book: added docs for status subresource

Author: droot

docs/book/SUMMARY.md

@@ -17,6 +17,7 @@
 * Resources
   * [What is a Resource](basics/what_is_a_resource.md)
   * [Resource Example](basics/simple_resource.md)
+  * [Status Subresource](basics/status_subresource.md)
 * Controllers
   * [What is a Controller](basics/what_is_a_controller.md)
   * [Controller Example](basics/simple_controller.md)

docs/book/basics/status_subresource.md

@@ -0,0 +1,125 @@
+# Status Subresource 
+By convention, the Kubernetes API makes a distinction between the specification
+of the desired state of an object (a nested object field called "spec") and the 
+status of the object at the current time (a nested object field called "status"). 
+The specification is a complete description of the desired state, including 
+configuration settings provided by the user, default values expanded by the 
+system, and properties initialized or otherwise changed after creation by other 
+ecosystem components (e.g., schedulers, auto-scalers), and is persisted in Etcd 
+with the API object. The status summarizes the current state of the object in 
+the system, and is usually persisted with the object by an automated processes 
+but may be generated on the fly. At some cost and perhaps some temporary 
+degradation in behavior, the status could be reconstructed by observation if it 
+were lost.
+
+The PUT and POST verbs on objects MUST ignore the "status" values, to avoid 
+accidentally overwriting the status in read-modify-write scenarios. A /status 
+subresource MUST be provided to enable system components to update statuses of 
+resources they manage.
+
+You can read more about the API convention in [Kubernetes API Convention
+doc](https://github.com/kubernetes/community/blob/master/contributors/devel/api-conventions.md#spec-and-status).
+
+{% panel style="info", title="Status subresource support in Kubernetes" %}
+Subresource support for CRD is enabled by default in 1.10+ releases
+, so ensure that you are running kubernetes with the minimum version. You can
+use `kubectl version` command to check the Kubernetes version.
+{% endpanel %}
+
+{% method %}
+## Enabling Status subresource in CRD definition
+First step is to enable status subresource in the CRD definition. This can be
+achieved by adding a comment `// +kubebuilder:subresource:status` just above the
+Go type definition as shown in example below.
+
+
+{% sample lang="go" %}
+```Go
+// MySQL is the Schema for the mysqls API
+// +k8s:openapi-gen=true
+// +kubebuilder:subresource:status
+type MySQL struct {
+	metav1.TypeMeta   `json:",inline"`
+	metav1.ObjectMeta `json:"metadata,omitempty"`
+
+	Spec   MySQLSpec   `json:"spec,omitempty"`
+	Status MySQLStatus `json:"status,omitempty"`
+}
+```
+
+CRD generation tool will use the `+kubebuilder:subresource:status` annotation to
+enable status subresource in the CRD definition. So, if you run, `make manifests`,
+it will regenerate the CRD manifests under `config/crds/<kind_types.yaml`. Here
+is an example manifests with status subresource enabled. Note the `subresources`
+section with an empty `status` field.
+
+```yaml
+apiVersion: apiextensions.k8s.io/v1beta1
+kind: CustomResourceDefinition
+metadata:
+  creationTimestamp: null
+  labels:
+    controller-tools.k8s.io: "1.0"
+  name: mysqls.myapps.examples.org
+spec:
+  group: myapps.examples.org
+  names:
+    kind: MySQL
+    plural: mysqls
+  scope: Namespaced
+  subresources:
+    status: {}
+  validation:
+    openAPIV3Schema:
+      properties:
+        apiVersion:
+          type: string
+        kind:
+          type: string
+        metadata:
+          type: object
+        spec:
+          type: object
+        status:
+          type: object
+  version: v1
+status:
+  acceptedNames:
+    kind: ""
+    plural: ""
+  conditions: []
+  storedVersions: []
+```
+
+Ensure you have updated the CRD definition in your cluster by running `kubectl
+apply -f config/crds`
+
+{% endmethod %}
+
+{% method %}
+## Getting and Updating status in Reconciler code
+
+In order to get the status subresource, you don't have do anything new. The
+`Get` client method returns the entire object which contains the status field.
+
+For updating the status subresource, compute the new status value and update it 
+in the object and then issue `client.Status().Update(context.Background(), &obj)` to update the
+status.
+
+{% sample lang="go" %}
+```go
+	instance := &myappsv1.MySQL{}
+	err := r.Get(context.TODO(), request.NamespacedName, instance)
+	if err != nil {
+		// handle err
+	}
+
+	// updating the status
+	instance.Status.SomeField = "new-value"
+	err = r.Status().Update(context.Background(), instance)
+	if err != nil {
+		return reconcile.Result{}, err
+	}
+```
+
+{% endmethod %}