Kubernetes has a scheduler to assign the container to the right node. In addition, you can easily scale out and scale down the number of containers. The Kubernetes scaling function will conduct the replication controller to adjust the number of containers.
Prepare the following YAML file, which is a simple replication controller to launch two nginx containers. Also, service will expose the TCP port 30080:
# cat nginx-rc-svc.yaml apiVersion: v1 kind: ReplicationController metadata: name: my-nginx spec: replicas: 2 selector: sel : my-selector template: metadata: labels: sel : my-selector spec: containers: - name: my-container image: nginx --- apiVersion: v1 kind: Service metadata: name: my-nginx spec: ports: - protocol: TCP port: 80 nodePort: 30080 type: NodePort selector: sel: my-selector
Use the kubectl command to create resources as follows:
# kubectl create -f nginx-service.yaml replicationcontroller "my-nginx" created service "my-nginx" created
Wait for a moment to completely launch two nginx containers as follows:
# kubectl get pods NAME READY STATUS RESTARTS AGE my-nginx-iarzy 1/1 Running 0 7m my-nginx-ulkvh 1/1 Running 0 7m # kubectl get services NAME CLUSTER_IP EXTERNAL_IP PORT(S) SELECTOR AGE kubernetes 192.168.0.1 <none> 443/TCP <none> 44d my-nginx 192.168.95.244 nodes 80/TCP sel=my-selector 7m
Kubernetes has a command that changes the number of replicas for service:
- Type the
kubectl scalecommand as follows to specify the desired replicas:# kubectl scale --replicas=4 rc my-nginx replicationcontroller "my-nginx" scaled
This example indicates that the replication controller, which is named
my-nginx, changes the replicas to4. - Type
kubectl get podsto confirm the result as follows:# kubectl get pods NAME READY STATUS RESTARTS AGE my-nginx-iarzy 1/1 Running 0 20m my-nginx-r5lnq 1/1 Running 0 1m my-nginx-uhe8r 1/1 Running 0 1m my-nginx-ulkvh 1/1 Running 0 20m
The kubectl scale feature can change the number of replicas; not only increase, but also decrease. For example, you can change back to two replicas as follows:
# kubectl scale --replicas=2 rc my-nginx replicationcontroller "my-nginx" scaled # kubectl get pods NAME READY STATUS RESTARTS AGE my-nginx-iarzy 0/1 Terminating 0 40m my-nginx-r5lnq 1/1 Running 0 21m my-nginx-uhe8r 1/1 Running 0 21m my-nginx-ulkvh 0/1 Terminating 0 40m # kubectl get pods NAME READY STATUS RESTARTS AGE my-nginx-r5lnq 1/1 Running 0 25m my-nginx-uhe8r 1/1 Running 0 25m
There is an option --current-replicas that specifies the expected current replicas. If it doesn't match, Kubernetes doesn't perform the scale function as follows:
//abort scaling, because current replica is 2, not 3 # kubectl scale --current-replicas=3 --replicas=4 rc my-nginx Expected replicas to be 3, was 2 # kubectl get pods NAME READY STATUS RESTARTS AGE my-nginx-r5lnq 1/1 Running 0 27m my-nginx-uhe8r 1/1 Running 0 27m
It will help prevent human error. By default, --current-replicas equals -1, which means bypass to check the current number of replicas:
//no matter current number of replicas, performs to change to 4 # kubectl scale --current-replicas=-1 --replicas=4 rc my-nginx replicationcontroller "my-nginx" scaled # kubectl get pods NAME READY STATUS RESTARTS AGE my-nginx-dimxj 1/1 Running 0 5s my-nginx-eem3a 1/1 Running 0 5s my-nginx-r5lnq 1/1 Running 0 35m my-nginx-uhe8r 1/1 Running 0 35m
This recipe described how to change the number of pods using the scaling option by the replication controller. It is useful to scale up and scale down your application quickly. To know more about how to update your container, refer to the following recipes:
- Updating live containers
- Ensuring flexible usage of your containers