When you create a brand new kube cluster, and then get a list of pods, you'll get:
$ kubectl get pods
No resources found.This is not strictly true. In Kubernetes we have a feature called namespaces that lets us segment/organise all our objects into a construct known as namespaces. Here's the current list of namespaces:
$ kubectl get namespaces
NAME STATUS AGE
default Active 11h
kube-public Active 11h
kube-system Active 11hThese 3 namespaces comes included in a fresh kubernetes install.
When we run the kubectl get pods earlier, we didn't specify a namespace, so kubectl by default will use the 'default' namspace. The 'kube-system' namespace store objects that are used by kubernetes internally. For example here's how to view what pods k8s uses internally:
$ kubectl get all --namespace=kube-system
NAME READY STATUS RESTARTS AGE
po/coredns-86c58d9df4-cnqnr 1/1 Running 0 11h
po/coredns-86c58d9df4-ct8fx 1/1 Running 0 11h
po/etcd-minikube 1/1 Running 0 11h
po/kube-addon-manager-minikube 1/1 Running 0 11h
po/kube-apiserver-minikube 1/1 Running 0 11h
po/kube-controller-manager-minikube 1/1 Running 0 11h
po/kube-proxy-8zwff 1/1 Running 0 11h
po/kube-scheduler-minikube 1/1 Running 0 11h
po/kubernetes-dashboard-ccc79bfc9-l66xk 1/1 Running 0 11h
po/storage-provisioner 1/1 Running 0 11h
NAME CLUSTER-IP EXTERNAL-IP PORT(S) AGE
svc/kube-dns 10.96.0.10 <none> 53/UDP,53/TCP 11h
svc/kubernetes-dashboard 10.103.63.176 <none> 80/TCP 11h
NAME DESIRED CURRENT UP-TO-DATE AVAILABLE AGE
deploy/coredns 2 2 2 2 11h
deploy/kubernetes-dashboard 1 1 1 1 11h
NAME DESIRED CURRENT READY AGE
rs/coredns-86c58d9df4 2 2 2 11h
rs/kubernetes-dashboard-ccc79bfc9 1 1 1 11hNamespaces lets you can organise your objects in various ways. For example, we can create namespaces called 'Prod' and 'Dev'. Here's a yaml example for a namespace:
---
apiVersion: v1
kind: Namespace
metadata:
name: ns-devNotice that we didn't need to specify a namespace.spec section.
After creating the namespace, we can view it like this:
$ kubectl get namespaces
NAME STATUS AGE
default Active 5h19m
kube-node-lease Active 5h19m
kube-public Active 5h19m
kube-system Active 5h19mYou can then create objects in your new namespace by using the kubectl apply --namespace -f .... However I prefer the declaritive approach, by specifing what namespace objects belong to using the xxx.metadata.namespace setting:
---
apiVersion: v1
kind: Pod
metadata:
name: pod-httpd
namespace: ns-dev # we add this line.
labels:
app: apache_webserver
spec:
containers:
- name: cntr-httpd
image: httpd
ports:
- containerPort: 80And for our service we have:
---
apiVersion: v1
kind: Service
metadata:
name: svc-nodeport-apache
namespace: ns-dev # we add this line.
spec:
type: NodePort
ports:
- port: 3050
targetPort: 80
nodePort: 31000
selector:
app: apache_webserverWe can apply them using the usual apply commands. And then we can check that they have been created by running:
$ kubectl get pods
No resources found.
$ kubectl get all -o wide --namespace=ns-dev
NAME READY STATUS RESTARTS AGE IP NODE NOMINATED NODE READINESS GATES
pod/pod-httpd 1/1 Running 0 107s 172.17.0.8 minikube <none> <none>
NAME TYPE CLUSTER-IP EXTERNAL-IP PORT(S) AGE SELECTOR
service/svc-nodeport-apache-webserver NodePort 10.107.98.250 <none> 3050:31000/TCP 62s app=apache_webserverCluster level objects
Some objects are cluster level objects and therefore can't be namespaced (i.e. organised into namespaces):
$ kubectl api-resources -o wide
NAME SHORTNAMES APIGROUP NAMESPACED KIND VERBS
bindings true Binding [create]
componentstatuses cs false ComponentStatus [get list]
configmaps cm true ConfigMap [create delete deletecollection get list patch update watch]
endpoints ep true Endpoints [create delete deletecollection get list patch update watch]
events ev true Event [create delete deletecollection get list patch update watch]
limitranges limits true LimitRange [create delete deletecollection get list patch update watch]
namespaces ns false Namespace [create delete get list patch update watch]
nodes no false Node [create delete deletecollection get list patch update watch]
...Set the namespace persistently
If you want to perform a lot of tasks against a particular namespace, then specifying namespaces on the command line every time can get quite tedious. However you can persistantly change namespaces by running:
$ kubectl config set-context $(kubectl config current-context) --namespace=ns-dev
Context "minikube" modified.This command is setting the user+cluster details via the current-context setting, and the sets the namespace to go with it. This command essentially makes a one-line change in your ~/.kube/config file.
$ diff ~/.kube/config ~/.kube/config-orig
26c26
< namespace: ns-dev
---
> namespace: kube-system
$ grep 'ns-dev' ~/.kube/config -A2 -B2
- context:
cluster: minikube
namespace: ns-dev
user: minikube
name: minikubeThis means the context called 'minikube' is telling kubectl which kubecluster to connect to, as which user, and which namespace to use as the default namespace (if the namespace flag isn't specified on the command line).
This results in:
$ kubectl config get-contexts
CURRENT NAME CLUSTER AUTHINFO NAMESPACE
default kubernetes chowdhus
docker-for-desktop docker-for-desktop-cluster docker-for-desktop
* minikube minikube minikube ns-devThis command essentially displays some of the the content extracted from ~/.kube/config in a more readable form.