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Kubernetes: Launching & accessing running applications inside the cluster

There are multiple ways to start an application inside a kubernetes cluster, and I’ll describe on this page how to run a simple one and the multiple ways to access it for debugging reasons. Please note that I will stay inside the cluster perimeter, and will not write about ClusterIP, LoadBalancer etc. in this one.

Starting a pod without a deployment

The simple way to run an app - a pod - in to use kubectl run:

$ kubectl get pods
No resources found.

$ kubectl run --generator=run-pod/v1 --image=pmarie/httpd-sample:v2 httpd-sample
pod/httpd-sample created

$ kubectl get pods
httpd-sample   1/1     Running   0          3s

The application I’m using, httpd-sample is a small golang debug httpd.

My pod is called httpd-sample and you can get its own description using the describe command:

$ kubectl describe pod httpd-sample
Name:               httpd-sample
Namespace:          default
Priority:           0
PriorityClassName:  <none>
Node:               kub5/
Start Time:         Thu, 16 May 2019 14:30:46 +0200
Labels:             run=httpd-sample
Annotations:        <none>
Status:             Running
  Type    Reason     Age    From               Message
  ----    ------     ----   ----               -------
  Normal  Scheduled  3m57s  default-scheduler  Successfully assigned default/httpd-sample to kub5
  Normal  Pulled     3m56s  kubelet, kub5      Container image "pmarie/httpd-sample:v2" already present on machine
  Normal  Created    3m56s  kubelet, kub5      Created container httpd-sample
  Normal  Started    3m56s  kubelet, kub5      Started container httpd-sample

You can also get its logs (ie: its output) using logs:

$ kubectl logs httpd-sample
2019/05/16 12:30:46 --- Starting up on 8080

Accessing the app

When an app is running, there are multiple ways to access it: through a proxy using the kubernetes API, using a port-forward or a service…

Running the Kubernetes kubectl proxy

The kubectl proxy allows to access applications through a REST API:

$ kubectl proxy
Starting to serve on

On another terminal:

$ curl
  "major": "1",
  "minor": "14",
  "gitVersion": "v1.14.1",
  "gitCommit": "b7394102d6ef778017f2ca4046abbaa23b88c290",
  "gitTreeState": "clean",
  "buildDate": "2019-04-08T17:02:58Z",
  "goVersion": "go1.12.1",
  "compiler": "gc",
  "platform": "linux/amd64"

Note that you can query all kubernetes' API endpoints this way. Feel free to query directly. Once reachable, you will be able to query any http port of your pods using the pod name and its port number:

$ export POD_NAME=httpd-sample
$ curl${POD_NAME}:8080/proxy/
Date: Thu May 16 09:19:23
RequestURI: /
User-agent: curl/7.29.0
Random-Number: 8674665223082153551

Accessing the app through port-forward

It is possible to forward any port using kubectl port-forward:

$ kubectl port-forward httpd-sample 60123:8080
Forwarding from -> 8080
Forwarding from [::1]:60123 -> 8080

Your app will be reachable using the newly opened port:

$ curl http://localhost:60123
Date: Thu May 16 09:22:04
Host: localhost:60123
RequestURI: /
User-agent: curl/7.29.0
Random-Number: 4037200794235010051

Running a shell inside the app’s container

As any docker container, you’ll be able ot run processes within it or even run a shell:

$ kubectl exec httpd-sample -ti /bin/sh
/app # ls