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controllers.md

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Controllers

ATM All controllers are based on the same framework except harbor-cluster.

The framework manages basic status fields: observedGeneration and conditions.

Resource status

Combination of observedGeneration and conditions leads to the current state of the object:

Observed generation

Expose the last generation reconciled by the controller (or currently reconciling).

Conditions

Kubernetes expose .status.conditions which describe the current state of the resource.

Each conditions have 3 states: unknown, true, false. On Reconciler side, unknown is considered the same way than false.

1. In progress

True if the controller is currently reconciling the resource.

Failed

True if kubernetes resource is in expected state.

Control loop

The control loop is triggered by a Kubernetes event when one of the Resource controlled by the operator changes.

Event received  +------------ +
+-------------> |  Reconcile  |
                +------+------+
                       |
     (1)           Deleted? -----------> Exit
                       |            True
                       v
     (2)     Set InProgress = True
        observedGeneration = generation
                       |
                       v
     (3)         Compute all
              resources to apply
                       |
                       v
     (4)       For each resource
                       |
                       v
     (4.1)    Apply if necessary*
                       |
                       v
     (4.1)     Is resource ready? -----> Set Failed = true
                       |           False         |
                       v                         v
     (5)      Set Failed = False           Exit with error
              InProgress = True
                       |
                       v
                      Exit

Note

  • Step (1): Nothing to do with deleted resources since all owned resources are handled by kubernetes GC: https://kubernetes.io/docs/concepts/workloads/controllers/garbage-collection/#owners-and-dependents

  • Step (2) and (3): Update observedGeneration and

  • Step (3): Resources to deploy are computed in memory and added to a graph of dependency.

    • Leaves are resources that should be applied and ready.
    • Roots are resources that own resources that should be applied.
    • Branches are leaves and roots: resources that should be applied and resources that own other resources.
    • The main root of the graph is the custom resource.
    • Other roots of the graph are external resources not created by the controller.

  • Step (4.1):

    • To check if a resource needs to be applied, the controller compute the checksum of the owner resource (probably the custom resource) and the checksum of the owner when the resource was last applied.
      • The checksum of the last owner is stored in annotations.
      • The checksum is computed thanks to .metadata.generation or .metadata.resourceVersion of the owner depending of the context.
    • The apply method use server side apply.