, twiddles its thumbs with pretty much no mindshare.

For one, if Kubernetes were the successor to Borg, they wouldn’t have hobbled it architecturally as much as they have by marrying it to both Docker (kinda) and etcd, and deciding in the beginning to do every cluster mutation via external consensus in etcd, because you know, that’s a great idea and a classic Google design. Remember when Kubernetes pushed all job state changes through consensus and a flapping job could OOM etcd? I do too. Someone cynical could argue its fundamental architectural limitations are intentional. (I would argue simply that it didn’t have Paul Menage and most of the other names on the Borg paper working on it, to my knowledge.) I hear keyboards getting angry to yammer about how it just works. Not at seven-digit machine scale, it doesn’t, and never will. I’m happy it works for you. It’s a toy for a large fleet, which I’ll revisit in another point. Everyone I am aware of at Borg or Mesos scale has ruled out or failed with Kubernetes. No, really.

Relatedly, if Kubernetes were the successor to Borg, it’d be in C++. It just would be, and that’s not a language flame war. Ever wonder what percentage of systems at Google are C++? Ever wonder what that number does when you qualify “infrastructural”?

For two, Google containers aren’t an entire operating system, unlike Docker without crazy gymnastics. Seriously, this paragraph could be an essay. To paraphrase Jeremy Clarkson, Docker looks like a proper container system described over a blurry fax. Maybe we do need seventy probably-not-deduplicated copies of getty on every machine, and I’m yelling at clouds. I doubt it, and it reeks of “disk is cheap, fuck it.”

For three, Kubernetes is several orders of magnitude behind Borg and Omega, if that’s still alive, in terms of scheduling performance and maximum “cluster” size (I quote cluster because Google identifies a Borg unit as a cell, and a cluster means something else). This is not fixable in Kubernetes, in my reasonably informed opinion, without doing consensus differently. To my point, Borg does consensus and cluster state much differently, and you know what? It’s fine. Anybody who has used fauxmaster will back me up on that, and John Wilkes even said yup, every time we hit a limit we manage to double it with no end in sight. Why would that suddenly change? etcd remains Kubernetes’ Achilles heel, and this is why messaging around Kubernetes has gravitated toward smaller, targeted clusters. Bonus: if you find a bug in etcd make sure it loudly affects Kubernetes so it gets properly prioritized. Double bonus: someone was brave enough to suggest Consul in #1957 and children. Go read and sigh.

For four, when’s the last time you ran a 10,000+ node MapReduce on Kubernetes? Surprise, the underpinnings of Borg handle both batch and interactive with the exact same control plane, which is where the billions of containers a day number they occasionally talk about comes from. I mean, several JARs of glue might get you to Hadoop scheduling via Kubernetes, but that’s a much different animal than the platform thinking in terms of jobs with different interactivity requirements.

For five, half of Borg is the shit around it. Borg works because everything behaves the same. Everything is a Web server. Everything exposes /statusz. Everything builds and monitors the same way. Everything speaks the same RPC to each other. All of this is implemented by forcing production systems at Google to choose from four (as of my tenure) languages which are well tended and manicured by hundreds of people. Google has a larger C++ standard library team than many startups have engineers. Borg works because apps let it work. They’re not black boxes. Unlike Kubernetes.

Which brings me to point the sixth, which is that the reason you haven’t seen open source Borg is (a) they’re not moving off it, like, ever (I’d bet my season tickets on it), because significant parts of every production system and tool would have to change and (b) they can’t unravel Borg and the rest of the google3 codebase, because it’s so fundamental to the Google ecosystem and half of Borg’s magic is wrapped up in other projects within Google which they aren’t keen to show you.

Link to this answer next time anyone gets tempted to relay what they’ve heard about Borg and Kubernetes, please. For years I’ve watched this tale evolve until it’s barely recognizable as factual. Saying Kubernetes is Borg’s proper successor not only drastically insults Borg and the hundreds (thousands?) who have worked on it, it also calls to mind thinking of a cotton candy machine as the successor to the automobile. They’re that different.