Exploring Object Specs

So lets discuss about a new command kubectl explain so that we don’t have to remember all YAML specs of kubernetes objects.

With kubectl explain subcommand , you can see the specification of each objects and can use that as a reference to write your YAML files.

Fist level spec

We will use kubectl explain Pod command to see the specifications of a Pod YAML.

$ kubectl explain Pod

Output

ubuntu@k8s-master-01:~$ kubectl explain pod
KIND:     Pod
VERSION:  v1

DESCRIPTION:
     Pod is a collection of containers that can run on a host. This resource is
     created by clients and scheduled onto hosts.

FIELDS:
   apiVersion	<string>
     APIVersion defines the versioned schema of this representation of an
     object. Servers should convert recognized schemas to the latest internal
     value, and may reject unrecognized values. More info:
     https://git.k8s.io/community/contributors/devel/api-conventions.md#resources

   kind	<string>
     Kind is a string value representing the REST resource this object
     represents. Servers may infer this from the endpoint the client submits
     requests to. Cannot be updated. In CamelCase. More info:
     https://git.k8s.io/community/contributors/devel/api-conventions.md#types-kinds

   metadata	<Object>
     Standard object's metadata. More info:
     https://git.k8s.io/community/contributors/devel/api-conventions.md#metadata

   spec	<Object>
     Specification of the desired behavior of the pod. More info:
     https://git.k8s.io/community/contributors/devel/api-conventions.md#spec-and-status

   status	<Object>
     Most recently observed status of the pod. This data may not be up to date.
     Populated by the system. Read-only. More info:
     https://git.k8s.io/community/contributors/devel/api-conventions.md#spec-and-status

ubuntu@k8s-master-01:~$

As we discussed earlier , the specification is very familiar.

Filed status is readonly and its system populated , so we don’t have to write anything for status.

Exploring inner fields

If we want to see the fields available in spec , then execute below command.

$ kubectl explain pod.spec

KIND:     Pod
VERSION:  v1

RESOURCE: spec <Object>

DESCRIPTION:
     Specification of the desired behavior of the pod. More info:
     https://git.k8s.io/community/contributors/devel/api-conventions.md#spec-and-status

     PodSpec is a description of a pod.

FIELDS:
...
containers	<[]Object> -required-
  List of containers belonging to the pod. Containers cannot currently be
  added or removed. There must be at least one container in a Pod. Cannot be
  updated.
...

How easy is that.

As you can see in spec the containers filed is -required- which indicates that this filed is mandatory.

<[]Object> indicates that its an array of objects , which means , you can put more than one element under containers

That make sense , because the Pod may contain more than one container.

In YAML we can use - infront of a filed to mark it as an array element.

Lets take a look at the YAML that we wrote earlier

apiVersion: v1
kind: Pod
metadata:
 name: coffee-app01
 labels:
  app: frontend
  run: coffee-app01
spec:
 containers:
 - name: demo-coffee
   image: ansilh/demo-coffee

There is a - under the fist filed of the containers. If we say that in words ; “containers is an array object which contains one array element with filed name and image”

If you want to add one more container in Pod , we will add one more array element with needed values.

apiVersion: v1
kind: Pod
metadata:
 name: coffee-app01
 labels:
  app: frontend
  run: coffee-app01
spec:
 containers:
 - name: demo-coffee
   image: ansilh/demo-coffee
 - name: demo-tea
   image: ansilh/demo-tea

Now the Pod have two containers .

How I know the containers array element need name and image ?

We will use explain command to get those details.

$ kubectl explain pod.spec.containers

Snipped Output

...
name	<string> -required-
  Name of the container specified as a DNS_LABEL. Each container in a pod
  must have a unique name (DNS_LABEL). Cannot be updated.

image	<string>
  Docker image name
...

As you can see , name and image are of type string which means , you have to provide a string value to it.