40 Days Of Kubernetes (18/40)

40 Days Of Kubernetes (18/40)

Day 18/40

Kubernetes Probes Explained | Liveness vs Readiness vs Startup Probes

Video Link
@piyushsachdeva
Git Repository
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It’s time to explain the Health Probes like:

Liveness Probe
Readiness Probe
Startup Probe
and so on.


(Photo from the video)

In Kubernetes, a probe is a mechanism used to determine the health and readiness of a container or application running within a pod. Probes are defined in the pod specification and are performed periodically to ensure the proper functioning of the application.

Liveness Probe: A liveness probe determines if a container is still running and functioning correctly.

Readiness Probe: A readiness probe determines if a container is ready to receive incoming network traffic and serve requests. It ensures that the container has completed its initialization process and is prepared to handle requests.

Startup Probe: A startup probe is used to determine if a container’s application has started successfully.

source

Demo

Sample1: liveness command

apiVersion: v1
kind: Pod
metadata:
labels:
test: liveness
name: liveness-exec
spec:
containers:
name: liveness
image: registry.k8s.io/busybox
args:
/bin/sh
-c
touch /tmp/healthy; sleep 30; rm -f /tmp/healthy; sleep 600
livenessProbe:
exec:
command:
cat
/tmp/healthy
initialDelaySeconds: 5
periodSeconds: 5
root@localhost:~# kubectl apply -f day18-liveness-command.yaml
pod/liveness-exec created
root@localhost:~# kubectl get pod –watch
NAME READY STATUS RESTARTS AGE
liveness-exec 1/1 Running 0 11s
liveness-exec 1/1 Running 1 (0s ago) 76s
liveness-exec 1/1 Running 2 (2s ago) 2m33s

Logs of the pod:

Events:
Type Reason Age From Message
—- —— —- —- ——-
Normal Scheduled 3m37s default-scheduler Successfully assigned default/liveness-exec to lucky-luke-worker
Normal Pulled 3m36s kubelet Successfully pulled image “registry.k8s.io/busybox” in 1.08s (1.08s including waiting). Image size: 1144547 bytes.
Normal Pulled 2m22s kubelet Successfully pulled image “registry.k8s.io/busybox” in 367ms (367ms including waiting). Image size: 1144547 bytes.
Normal Pulling 67s (x3 over 3m37s) kubelet Pulling image “registry.k8s.io/busybox”
Normal Created 67s (x3 over 3m36s) kubelet Created container liveness
Normal Pulled 67s kubelet Successfully pulled image “registry.k8s.io/busybox” in 503ms (505ms including waiting). Image size: 1144547 bytes.
Normal Started 66s (x3 over 3m36s) kubelet Started container liveness
Warning Unhealthy 22s (x9 over 3m2s) kubelet Liveness probe failed: cat: can’t open ‘/tmp/healthy’: No such file or directory
Normal Killing 22s (x3 over 2m52s) kubelet Container liveness failed liveness probe, will be restarted

Sample2: liveness http and readiness http

apiVersion: v1
kind: Pod
metadata:
name: hello
spec:
containers:
name: liveness
image: registry.k8s.io/e2e-test-images/agnhost:2.40
args:
liveness
livenessProbe:
httpGet:
path: /healthz
port: 8080
initialDelaySeconds: 3
periodSeconds: 3
readinessProbe:
httpGet:
path: /healthz
port: 8080
initialDelaySeconds: 15
periodSeconds: 10
root@localhost:~# kubectl apply -f day18-liveness-http.yaml
pod/hello created
root@localhost:~# kubectl get pod
NAME READY STATUS RESTARTS AGE
hello 0/1 Running 0 10s
root@localhost:~# kubectl get pod –watch
NAME READY STATUS RESTARTS AGE
hello 0/1 Running 0 16s
hello 0/1 Running 1 (1s ago) 22s
hello 0/1 Running 2 (1s ago) 40s
Events:
Type Reason Age From Message
—- —— —- —- ——-
Normal Scheduled 2m11s default-scheduler Successfully assigned default/hello to lucky-luke-worker
Normal Pulling 2m11s kubelet Pulling image “registry.k8s.io/e2e-test-images/agnhost:2.40”
Normal Pulled 2m8s kubelet Successfully pulled image “registry.k8s.io/e2e-test-images/agnhost:2.40” in 3.022s (3.022s including waiting). Image size: 51155161 bytes.
Normal Created 92s (x3 over 2m8s) kubelet Created container liveness
Normal Started 92s (x3 over 2m7s) kubelet Started container liveness
Warning Unhealthy 92s (x3 over 111s) kubelet Readiness probe failed: HTTP probe failed with statuscode: 500
Normal Pulled 92s (x2 over 110s) kubelet Container image “registry.k8s.io/e2e-test-images/agnhost:2.40” already present on machine
Warning Unhealthy 74s (x9 over 116s) kubelet Liveness probe failed: HTTP probe failed with statuscode: 500
Normal Killing 74s (x3 over 110s) kubelet Container liveness failed liveness probe, will be restarted

