Difference between targetPort and port in Kubernetes Service definition

Service: This directs the traffic to a pod.

TargetPort: This is the actual port on which your application is running inside the container.

Port: Some times your application inside container serves different services on a different port.

Example: The actual application can run 8080 and health checks for this application can run on 8089 port of the container. So if you hit the service without port it doesn't know to which port of the container it should redirect the request. Service needs to have a mapping so that it can hit the specific port of the container.

kind: Service
apiVersion: v1
metadata:
  name: my-service
spec:
  selector:
    app: MyApp
  ports:
    - name: http
      nodePort: 30475
      port: 8089
      protocol: TCP
      targetPort: 8080
    - name: metrics
      nodePort: 31261
      port: 5555
      protocol: TCP
      targetPort: 5555
    - name: health
      nodePort: 30013
      port: 8443
      protocol: TCP
      targetPort: 8085 

if you hit the my-service:8089 the traffic is routed to 8080 of the container(targetPort). Similarly, if you hit my-service:8443 then it is redirected to 8085 of the container(targetPort). But this myservice:8089 is internal to the kubernetes cluster and can be used when one application wants to communicate with another application. So to hit the service from outside the cluster someone needs to expose the port on the host machine on which kubernetes is running so that the traffic is redirected to a port of the container. This is node port(port exposed on the host machine). From the above example, you can hit the service from outside the cluster(Postman or any rest-client) by host_ip:nodePort

Say your host machine ip is 10.10.20.20 you can hit the http, metrics, health services by 10.10.20.20:30475, 10.10.20.20:31261, 10.10.20.20:30013.

Edits: Edited as per Raedwald comment.

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It helps me to think of things from the perspective of the service.

nodePort: The port on the node where external traffic will come in on

port: The port of this service

targetPort The target port on the pod(s) to forward traffic to

Traffic comes in on nodePort, forwards to port on the service which then routes to targetPort on the pod(s).

It's worth emphasizing that nodePort is for external traffic. Other pods in the cluster that may need to access the service will just use port, not nodePort as it's internal only access to the service.

Also worth noting that if targetPort is not set, it will default to the same value as port. E.g. 80:80 for service port 80 targeting container port 80.

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In nutshell

nodeport: Listens for external request on all worker nodes on nodeip:nodeport and forwards the request to port.

ClusterIP: Request comes through ingress and points to service name and port.

port: Internal cluster service port for container and listens for incoming request from the nodeport and forwards to targetPort.

targetPort: Receives the request from port and forwards to container pod(port) where it's listening. Even if you don't specify this will get by default assigned the same port numbers as port.

So the traffic flows Ingress-->Service-->Endpoint(Basically has POD IP)-->POD

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The answer given above by @Manikanta P is correct. However, the explanation of "Port" might be a little unclear at first reading. I will explain with an example:

Consider a Web-Application with its static content (front-page, images etc) hosted by httpd and the dynamic content (eg. response to requests, etc.) hosted by tomcat. The Webserver (or the static content) is served by httpd at port 80 while Appserver (or the dynamic content) is served by tomcat at port 8080.

What a developer wants: User should be able to access the Webserver from outside BUT not the Appserver from outside.

Solution: The service-type of Webserver in its service.yml will be NodePort while the service-type of Appserver in its service.yml will be ClusterIP.

Code for webserver's service.yml:

spec:
  selector:
    app: Webserver
  type: NodePort        // written to make this service accessible from outside.
  ports:
    - nodePort: 30475   // To access from outside, type <host_IP>:30475 in browser.
      port: 5050        // (ignore for now, I will explain below).
      protocol: TCP
      targetPort: 80  // port where httpd runs inside the webserver pod.

Code for Appserver's service.yml

spec:
  selector:
    app: appserver
  type: ClusterIP        // written to make this service NOT accessible from outside.
  ports:
    - port: 5050         // port to access this container internally
      protocol: TCP
      targetPort: 8080   // port where tomcat runs inside the appserver pod.

