Update ClusterVersion with a desired field and a history field

The current field is not useful because it doesn't represent level
driven flows well. Instead, track the version the cluster is working
towards as `status.desired` and each time that target changes update
the `status.history` field. The `status.history` field allows us to
know what previous updates were made and what times those completed.

Even though rollback is not desirable, we need to report a field that
indicates what the last successfully applied version was (user starts
at 4.0.0, then upgrades to 4.0.1, which fails, then a new 4.0.2
version is released that also fails, we need to report 4.0.0 as the
version to go back to).

config/v1/types_cluster_version.go

@@ -73,11 +73,19 @@ type ClusterVersionSpec struct {
 // progress, or is failing.
 // +k8s:deepcopy-gen=true
 type ClusterVersionStatus struct {
-	// current is the version that the cluster will be reconciled to. This
-	// value may be empty during cluster startup, and then will be set whenever
-	// a new update is being applied. Use the conditions array to know whether
-	// the update is complete.
-	Current Update `json:"current"`
+	// desired is the version that the cluster is reconciling towards.
+	// If the cluster is not yet fully initialized desired will be set
+	// with the information available, which may be a payload or a tag.
+	Desired Update `json:"desired"`
+
+	// history contains a list of the most recent versions applied to the cluster.
+	// This value may be empty during cluster startup, and then will be updated
+	// when a new update is being applied. The newest update is first in the
+	// list and it is ordered by recency. Updates in the history have state
+	// Completed if the rollout completed - if an update was failing or halfway
+	// applied the state will be Partial. Only a limited amount of update history
+	// is preserved.
+	History []UpdateHistory `json:"history"`
 
 	// generation reports which version of the spec is being processed.
 	// If this value is not equal to metadata.generation, then the
@@ -106,6 +114,46 @@ type ClusterVersionStatus struct {
 	AvailableUpdates []Update `json:"availableUpdates"`
 }
 
+// UpdateState is a constant representing whether an update was successfully
+// applied to the cluster or not.
+type UpdateState string
+
+const (
+	// CompletedUpdate indicates an update was successfully applied
+	// to the cluster (all resource updates were successful).
+	CompletedUpdate UpdateState = "Completed"
+	// PartialUpdate indicates an update was never completely applied
+	// or is currently being applied.
+	PartialUpdate UpdateState = "Partial"
+)
+
+// UpdateHistory is a single attempted update to the cluster.
+type UpdateHistory struct {
+	// state reflects whether the update was fully applied. The Partial state
+	// indicates the update is not fully applied, while the Completed state
+	// indicates the update was successfully rolled out at least once (all
+	// parts of the update successfully applied).
+	State UpdateState `json:"state"`
+
+	// startedTime is the time at which the update was started.
+	StartedTime metav1.Time `json:"startedTime"`
+	// completionTime, if set, is when the update was fully applied. The update
+	// that is currently being applied will have a null completion time.
+	// Completion time will always be set for entries that are not the current
+	// update (usually to the started time of the next update).
+	CompletionTime *metav1.Time `json:"completionTime"`
+
+	// version is a semantic versioning identifying the update version. If the
+	// requested payload does not define a version, or if a failure occurs
+	// retrieving the payload, this value may be empty.
+	//
+	// +optional
+	Version string `json:"version"`
+	// payload is a container image location that contains the update. This value
+	// is always populated.
+	Payload string `json:"payload"`
+}
+
 // ClusterID is string RFC4122 uuid.
 type ClusterID string