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Objects In Kubernetes
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:
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
in the 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:
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 objectkind- What kind of object you want to createmetadata- Data that helps uniquely identify the object, including anamestring,UID, and optionalnamespacespec- 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.
Server side field validation
Starting with Kubernetes v1.25, the API server offers server side
field validation
that detects unrecognized or duplicate fields in an object. It provides all the functionality
of kubectl --validate on the server side.
The kubectl tool uses the --validate flag to set the level of field validation. It accepts the
values ignore, warn, and strict while also accepting the values true (equivalent to strict)
and false (equivalent to ignore). The default validation setting for kubectl is --validate=true.
Strict- Strict field validation, errors on validation failure
Warn- Field validation is performed, but errors are exposed as warnings rather than failing the request
Ignore- No server side field validation is performed
When kubectl cannot connect to an API server that supports field validation it will fall back
to using client-side validation. Kubernetes 1.27 and later versions always offer field validation;
older Kubernetes releases might not. If your cluster is older than v1.27, check the documentation
for your version of Kubernetes.
What's next
If you're new to Kubernetes, read more about the following:
- Pods which are the most important basic Kubernetes objects.
- Deployment objects.
- Controllers in Kubernetes.
- kubectl and kubectl commands.
To learn about the Kubernetes API in general, visit:
To learn about objects in Kubernetes in more depth, read other pages in this section: