On the Kubernetes master, we can use kubectl run to create a certain number of containers. The Kubernetes master will then schedule the pods for the nodes to run, with general command formatting, as follows:
$ kubectl run <replication controller name> --image=<image name> --replicas=<number of replicas> [--port=<exposing port>]
The following example will create two replicas with the name my-first-nginx from the nginx image and expose port 80. We can deploy one or more containers in what is referred to as a pod. In this case, we will deploy one container per pod. Just like a normal Docker behavior, if the nginx image doesn't exist locally, it will pull it from the Docker Hub by default:
// run a deployment with 2 replicas for the image nginx and expose the container port 80
$ kubectl run my-first-nginx --image=nginx --replicas=2 --port=80
deployment "my-first-nginx" created
The name of deployment <my-first-nginx> cannot be duplicated
The resource (pods, services, deployment, and so on) in one Kubernetes namespace cannot be duplicated. If you run the preceding command twice, the following error will pop up:
Error from server (AlreadyExists): deployments.extensions "my-first-nginx" already exists
Let's move on and see the current status of all the pods by kubectl get pods. Normally the status of the pods will hold on Pending for a while, since it takes some time for the nodes to pull the image from the registry:
// get all pods
$ kubectl get pods
NAME READY STATUS RESTARTS AGE
my-first-nginx-7dcd87d4bf-jp572 1/1 Running 0 7m
my-first-nginx-7dcd87d4bf-ns7h4 1/1 Running 0 7m
If the pod status is not running for a long time
You could always use kubectl get pods to check the current status of the pods, and kubectl describe pods $pod_name to check the detailed information in a pod. If you make a typo of the image name, you might get the ErrImagePull error message, and if you are pulling the images from a private repository or registry without proper credentials, you might get the ImagePullBackOff message. If you get the Pending status for a long time and check out the node capacity, make sure you don't run too many replicas that exceed the node capacity. If there are other unexpected error messages, you could either stop the pods or the entire replication controller to force the master to schedule the tasks again.
You can also check the details about the deployment to see whether all the pods are ready:
// check the status of your deployment
$ kubectl get deployment
NAME DESIRED CURRENT UP-TO-DATE AVAILABLE AGE
my-first-nginx 2 2 2 2 2m