What is an 'endpoint' in Kubernetes?

Question

I am new to Kubernetes and started reading through the documentation. There often the term \'endpoint\' is used but the documentation lacks an explicit definition.

What

Answer 1

I'll answer your questions one by one:

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What is an 'endpoint' in terms of Kubernetes?

(The resource name in K8S is Endpoints).

Endpoints is an object in kubernetes which represents a… List of Endpoints.
Those endpoints can be:

1. An internal pod running inside the cluster - this is the form that is more familiar.
   It is created automatically behind the scenes for us when we create service and pods and match the service label selector to the pods labels.

2. An external IP which is not a pod - this is the least known option.

The external IP can reside outside the cluster - for example external web server or database.
It can also reside in a different namespace - if you want to point your Service to a Service in a different Namespace inside your cluster.

Regarding external Endpoints - If you do not specify a label selector in your service - Kubernetes can’t create the list of endpoints because he doesn’t know which pods should be included and proxied by the service.

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Where is it located?

Like the great diagrams provided here are shown - it sits between the service and an internal (pods) or external (web server, databases etc’) resources.

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I could image the 'endpoint' is some kind of access point for an individual 'node' Its an access point to a resource that sits inside one of the nodes in your cluster.

An Endpoint can reside inside one of the nodes in your cluster, or outside your cluster / environment.

If its an internal endpoint (which means that the pod label matches a service label selector) - you can reach it with:

$kubectl describe svc/my-service


Name:                     my-service
Namespace:                default
Labels:                   
Annotations:              kubectl.kubernetes.io/last-applied-configuration:
                            {"apiVersion":"v1","kind":"Service","metadata":{"annotations":{},"name":" my-service","namespace":"...
Selector:                 run=some-run
Type:                     NodePort
IP:                       10.100.92.162
Port:                       8080/TCP
TargetPort:               80/TCP
NodePort:                   31300/TCP
Endpoints:                172.21.21.2:80,172.21.38.56:80,172.21.39.160:80
Session Affinity:         None
External Traffic Policy:  Cluster
Events:                   

Or directly with:

$kubectl get endpoints my-service

NAME           ENDPOINTS                                       AGE
my-service   172.21.21.2:80,172.21.38.56:80,172.21.39.160:80   63d

Regarding external Enpoints:

You create a service without a label selector:

apiVersion: v1
kind: Service
metadata:
  name: my-service #<------ Should match the name of Endpoints object
spec:
  ports:
    - protocol: TCP
      port: 8080
      targetPort: 9376

So the corresponding Endpoint object will not be created automatically and you manually add the Endpoints object and map the Service to the desired network address and port where the external resource is running:

apiVersion: v1
kind: Endpoints
metadata:
  name: my-service #<------ Should match the name of Service
subsets:
  - addresses:
      - ip: 192.0.2.45
    ports:
      - port: 9376

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(Notice:) I used the term internal for the auto-generated Endpoints for pods that has a match with the label selector and the term external for Endpoints that were created manually.

I could used the terms auto-generated and manual instead - that would have been more accurate but I think more confusing also.

In most cases, when the Endpoints are related to pods inside our cluster - we would want them to be also managed by K8S - in this case they will also need to be generated by K8S.