Kubernetes pod queueing

[cioc]

Investigate how kubernetes queues pod requests.

[cioc]

Overall, yes there is queuing, but no, don't depend on ordering. The backing store is etcd and we rely on their watching mechanism at the lowest level.

Queueing is implied via the scheduler docs, " Its interface to the API server is to watch for Pods with an empty PodSpec.NodeName, and for each Pod, it posts a Binding indicating where the Pod should be scheduled. ... The scheduler tries to find a node for each Pod, one at a time, as it notices these Pods via watch." So, the queueing mechanism is kind of the pod meta data system.

https://github.com/kubernetes/kubernetes/blob/b829d4d4ef68e64b9b7ae42b46877ee75bb2bfd9/docs/devel/scheduler.md

Here is the queue that feeds the scheduler:

https://github.com/kubernetes/kubernetes/blob/1c5af3e72feb00cf6510c8c10abb991a0895e620/plugin/pkg/scheduler/factory/factory.go#L58

This is how it is filled:

https://github.com/kubernetes/kubernetes/blob/1c5af3e72feb00cf6510c8c10abb991a0895e620/plugin/pkg/scheduler/factory/factory.go#L388

That is actually a:

https://github.com/kubernetes/kubernetes/blob/1c5af3e72feb00cf6510c8c10abb991a0895e620/plugin/pkg/scheduler/factory/factory.go#L464

Which uses a ListWatch to actually determine the state changes:

https://github.com/kubernetes/kubernetes/blob/f2ddd60eb9e7e9e29f7a105a9a8fa020042e8e52/pkg/client/cache/listwatch.go#L48

Here's some etcd info on watching:

https://coreos.com/etcd/docs/latest/getting-started-with-etcd.html

This use of etcd is referenced in the kubernetes architecture docs:

https://github.com/eBay/Kubernetes/blob/master/docs/design/architecture.md#etcd