Ports

 In all the previous sections, we have seen creating the pods, replicasets, and deployments, but we haven't gone through how to access pods inside the K8's cluster or outside the cluster. Services will come in handy to access pods outside the cluster. I will explain the terminologies first before we dive into it.

Let us use this pod.yaml to demonstrate the different types of ports available in the Kubernetes cluster. It often confuses most of them, which I am going to explain to you with examples.

download code here Github: portsdemo.yaml

apiVersion: v1
kind: Pod
metadata:
  name: portsdemo
spec:
  containers:
  - iam7hills/learnkubernetes:podsdemo-1.0
    name: portsdemo
    ports:
    - containerPort: 80

ContainerPort:

    In my example, I have used "podsdemo-1.0", which is a nginx container running on the listenerport "80". In case, if you are using the Tomcat app running on port 8080, then your containerPort should be same as your listener port 8080. In simple words, your listener port should match with your containerPort in your yaml file. Below is my nginx configuration, that is how I know that my listener port is 80.

nginx configuration file - FYI

server {
    listen 80;
    root /usr/share/nginx/html;
    }

hostNetwork: true

Let us assume that I need to access the below pod that was created using "portsdemo.yaml"

kubectl get pods portsdemo -o wide

NAME       READY   STATUS    RESTARTS   AGE   IP               NODE        NOMINATED NODE  

portsdemo   1/1     Running   0          29s   192.168.181.41   iam7hills   <none>           <none>

In the above example, you cannot access your application, because your pods running inside the Kubernetes cluster is not visible outside the cluster. You will get the connection refused. To fix this problem, I am going to add "hostNetwork: true" within the spec in the yaml as shown below. And then rerun the yaml.

apiVersion: v1
kind: Pod
metadata:
  name: portsdemo
spec:
  hostNetwork: true
  containers:
  - iam7hills/learnkubernetes:podsdemo-1.0
    name: portsdemo
    ports:
    - containerPort: 80

From your user browser, you need to access your application as http://<workernode>:80. In my case, it is going to be http://192.168.86.30:80; 80 is my nginx listener port waiting for the request to handle.

hostPort: <anyport>

If you want to map your listenerport to any custom port and then to expose outside your Kubernetes cluster, then you can use hostPort in your yaml file as below. In my example, I have used 8080 as my mapped port.

Drawbacks in using hostNetwork: true and hostPort: <customport>:

In both hostNetwork and hostPort, we could easily get into port conflict. The reason is that the pods are accessed by the hostIP of the worker node. The worker node will be hosting several other pods, it could be in hundreds and thousands. And everytime time when you allocate some port for each port, then you need to remember which port should I use for the next pod. This will become a nightmare to manage over the period. This is why, the Service object in the Kubernetes can rescue in such scenarios.