Kubernetes Borg/Omega history topic 5: Asynchronous controllers. Borgmaster had synchronous, transactional, edge-triggered state machines. We had challenges scaling, evolving, and extending them.
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We also fully embraced the controller model, even for Kubelet, by making Kubelets report back to apiserver (http://issues.k8s.io/156 ) and patch status (http://issues.k8s.io/2726 ) so that the API could be used as the source of truth by other controllers.
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Rather than rigid fine-grained state enumerations that couldn't be evolved, we initially adopted simple basic states that could report open-ended reasons for being in each state (http://issues.k8s.io/1146 ), and later non-orthogonal, extensible conditions (http://issues.k8s.io/7856 ).
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The entire system can now be described as an unbounded number of independent asynchronous control loops reading and writing from/to a schematized resource store as the source of truth. This model has proven to be very resilient, evolvable, and extensible.
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