What is System State
To have a stable system, you need a repeatable process, and to that end the control server uses functional paradigms to decide the generated Kubernetes resources, change database state, or report usage stats. Those decisions on what to do next are derived from the summary state of the world. System State is the system we use to get an atomic summary to drive other parts of the system.
The main struct is KubeExt.SystemState.StateSummary. The struct is a carrier of many different parts that are all gathered together simultaneously when ControlServer.SystemState.Summarizer is called.
If you need a fresh and up-to-date StateSummary, call &new/1. If your use case tolerates staleness, you can use &cached/1. If you only need a small part of the summary, then call &cached_field/2
Some code may need derived data from the system state. That code lives in platform_umbrella/apps/kube_services/lib/kube_services/system_state and notable examples include:
- What the current best service to connect to the kubelet is (
platform_umbrella/apps/kube_services/lib/kube_services/system_state/monitoring.ex) - What is the best hostname to give out of installed battery systems (
platform_umbrella/apps/kube_services/lib/kube_services/system_state/hosts.ex) - What the configured namespaces are (
platform_umbrella/apps/kube_services/lib/kube_services/system_state/namespaces.ex)
Internals
ControlServer.SystemState.Summarizer is a genserver started in the control server app. Its leading utility is to run a transaction listing all of the database tables containing requested or configured state and snapshotting the current kube_state ETS table.
That result is then cached and sent to you.