Understanding Kubernetes Objects

This page explains how Kubernetes objects are represented in the Kubernetes API, and how you can express them in .yaml format.

Understanding Kubernetes objects

Kubernetes objects are persistent entities in the Kubernetes system. Kubernetes uses these entities to represent the state of your cluster. Specifically, they can describe:

  • What containerized applications are running (and on which nodes)
  • The resources available to those applications
  • The policies around how those applications behave, such as restart policies, upgrades, and fault-tolerance

A Kubernetes object is a “record of intent”—once you create the object, the Kubernetes system will constantly work to ensure that object exists. By creating an object, you’re effectively telling the Kubernetes system what you want your cluster’s workload to look like; this is your cluster’s desired state.

To work with Kubernetes objects—whether to create, modify, or delete them—you’ll need to use the Kubernetes API. When you use the kubectl command-line interface, for example, the CLI makes the necessary Kubernetes API calls for you. You can also use the Kubernetes API directly in your own programs using one of the Client Libraries.

Object Spec and Status

Almost every Kubernetes object includes two nested object fields that govern the object’s configuration: the object spec and the object status. For objects that have a spec, you have to set this when you create the object, providing a description of the characteristics you want the resource to have: its desired state.

The status describes the current state of the object, supplied and updated by the Kubernetes system and its components. The Kubernetes control plane continually and actively manages every object’s actual state to match the desired state you supplied.

For example: in Kubernetes, a Deployment is an object that can represent an application running on your cluster. When you create the Deployment, you might set the Deployment spec to specify that you want three replicas of the application to be running. The Kubernetes system reads the Deployment spec and starts three instances of your desired application—updating the status to match your spec. If any of those instances should fail (a status change), the Kubernetes system responds to the difference between spec and status by making a correction—in this case, starting a replacement instance.

For more information on the object spec, status, and metadata, see the Kubernetes API Conventions.

Describing a Kubernetes object

When you create an object in Kubernetes, you must provide the object spec that describes its desired state, as well as some basic information about the object (such as a name). When you use the Kubernetes API to create the object (either directly or via kubectl), that API request must include that information as JSON in the request body. Most often, you provide the information to kubectl in a .yaml file. kubectl converts the information to JSON when making the API request.

Here’s an example .yaml file that shows the required fields and object spec for a Kubernetes Deployment:

application/deployment.yaml

  1. apiVersion: apps/v1
  2. kind: Deployment
  3. metadata:
  4.   name: nginx-deployment
  5. spec:
  6.   selector:
  7.     matchLabels:
  8.       app: nginx
  9.   replicas: 2 # tells deployment to run 2 pods matching the template
  10.   template:
  11.     metadata:
  12.       labels:
  13.         app: nginx
  14.     spec:
  15.       containers:
  16.       - name: nginx
  17.         image: nginx:1.14.2
  18.         ports:
  19.         - containerPort: 80

One way to create a Deployment using a .yaml file like the one above is to use the kubectl apply command in the kubectl command-line interface, passing the .yaml file as an argument. Here’s an example:

  1. kubectl apply -f https://k8s.io/examples/application/deployment.yaml

The output is similar to this:

  1. deployment.apps/nginx-deployment created

Required Fields

In the .yaml file for the Kubernetes object you want to create, you’ll need to set values for the following fields:

  • apiVersion - Which version of the Kubernetes API you’re using to create this object
  • kind - What kind of object you want to create
  • metadata - Data that helps uniquely identify the object, including a name string, UID, and optional namespace
  • spec - What state you desire for the object

The precise format of the object spec is different for every Kubernetes object, and contains nested fields specific to that object. The Kubernetes API Reference can help you find the spec format for all of the objects you can create using Kubernetes.

For example, see the spec field for the Pod API reference. For each Pod, the .spec field specifies the pod and its desired state (such as the container image name for each container within that pod). Another example of an object specification is the spec field for the StatefulSet API. For StatefulSet, the .spec field specifies the StatefulSet and its desired state. Within the .spec of a StatefulSet is a template for Pod objects. That template describes Pods that the StatefulSet controller will create in order to satisfy the StatefulSet specification. Different kinds of object can also have different .status; again, the API reference pages detail the structure of that .status field, and its content for each different type of object.

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