Also Note, in the httpd.conf file of the Webserver, we will write the IP that redirects a user's request to the appserver. This IP will be: host_IP:5050.

What exactly is happening here? A user writes hostIP:30475 and sees the Webserver's page. This is because it is being served by httpd at port 80 (targetport). When a user clicks a button, a request is made. This request is redirected to the Appserver because in httpd.conf file, the port 5050 is mentioned and this is the port where Appserver's container and Webserver's conatainer communicate internally. When the appserver receives the request, it is able to serve the request because of tomcat running inside it at port 8080.

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if container listens on port 9376, then targetPort: 9376

if a service listens on port 80, then port: 80

Then service ports config looks like below

ports:
 - protocol: TCP
   port: 80
   targetPort: 9376

Finally, request received to the service’s port, and forwarded on the targetPort of the pod.

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This answer is to reference Kubernetes' documentation in addition to the other answers:

https://kubernetes.io/docs/concepts/services-networking/connect-applications-service/:

targetPort: is the port the container accepts traffic on,

port: is the abstracted Service port, which can be any port other pods use to access the Service

https://kubernetes.io/docs/concepts/services-networking/service/:

Port definitions in Pods have names, and you can reference these names in the targetPort attribute of a Service. This works even if there is a mixture of Pods in the Service using a single configured name, with the same network protocol available via different port numbers.

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Case 1:

Let's assume that there is no nodPort or port, now you want to run your aplication and expose it to outside, what you will need:

An Ingress controller which will use a servicePort for redirecting to our desired service based on routing. A cluster IP service with a defined target to your application port (which also called targetPort) A network port which will identifies the application or service running on the computer (in other words application port).

So, to get access from outside we found three port needed.

servicePort (Ingress controller) targetPort (Cluster Ip Service) networkPort (application port)

to work everything properly : servicePort === targetPort === networkPort

Case 2: Now assume that one service communicate with another service in our cluster, or let's assume one service received a request from out side and it emits an event which triggered another service inside of our cluster.

Suppose Service X is exposed outside by using nodePort Service, after receving a request, X service want to communicate with Y service.

Y service need following ports

A ClusterIP port, by which X service will forward request A ClusterIP targetPort by which Y service will determine in which port applicaiton is running. An application port

port === any

targetPort === application port

Inside Service X:

app.post('/posts/create', async (req, res) => {
  const id = randomBytes(4).toString('hex');
  const { title } = req.body;

  posts[id] = {
    id,
    title
  };

  await axios.post('http://event-bus-srv:4010/events', {
    type: 'PostCreated',
    data: {
      id,
      title
    }
  });

  res.status(201).send(posts[id]);
});

Configuration and Inside of Service Y

apiVersion: v1
kind: Service
metadata:
  name: event-bus-srv
spec:
  selector:
    app: event-bus
  type: ClusterIP
  ports:
    - name: event-bus
      protocol: TCP
      port: 4010
      targetPort: 4009

app.listen(4009, () => {

  console.log('Listening on 4009');
});

https://i.stack.imgur.com/2bVEz.png

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Since people have explained port and targetPort in Kubernetes Service definition, I'll add information on how Dockerfile, Kubernetes Deployment and Kubernetes Ingress come into picture because they are part of a common workflow.

Part 1 - Applications and their Dockerfile

Lets say you're running a Flask server on port 3000 and a Golang server on port 4000. When you containerize these applications using Docker, you'll have to expose ports 3000 and 4000 in their Dockerfiles:

Python

Application

...
...
if __name__ == "__main__":
    app.run(host='0.0.0.0', port=3000)

Dockerfile

FROM python:3.10-alpine

...

...

EXPOSE 3000

CMD ...

Golang

Application

...
...
func main() {
    ...
    log.Fatal(http.ListenAndServe(":4000", nil))
}

Dockerfile

FROM golang:1.18-alpine

...

