Kubernetes Container Probes
External
- https://kubernetes.io/docs/tasks/configure-pod-container/configure-liveness-readiness-probes
- https://kubernetes.io/docs/concepts/workloads/pods/pod-lifecycle/#container-probes
Internal
Overview
A probe is a diagnostic performed periodically by the kubelet on a container. The diagnostic is sometimes referred to as a probe. To perform the diagnostic, the kubelet calls a handler, that must be declared and implemented by the container. Each probe has one of these results:
- Success - the container passed the diagnostic
- Failure - the container failed the diagnostic
- Unknown - the diagnostic itself filed so no action should be taken.
There are three kinds of probes: startup, liveness and readiness.
Handlers
A handler is a piece of logic declared and implemented by the container, which is used by Kubernetes control mechanism to draw conclusions about the state the container is in There are three types of handlers, described below. Any of these handlers can be used to perform startup, liveness and readiness checks.
ExecAction
The exec action (declared as "exec:") executes a specified command inside the container. The diagnostic is considered successful if the command exits with a status code of 0.
TCPSocketAction
Performs a TCP check against the container’s IP address on a specified port. The diagnostic is considered successful if the port is open.
HTTPGetAction
Performs an HTTP GET request against the container’s IP address on a specified port and path. The diagnostic is considered successful if the response has a status code greater than or equal to 200 and less than 400
Container Startup Check
The startup check is performed by a startup probe.
Container Liveness Check
The result of the container liveness check is used by Kubernetes to know when to restart the container (Not the pod? How about atomicity?)
Container Readiness Check
The result of the container readiness check is used by Kubernetes to know when the container is ready to accept traffic. The pod is considered ready wen all of its containers are ready. The readiness check is used by services to decide whether to send traffic into the pod or not. Container will be removed from service endpoints if the probe fails.
Probe Template
kind: Pod
spec:
containers:
- name: ...
readinessProbe|livenessProbe:
exec:
Example:
readinessProbe:
exec:
command:
- /bin/sh
- -c
- nodetool status | grep -E "^UN\s+${POD_IP}"
failureThreshold: 3
initialDelaySeconds: 90
periodSeconds: 30
successThreshold: 1
timeoutSeconds: 5
Elements
initialDelaySeconds
Specifies the number of seconds after the container has started before liveness probes are initiated.