Binding workload with WEC

This document is about associating WECs with workload objects. The primary concept is sometimes called "downsync", which confusingly refers to both the propagation and transformation of desired state from core to WECs and the propagation and summarization of reported state from WECs to core.

Binding Basics

The user controls downsync primarily through API objects of kinds BindingPolicy and Binding. These go in a WDS and associate workload objects in that WDS with WECs, along with adding some modulations on how downsync is done.

BindingPolicy is a higher level concept than Binding. KubeStellar has a controller that translates each BindingPolicy to a Binding. A user could eschew the BindingPolicy and directly maintain a Binding object or let a different controller maintain the Binding object (TODO: check that this is true). The Binding object shows which workload objects and which WECs matched the predicates in the BindingPolicy and so is also useful as feedback to the user about that.

BindingPolicy

The spec of a BindingPolicy has two predicates that (1) identify a subset of the WECs in the inventory of the ITS associated with the WDS and (2) identify a subset of the workload objects in the WDS. The primary function of the BindingPolicy is to assert the desired association between (1) and (2). A BindingPolicy can also add some modulations on how those workload objects are downsynced to/from those WECs.

The WEC-selecting predicate is an array of label selectors in spec.clusterSelectors. These label selectors test the labels of the inventory objects describing the WECs. The bound WECs are the ones whose inventory object passes at least one of the the label selectors in spec.clusterSelectors.

The workload object selection predicate is in spec.downsync, which holds a list of DownsyncPolicyClauses; each includes both a workload object selection predicate and also three kinds of information that modulate the downsync. Note that each such clause must have at least one field specifying part of the workload selection predicate.

For more definitional details about a BindingPolicy, see the API reference.

Following is an example of a BindingPolicy object, used in the end-to-end test of createOnly functionality.

apiVersion: control.kubestellar.io/v1alpha1
kind: BindingPolicy
metadata:
  name: nginx
spec:
  clusterSelectors:
  - matchLabels:
      location-group: edge
  downsync:
  - objectSelectors:
    - matchLabels:
        app.kubernetes.io/name: nginx
    resources:
    - namespaces
  - createOnly: true
    objectSelectors:
    - matchLabels:
        app.kubernetes.io/name: nginx
    resources:
    - deployments

Binding

TODO: write this