...

EXPOSE 4000

CMD ...

Part 2 - Dockerfiles and Kubernetes Deployments

The exposed ports in Dockerfiles will have to match the containerPort in the deployment manifests.

Python Deployment Manifest

---
apiVersion: apps/v1
kind: Deployment
metadata:
  name: python-flask-api
spec:
...
...
      app: flask-api
  template:
    metadata:
      labels:
        app: flask-api
    spec:
      containers:
        - name: flask-api
          image: ...
          ports:
            - containerPort: 3000
...
...

Golang Deployment Manifest

---
apiVersion: apps/v1
kind: Deployment
metadata:
  name: go-backend-api
spec:
...
...
      app: go-api
  template:
    metadata:
      labels:
        app: go-api
    spec:
      containers:
        - name: go-api
          image: ...
          ports:
            - containerPort: 4000
...
...

Part 3 - Kubernetes Deployments and Services

The containerPort in the deployment manifests will have to match the targetPort in the service manifests

Python Service Manifest

apiVersion: v1
kind: Service
metadata:
  name: flask-api-service
spec:
  type: NodePort
  selector:
    app: flask-api
  ports:
    - port: 80
      targetPort: 3000

Golang Service Manifest

apiVersion: v1
kind: Service
metadata:
  name: go-api-service
spec:
  type: NodePort
  selector:
    app: go-api
  ports:
    - port: 443
      targetPort: 4000

Part 4 - Kubernetes Service and Ingress

The port in the service manifest will have to match the port number in the ingress

AWS Ingress for Python and Golang application

apiVersion: networking.k8s.io/v1
kind: Ingress
metadata:
  name: microservice-app-ingress
  annotations:
    kubernetes.io/ingress.class: alb
    alb.ingress.kubernetes.io/scheme: internet-facing
    alb.ingress.kubernetes.io/target-type: ip
spec:
  rules:
    - host: foo.biz.com
      http:
        paths:
          - path: /
            pathType: Prefix
            backend:
              service:
                name: go-api-service
                port:
                  number: 443
    - host: bar.biz.com
      http:
        paths:
          - path: /
            pathType: Prefix
            backend:
              service:
                name: flask-api-service
                port:
                  number: 80

Flow

An incoming request hits the ingress on a port number

The ingress forwards this request to the service port

The service port maps the port to a targetPort

From the service targetPort the request goes to the deployment containerPort

The deployment containerPort is the application's Docker image, which has this corresponding port exposed in its Dockerfile

And finally, the exposed port in Dockerfile sends the request to the application

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targetport: One or more ports on which a container listens within a pod.

nodeport: Used primarily to accept consumer requests. (Eg: HTTP request from consumers to a webserver running in a container)

nodeport is listened on all nodes on all interfaces i.e 0.0.0.0:nodeport. Consumer service requests sent to nodeport is routed to container's targetport so that the container can fulfill the request.

port: Port used within the kubernetes pod network, primarily used to exchange requests between pods. Here as well, requests to a pod from another, is routed to the corrosponding pod's container targetport.

Summary: all requests end up in the targetport. nodeport is used if request from outside k8s network & port if from within.

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"Target port" is the port on which your container is running.

Port : port redirects the traffic to the container from the service.

Exposing the deployment

  master $ kubectl get deployments
NAME         READY   UP-TO-DATE   AVAILABLE   AGE

nginx        1/1     1            1           31s
master $ kubectl expose deployment nginx --name=nginx-svc --port=8080 --target-port=80
service/nginx-svc exposed

master $ kubectl get svc

NAME         TYPE        CLUSTER-IP       EXTERNAL-IP   PORT(S)    AGE

nginx-svc    ClusterIP   10.107.209.151   <none>        8080/TCP   5s

NodePort : is the port that enables the service to access the externally.

Hope this answers.

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I think image describes the best.

https://i.stack.imgur.com/O9e5U.png

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