Health Checking Probes

Kubernetes Liveness and Readiness probes are useful for checking the health of pods. Readiness probes determine if a pod is ready to receive traffic, and Liveness probes signals if a pod's containers should be restarted.

Both liveness and readiness probes support a variety of action types to determine if something is healhty:

• HTTP: Healthy means a request to some specified HTTP endpoint returned a response between 200 to 399
• Command: - Healhty means a command executed successfully (return code 0)
• TCP: Healthy means a specific TCP socket was successfully opened

Liveness Probe Example

Let's go through an example of a liveness probe implemented via a command

• Create a new application
• Create a new component, with busybox as image
• Add the command /bin/sh -c 'touch /tmp/healthy; sleep 10000'

This creates a file upon startup, in this case representing the health of our Component.

• Click the 'Health' Tab
• Select Command from the Liveness Probe dropdown.
• Enter cat /tmp/healthy as the command

The cat command will execute successfully if the file exists.

• Decrease the number of consequtive tests from 3 to 1. This will save us some time to see the results
• Click Deploy Component

The pod should spin up successfully. Now let's delete the file by opening a shell.

rm /tmp/healthy

Within 20 seconds, the Terminal will become disconnectd because the container is deleted. Go back to the Component view, and you will see the number of Restarts increase from 0 to 1.

By restarting the pod, the "problem" of the missing /tmp/healthy is fixed, as the file is created by the startup command. This demonstrates the purpose of the livenessProbe: triggering automatic restarts in an attempt to fix problematic pods.

Readiness Probe Example

Readiness Probes are very similar. Let's create one with an HTTP action.

• Create a new component with quay.io/openshiftlabs/simpleservice:0.5.0 as the image
• In the Networking tab, Add a port named healthport with Container Port set to 9876
• In the **Health Tabcreate a HTTP liveness probe with /health and 9876 for the port

The pod should be ready according to the probe.

The image we are using allows us to insert an artificial delay to /health by adding an environment variable:

• add an environment variable HEALTH_MAX with the value 5000 which means there's a 5 second delay for the timeout

Probe should Now fail

• Remove the environment variable, and the probe should start working again.