k8s - livenessProbe vs readinessProbe

The liveness probes are to check if the container is started and alive. If this isn’t the case, kubernetes will eventually restart the container.

The readiness probes in turn also check dependencies like database connections or other services your container is depending on to fulfill it’s work. As a developer you have to invest here more time into the implementation than just for the liveness probes. You have to expose an endpoint which is also checking the mentioned dependencies when queried.

Your current configuration uses a health endpoint which are usually used by liveness probes. It probably doesn’t check if your services is really ready to take traffic.

Kubernetes relies on the readiness probes. During a rolling update, it will keep the old container up and running until the new service declares that it is ready to take traffic. Therefore the readiness probes have to be implemented correctly.

I'm starting from the second problem to answer. The second question is:

Does liveness probe start working only after the pod becomes ready? In other words, I assume readiness probe job is complete once the POD is ready. After that livenessProbe takes care of health check.

Our initial understanding is that liveness probe will start to check after readiness probe was succeeded but it turn out not to be like that. It has opened an issue for this challenge.Yon can look up to here. Then It was solved this problem by adding startup probes.

To sum up:

• livenessProbe

livenessProbe: Indicates whether the Container is running. If the liveness probe fails, the kubelet kills the Container, and the Container is subjected to its restart policy. If a Container does not provide a liveness probe, the default state is Success.

• readinessProbe

readinessProbe: Indicates whether the Container is ready to service requests. If the readiness probe fails, the endpoints controller removes the Pod’s IP address from the endpoints of all Services that match the Pod. The default state of readiness before the initial delay is Failure. If a Container does not provide a readiness probe, the default state is Success.

• startupProbe

startupProbe: Indicates whether the application within the Container is started. All other probes are disabled if a startup probe is provided, until it succeeds. If the startup probe fails, the kubelet kills the Container, and the Container is subjected to its restart policy. If a Container does not provide a startup probe, the default state is Success

look up here.

Both readiness probe and liveness probe seem to have same behavior. They do same type of checks. But the action they take in case of failures is different.

Readiness Probe shuts the traffic from service down. so that service can always the send the request to healthy pod whereas the liveness probe restarts the pod in case of failure. It does not do anything for the service. Service continues to send the request to the pods as usual if it is in ‘available’ status.

It is recommended to use both probes!!

Check here for detailed explanation with code samples.