The primary function of a docker container is to run an executable (e.g. a binary, command, or shell script) along with some optional arguments. The executable+arguments are both specified in the Dockerfile with the following settings:
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- CMD – There’s three ways to use this setting – the 2nd approach, which is to use CMD in conjunction with the ENTRYPOINT is the recommended way. This may look unintuitive but it ensures that the primary process (PID 1) inside the container isn’t the bash/sh session wrapper, but is the primary executable.
This entrypoint’s binary/command/script can be used to run:
- shortlived workloads – if the container is supposed to perform a specific task.
- ongoing workloads – E.g. running the apache httpd binary to provide an ongoing web service.
Shortlived Workloads (eg1-shortlived)
Pods are designed for running containers with ongoing workloads. However, if your pod is built from an image, whose entrypoint is a shortlived (e.g. centos):
--- apiVersion: v1 kind: Pod metadata: name: pod-centos labels: component: centos spec: containers: - name: cntr-centos image: centos
Then the pod is created, it runs it’s command, then shuts down again within a few seconds.
$ kubectl get pods NAME READY STATUS RESTARTS AGE pod-centos 0/1 ContainerCreating 0 2s $ kubectl get pods NAME READY STATUS RESTARTS AGE pod-centos 0/1 Completed 0 5s $ kubectl get pods NAME READY STATUS RESTARTS AGE pod-centos 0/1 CrashLoopBackOff 1 9s $ kubectl get pods NAME READY STATUS RESTARTS AGE pod-centos 0/1 CrashLoopBackOff 1 12s $ kubectl get pods NAME READY STATUS RESTARTS AGE pod-centos 0/1 CrashLoopBackOff 1 21s $ kubectl get pods NAME READY STATUS RESTARTS AGE pod-centos 0/1 Completed 2 26s $
Here the pods ran for less than a second before shutting down, kubernetes thought something went wrong and restarted the container, and keeps restarting it in an endless cycle:
$ kubectl describe pod pod-centos ... Events: Type Reason Age From Message ---- ------ ---- ---- ------- Normal Scheduled 15m default-scheduler Successfully assigned default/pod-centos to minikube Normal Pulling 14m (x4 over 15m) kubelet, minikube pulling image "centos" Normal Pulled 14m (x4 over 15m) kubelet, minikube Successfully pulled image "centos" Normal Created 14m (x4 over 15m) kubelet, minikube Created container Normal Started 14m (x4 over 15m) kubelet, minikube Started container Warning BackOff 33s (x70 over 15m) kubelet, minikube Back-off restarting failed container
To run containers that have shortlived workloads, you should run them as Kubernetes a jobs or cronjobs object. We’ll cover them later.
Ongoing Workloads (eg2-ongoing)
Lets say you still want to use the centos image for the primary container in your pod. That’s still possible, by overriding the centos image’s default ENTRYPOINT/CMD, with an ongoing command/script using the command+args settings:
--- apiVersion: v1 kind: Pod metadata: name: pod-centos labels: component: centos spec: containers: - name: cntr-centos image: centos command: ["/bin/bash", "-c"] # this starts a bash terminal and feeds the args content into it args: # the args section here is used to store a small shell script - | while true ; do date sleep 10 done
Here we used the following settings:
pod.spec.containers.command– This overrides/adds the docker images ‘ENTRYPOINT’ setting.
pod.spec.containers.args– This override/adds the docker images ‘CMD’ setting
Here we’re feeding an infinite while loop to keep the pod running continuously:
$ kubectl get pod NAME READY STATUS RESTARTS AGE pod-centos 1/1 Running 0 15s
We specified the ‘date’ command in the while loop, so you can monitor the pods standard output for this info:
$ kubectl logs pod-centos -c cntr-centos Mon Mar 11 12:14:01 UTC 2019 Mon Mar 11 12:14:11 UTC 2019 Mon Mar 11 12:14:21 UTC 2019 Mon Mar 11 12:14:31 UTC 2019 Mon Mar 11 12:14:41 UTC 2019 Mon Mar 11 12:14:51 UTC 2019 Mon Mar 11 12:15:01 UTC 2019 Mon Mar 11 12:15:11 UTC 2019 Mon Mar 11 12:15:21 UTC 2019 Mon Mar 11 12:15:31 UTC 2019 Mon Mar 11 12:15:41 UTC 2019 Mon Mar 11 12:15:51 UTC 2019
You can also monitor the pods standard output in realtime by using the logs -f flag:
kubectl logs -f pod-centos -c cntr-centos
Or connect your bash terminal directly to the pod’s standard output using the ‘attach’ command:
$ kubectl attach pod-centos Defaulting container name to cntr-centos. Use 'kubectl describe pod/ -n default' to see all of the containers in this pod. If you don't see a command prompt, try pressing enter. Mon Mar 11 12:21:51 UTC 2019 Mon Mar 11 12:22:01 UTC 2019 Mon Mar 11 12:22:11 UTC 2019 Mon Mar 11 12:22:21 UTC 2019 Mon Mar 11 12:22:31 UTC 2019 Mon Mar 11 12:22:41 UTC 2019
In this demo we use an image with an shortlived workload. However you can use this approach to replace a ongoing workload with another ongoing workload.
There could be times when you want to run commands/scripts in addition to the docker image’s CMD/Entrypoint, rather than over-riding it. Luckily there are other ways to inject commands/shellscripts into pods, using Poststart/PreStop hooks. We’ll cover them later.
You can also run non-primary containers with shortlived workloads using
pod.spec.initContainers, which will cover later.
You can also run other commands that periodically monitors the health of your pod’s containers. These are known as liveness and readiness probes. We’ll cover these later too.