vagrant @ ubuntu: ~ $ sleep 20

vagrant @ ubuntu: ~ $ sudo docker ps

CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES

c0041a8d973e ubuntu "bash -c 'sleep 1000'" 38 seconds ago Up 37 seconds (unhealthy) healt

vagrant @ ubuntu: ~ $ sudo docker rm -f healt

healt

If at least one of the checks worked, then the container is marked as healthy immediately:

vagrant @ ubuntu: ~ $ sudo docker run \

–d –name healt \

–-health-timeout = 0s \

–-health-interval = 5s \

–-health-retries = 3 \

–-health-cmd = "ls / halth" \

ubuntu bash -c 'touch / halth && sleep 1000'

vagrant @ ubuntu: ~ $ sudo docker ps

CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES

160820d11933 ubuntu "bash -c 'touch / hal …" 4 seconds ago Up 2 seconds (health: starting) healt

vagrant @ ubuntu: ~ $ sudo docker ps

CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES

160820d11933 ubuntu "bash -c 'touch / hal …" 6 seconds ago Up 5 seconds (healthy) healt

vagrant @ ubuntu: ~ $ sudo docker rm -f healt

healt

In this case, the checks are repeated all the time at a given interval:

vagrant @ ubuntu: ~ $ sudo docker run \

–d –name healt \

–-health-timeout = 0s \

–-health-interval = 5s \

–-health-retries = 3 \

–-health-cmd = "ls / halth" \

ubuntu bash -c 'touch / halth && sleep 60 && rm -f / halth && sleep 60'

vagrant @ ubuntu: ~ $ sudo docker ps

CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES

8ec3a4abf74b ubuntu "bash -c 'touch / hal …" 7 seconds ago Up 5 seconds (health: starting) healt

vagrant @ ubuntu: ~ $ sudo docker ps

CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES

8ec3a4abf74b ubuntu "bash -c 'touch / hal …" 24 seconds ago Up 22 seconds (healthy) healt

vagrant @ ubuntu: ~ $ sleep 60

vagrant @ ubuntu: ~ $ sudo docker ps

CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES

8ec3a4abf74b ubuntu "bash -c 'touch / hal …" About a minute ago Up About a minute (unhealthy) healt

Kubernetes provides (kubernetes.io/docs/tasks/configure-POD-container/configure-liveness-readiness-probes/) three tools that check the state of a container from outside. They are more important because they serve not only to inform, but also to manage the application life cycle, roll-forward and rollback of updates. Configuring them incorrectly can, and often does, cause the application to malfunction. So, if the liveness test is triggered before the application starts working, Kubernetes will kill the container, not allowing it to rise. Let's consider it in more detail. The liveness probe is used to determine the health of the application, and if the application crashes and does not respond to the liveness probe, Kubernetes reloads the container. As an example, we will take a shell test, due to the simplicity of the demonstration of work, but in practice it should be used only in extreme cases, for example, if the container is started not as a long-lived server, but as a JOB, doing its job and ending its existence, having achieved the result … For server checks, it is better to use HTTP probes, which already have a built-in dedicated proxy and do not require curl in the container and do not depend on external kube-proxy settings. When using databases, you must use a TCP probe, as they usually do not support the HTTP protocol. Let's create a long-lived container at www.katacoda.com/courses/kubernetes/playground:

controlplane $ cat << EOF> liveness.yaml

apiVersion: v1

kind: Pod

metadata:

name: liveness

spec:

containers:

– name: healtcheck

image: alpine: 3.5

args:

– / bin / sh

– -c

– touch / tmp / healthy; sleep 10; rm -rf / tmp / healthy; sleep 60

livenessProbe:

exec:

command:

– cat

– / tmp / healthy

initialDelaySeconds: 15

periodSeconds: 5

EOF

controlplane $ kubectl create -f liveness.yaml

pod / liveness created

controlplane $ kubectl get pods

NAME READY STATUS RESTARTS AGE

liveness 1/1 Running 2 2m11s

controlplane $ kubectl describe pod / liveness | tail -n 10

Type Reason Age From Message

–– – – – –

Normal Scheduled 2m37s default-scheduler Successfully assigned default / liveness to node01

Normal Pulling 2m33s kubelet, node01 Pulling image "alpine: 3.5"

Normal Pulled 2m30s kubelet, node01 Successfully pulled image "alpine: 3.5"

Normal Created 33s (x3 over 2m30s) kubelet, node01 Created container healtcheck

Normal Started 33s (x3 over 2m30s) kubelet, node01 Started container healtcheck

Normal Pulled 33s (x2 over 93s) kubelet, node01 Container image "alpine: 3.5" already present on machine

Warning Unhealthy 3s (x9 over 2m13s) kubelet, node01 Liveness probe failed: cat: can't open '/ tmp / healthy': No such file or directory

Normal Killing 3s (x3 over 2m3s) kubelet, node01 Container healtcheck failed liveness probe, will be restarted