Sample3: liveness tcp

apiVersion: v1
kind: Pod
metadata:
name: tcp-pod
labels:
app: tcp-pod
spec:
containers:
name: goproxy
image: registry.k8s.io/goproxy:0.1
ports:
containerPort: 8080
livenessProbe:
tcpSocket:
port: 3000
initialDelaySeconds: 10
periodSeconds: 5
root@localhost:~# kubectl apply -f day18-liveness-tcp.yaml
pod/tcp-pod created
root@localhost:~# kubectl get pod –watch
NAME READY STATUS RESTARTS AGE
hello 0/1 CrashLoopBackOff 9 (2m13s ago) 15m
tcp-pod 1/1 Running 0 10s
tcp-pod 1/1 Running 1 (1s ago) 27s
tcp-pod 1/1 Running 2 (1s ago) 47s
tcp-pod 1/1 Running 3 (0s ago) 66s
hello 0/1 Terminating 9 (3m18s ago) 16m
hello 0/1 Terminating 9 16m
hello 0/1 Terminating 9 16m
hello 0/1 Terminating 9 16m
hello 0/1 Terminating 9 16m
hello 0/1 Terminating 9 16m
tcp-pod 1/1 Running 4 (0s ago) 86s

Events:
Type Reason Age From Message
—- —— —- —- ——-
Normal Scheduled 111s default-scheduler Successfully assigned default/tcp-pod to lucky-luke-worker
Normal Pulling 111s kubelet Pulling image “registry.k8s.io/goproxy:0.1”
Normal Pulled 110s kubelet Successfully pulled image “registry.k8s.io/goproxy:0.1” in 695ms (695ms including waiting). Image size: 1698862 bytes.
Normal Created 46s (x4 over 110s) kubelet Created container goproxy
Normal Started 46s (x4 over 110s) kubelet Started container goproxy
Warning Unhealthy 46s (x9 over 96s) kubelet Liveness probe failed: dial tcp 10.244.1.35:3000: connect: connection refused
Normal Killing 46s (x3 over 86s) kubelet Container goproxy failed liveness probe, will be restarted
Normal Pulled 46s (x3 over 86s) kubelet Container image “registry.k8s.io/goproxy:0.1” already present on machine

Note As you can see in the part of running pod yaml file, there are some option for livenessProbe

failureThreshold: 3 which says after 3 failed tests, it can be assumed as a failuer and each test perfomes after periodSeconds.

successThreshold: 1 which says only after one success test, it can be assumed as successful pod.


spec:
containers:
image: registry.k8s.io/goproxy:0.1
imagePullPolicy: IfNotPresent
livenessProbe:
failureThreshold: 3
initialDelaySeconds: 10
periodSeconds: 5
successThreshold: 1
tcpSocket:
port: 3000
timeoutSeconds: 1

Note We can see some tolerations added to the pod like:


tolerations:
effect: NoExecute
key: node.kubernetes.io/not-ready
operator: Exists
tolerationSeconds: 300
effect: NoExecute
key: node.kubernetes.io/unreachable
operator: Exists
tolerationSeconds: 300

These are automatically added by Kubernetes and if node get this taint, pod will be evicted from this node and move to another node.

node.kubernetes.io/not-ready
Type: Taint
Example: node.kubernetes.io/not-ready: “NoExecute”
Used on: Node
The Node controller detects whether a Node is ready by monitoring its health and adds or removes this taint accordingly.
source

node.kubernetes.io/unreachable
Type: Taint
Example: node.kubernetes.io/unreachable: “NoExecute”
Used on: Node
The Node controller adds the taint to a Node corresponding to the NodeCondition Ready being Unknown.
source

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