Concepts
Table of Contents
1 Kubernetes Objects
Kubernetes Objects are persistent entities in the Kubernetes system. Kubernetes uses these entities to represent the state of your cluster.
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.
1.1 Object Spec and Status
Every Kubernetes object includes two nested object fields that govern the object’s configuration: the object spec and the object status. The spec, which you must provide, describes your desired state for the object–the characteristics that you want the object to have. The status describes the actual state of the object, and is supplied and updated by the Kubernetes system. At any given time, the Kubernetes Control Plane actively manages an object’s actual state to match the desired state you supplied.
1.2 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).
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
apiVersion: apps/v1 # for versions before 1.9.0 use apps/v1beta2
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:1.7.9
ports:
- containerPort: 80
Create the Deployment object with the yaml file.
kubectl apply -f https://k8s.io/examples/application/deployment.yaml --record
The output is similar to this:
deployment.apps/nginx-deployment created
1.3 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
You’ll also need to provide the object spec field. The precise format of the object spec is different for every Kubernetes object, and contains nested fields specific to that object.
2 UIDs
A Kubernetes systems-generated string to uniquely identify objects.
Every object created over the whole lifetime of a Kubernetes cluster has a distinct UID. It is intended to distinguish between historical occurrences of similar entities.
3 Namespaces
Kubernetes supports multiple virtual clusters backed by the same physical cluster. These virtual clusters are called namespaces.
Namespaces provide a scope for names. Names of resources need to be unique within a namespace, but not across namespaces. Namespaces can not be nested inside one another and each Kubernetes resources can only be in one namespace.
Namespaces are a way to divide cluster resources between multiple users (via resource quota).
3.1 Working with Namespaces
3.2 Viewing namespaces
kubectl get namespace
Kubernetes starts with three initial namespaces:
- default The default namespace for objects with no other namespace
- kube-system The namespace for objects created by the Kubernetes system
- kube-public This namespace is created automatically and is readable by all users (including those not authenticated). This namespace is mostly reserved for cluster usage, in case that some resources should be visible and readable publicly throughout the whole cluster. The public aspect of this namespace is only a convention, not a requirement.
3.3 Setting the namespace for a request
To temporarily set the namespace for a request, use the --namespace flag.
For example:
kubectl --namespace=<insert-namespace-name-here> run nginx --image=nginx
3.4 Setting the namespace preference
You can permanently save the namespace for all subsequent kubectl commands in that context.
kubectl config set-context $(kubectl config current-context) --namespace=<insert-namespace-name-here>
# Validate it
kubectl config view | grep namespace:
3.5 Namespaces and DNS
When you create a Service, it creates a corresponding DNS entry. This entry is of the form <service-name>.<namespace-name>.svc.cluster.local, which means that if a container just uses <service-name>, it will resolve to the service which is local to a namespace. This is useful for using the same configuration across multiple namespaces such as Development, Staging and Production. If you want to reach across namespaces, you need to use the fully qualified domain name (FQDN).
3.6 Not All Objects are in a Namespace
Most Kubernetes resources (e.g. pods, services, replication controllers, and others) are in some namespaces. However namespace resources are not themselves in a namespace. And low-level resources, such as nodes and persistentVolumes, are not in any namespace.
To see which Kubernetes resources are and aren’t in a namespace:
# In a namespace
kubectl api-resources --namespaced=true
# Not in a namespace
kubectl api-resources --namespaced=false
4 Labels and Selectors
Labels are key/value pairs that are attached to objects, such as pods. Labels are intended to be used to specify identifying attributes of objects.
Labels can be used to organize and to select subsets of objects. Labels can be attached to objects at creation time and subsequently added and modified at any time. Each object can have a set of key/value labels defined. Each Key must be unique for a given object.
"metadata": {
"labels": {
"key1" : "value1",
"key2" : "value2"
}
}
Labels enable users to map their own organizational structures onto system objects in a loosely coupled fashion, without requiring clients to store these mappings. Service deployments and batch processing pipelines are often multi-dimensional entities (e.g., multiple partitions or deployments, multiple release tracks, multiple tiers, multiple micro-services per tier). Management often requires cross-cutting operations, which breaks encapsulation of strictly hierarchical representations, especially rigid hierarchies determined by the infrastructure rather than by users.
Example labels:
"release" : "stable", "release" : "canary"
"environment" : "dev", "environment" : "qa", "environment" : "production"
"tier" : "frontend", "tier" : "backend", "tier" : "cache"
"partition" : "customerA", "partition" : "customerB"
"track" : "daily", "track" : "weekly"
Kubectl get pods –selector app=App1
4.1 Labels
Labels are key/value pairs. Valid label keys have two segments: an optional prefix and name, separated by a slash 
.
The prefix is optional. If specified, the prefix must be a DNS subdomain: a series of DNS labels separated by dots. If the prefix is omitted, the label Key is presumed to be private to the user. Automated system components (e.g. kube-scheduler, or other third-party automation) which add labels to end-user objects must specify a prefix. The kubernetes.io/ and k8s.io/ prefixes are reserved for Kubernetes core components.
4.2 Label selectors
Via a label selector, the client/user can identify a set of objects. The label selector is the core grouping primitive in Kubernetes.
4.3 Equality-based requirement
Three kinds of operators are admitted =,=,!=.
environment = production
tier != frontend
4.4 Set-based requirement
Set-based label requirements allow filtering keys according to a set of values. Three kinds of operators are supported: in,notin and exists (only the key identifier). For example:
environment in (production, qa)
tier notin (frontend, backend)
partition
!partition
The third example selects all resources including a label with key partition; no values are checked.
4.5 API
For example
kubectl get pods -l environment=production,tier=frontend
kubectl get pods -l 'environment in (production),tier in (frontend)'
4.6 Service and ReplicationController
The set of pods that a service targets is defined with a label selector and only equality-based requirement selectors are supported. For example in yaml file for service：
selector:
component: redis
4.7 Resources that support set-based requirements
Newer resources, such as Job, Deployment, Replica Set, and Daemon Set, support set-based requirements as well.
selector:
matchLabels:
component: redis
matchExpressions:
- {key: tier, operator: In, values: [cache]}
- {key: environment, operator: NotIn, values: [dev]}
5 Annotations
You can use Kubernetes annotations to attach arbitrary non-identifying metadata to objects. Clients such as tools and libraries can retrieve this metadata.
"metadata": {
"annotations": {
"key1" : "value1",
"key2" : "value2"
}
}
6 Field Selectors
Field selectors let you select Kubernetes resources based on the value of one or more resource fields. Here are some example field selector queries:
metadata.name=my-service
metadata.namespace!=default
status.phase=Pending
This kubectl command selects all Pods for which the value of the status.phase field is Running:
kubectl get pods --field-selector status.phase=Running
