Use a Service to Access an Application in a Cluster#

This page shows how to create a Kubernetes Service object that external clients can use to access an application running in a cluster. The Service provides load balancing for an application that has two running instances.

objectives#

• Run two instances of a Hello World application.
• Create a Service object that exposes a node port.
• Use the Service object to access the running application.

Creating a service for an application running in two pods#

Here is the configuration file for the application Deployment:

{{% code_sample file="service/access/hello-application.yaml" %}}

1. Run a Hello World application in your cluster: Create the application Deployment using the file above:

   kubectl apply -f https://k8s.io/examples/service/access/hello-application.yaml

   The preceding command creates a {{< glossary_tooltip text="Deployment" term_id="deployment" >}} and an associated {{< glossary_tooltip term_id="replica-set" text="ReplicaSet" >}}. The ReplicaSet has two {{< glossary_tooltip text="Pods" term_id="pod" >}} each of which runs the Hello World application.

2. Display information about the Deployment:

   kubectl get deployments hello-world
   kubectl describe deployments hello-world

3. Display information about your ReplicaSet objects:

   kubectl get replicasets
   kubectl describe replicasets

4. Create a Service object that exposes the deployment:

   kubectl expose deployment hello-world --type=NodePort --name=example-service

5. Display information about the Service:

   kubectl describe services example-service

   The output is similar to this:

   Name:                   example-service
   Namespace:              default
   Labels:                 run=load-balancer-example
   Annotations:            <none>
   Selector:               run=load-balancer-example
   Type:                   NodePort
   IP:                     10.32.0.16
   Port:                   <unset> 8080/TCP
   TargetPort:             8080/TCP
   NodePort:               <unset> 31496/TCP
   Endpoints:              10.200.1.4:8080,10.200.2.5:8080
   Session Affinity:       None
   Events:                 <none>

   Make a note of the NodePort value for the Service. For example, in the preceding output, the NodePort value is 31496.

6. List the pods that are running the Hello World application:

   kubectl get pods --selector="run=load-balancer-example" --output=wide

   The output is similar to this:

   NAME                           READY   STATUS    ...  IP           NODE
   hello-world-2895499144-bsbk5   1/1     Running   ...  10.200.1.4   worker1
   hello-world-2895499144-m1pwt   1/1     Running   ...  10.200.2.5   worker2

7. Get the public IP address of one of your nodes that is running a Hello World pod. How you get this address depends on how you set up your cluster. For example, if you are using Minikube, you can see the node address by running kubectl cluster-info. If you are using Google Compute Engine instances, you can use the gcloud compute instances list command to see the public addresses of your nodes.

8. On your chosen node, create a firewall rule that allows TCP traffic on your node port. For example, if your Service has a NodePort value of 31568, create a firewall rule that allows TCP traffic on port 31568. Different cloud providers offer different ways of configuring firewall rules.

9. Use the node address and node port to access the Hello World application:

   curl http://<public-node-ip>:<node-port>

   where <public-node-ip> is the public IP address of your node, and <node-port> is the NodePort value for your service. The response to a successful request is a hello message:

   Hello, world!
   Version: 2.0.0
   Hostname: hello-world-cdd4458f4-m47c8

Using a service configuration file#

As an alternative to using kubectl expose, you can use a service configuration file to create a Service.

{{% heading "cleanup" %}}#

To delete the Service, enter this command:

To delete the Deployment, the ReplicaSet, and the Pods that are running the Hello World application, enter this command:

{{% heading "whatsnext" %}}#

Follow the Connecting Applications with Services tutorial.