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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
• Before you begin
• Creating a service for an application running in two pods
• Using a service configuration file
• Cleaning up
• What’s next

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.

Before you begin

You need to have a Kubernetes cluster, and the kubectl command-line tool must be configured to communicate with your cluster. If you do not already have a cluster, you can create one by using Minikube, or you can use one of these Kubernetes playgrounds:

• Katacoda
• Play with Kubernetes

To check the version, enter kubectl version.

Creating a service for an application running in two pods

1. Run a Hello World application in your cluster:

   kubectl run hello-world --replicas=2 --labels="run=load-balancer-example" --image=gcr.io/google-samples/node-hello:1.0  --port=8080
   

   The preceding command creates a Deployment object and an associated ReplicaSet object. The ReplicaSet has two Pods, 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
    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. For more information about this command, see the GCE documentation.

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. See the GCE documentation on firewall rules, for example.

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 Kubernetes!
   

Using a service configuration file

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

Cleaning up

To delete the Service, enter this command:

kubectl delete services example-service

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

kubectl delete deployment hello-world

What’s next

Learn more about connecting applications with services.