Understanding Kubernetes Objects
This page explains how Kubernetes objects are represented in the Kubernetes API, and how you can express them in
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
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.
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
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:
apiVersion: apps/v1 kind: Deployment metadata: name: nginx-deployment spec: selector: matchLabels: app: nginx replicas: 2 # tells deployment to run 2 pods matching the template template: metadata: labels: app: nginx spec: containers: - name: nginx image: nginx:1.14.2 ports: - containerPort: 80
One way to create a Deployment using a
.yaml file like the one above is to use the
kubectl apply command
kubectl command-line interface, passing the
.yaml file as an argument. Here's an example:
kubectl apply -f https://k8s.io/examples/application/deployment.yaml
The output is similar to this:
.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
UID, and optional
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
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
for the StatefulSet API. For StatefulSet, the
.spec field specifies the StatefulSet and
its desired state.
.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.