OpenShift Container Probes

Internal

• OpenShift Concepts
• Pod Operations
• Pod Definition

Networking

The pod IP address is routable by default from any other pod in the project. Depending on the network plugin configured for a specific cluster, pods may also be reachable across the entire cluster. The default addresses are part of the 10.x.x.x set. The containers on a pod share the IP address and TCP ports, because they share the pod's virtual network device.

Pod Placement

https://docs.openshift.com/container-platform/3.5/admin_guide/scheduler.html#controlling-pod-placement

Pods can be configured to execute on a specific node, defined by the node name, or on nodes that match a specific node selector.

To assign a pod to a specific node, TODO https://docs.openshift.com/container-platform/3.5/admin_guide/scheduler.html#constraining-pod-placement-labels

To assign a pod to nodes that match a node selector, add the "nodeSelector" element in the pod configuration, with a value consisting in key/value pairs, as described here:

Assigning a Pod to Nodes that Match a Node Selector

After a successful placement, either by a replication controller or by a DaemonSet, the pod records the successful node selector expression as part of its definition, which can be rendered with oc get pod -o yaml:

spec:
  ...
  nodeSelector:
   logging: "true"
  ...

TODO Consolidate with OpenShift_Concepts#Node_Selector

Once bound to a node, a Pod will never be rebound to another node.

Container Probe

https://docs.openshift.com/container-platform/latest/dev_guide/application_health.html

https://kubernetes.io/docs/concepts/workloads/pods/pod-lifecycle/#container-probes

Users can configure container probes for liveness or readiness. Sometimes they are referred as "pod probes", but they are configured at container-level, not pod-level. Each container can have its own probe set, which are exercised, and return results, independently. They are specified in the pod template.

A probe is executed periodically by Kubernetes, and consists in a diagnostic on the container, which may have one of the following results: Success, which means the container passed the diagnostic, Failure, meaning that the container failed the diagnostic and Unknown, which means the diagnostic execution itself failed and no action should be taken.

Liveness Probe

A liveness probe indicates whether the container is running. If the liveness probe fails, Kubernetes kills the container, and the container is subjected to its restart policy, as described in Liveness Probe Failure. If a container does not provide a liveness probe, the liveness diagnostic is considered successful by default.

The following sequence should go in the container declaration from the pod template, at the same level as "name":

livenessProbe: 
 
  initialDelaySeconds: 30
  timeoutSeconds: 1
  successThreshold: 1
  failureThreshold: 3
  periodSeconds: 10

  tcpSocket: 
      port: 5432

Readiness Probe

A readiness probe is deployed in a container to expose whether the container is ready to service requests. If a container does not provide a readiness probe, the readiness state after creation is by default "Success". On readiness probe failure, Kubernetes will stop sending traffic into that specific pod, by removing the corresponding endpoint form the service, as described in the readiness probe failure section. What about router?. A readiness probe is useful when we want to automatically stop sending traffic if a pod enters an unstable state, and resume sending traffic into it if, and when it recovers. This could also be used in implementing a mechanism to allow taking the container down for maintenance. Note that if you just want to be able to drain requests when the pod is deleted, you do not necessarily need a readiness probe; on deletion, the pod automatically puts itself into an unready state regardless of whether the readiness probe exists. The pod remains in the unready state while it waits for the containers in the pod to stop.

The following sequence should go in the container declaration from the pod template, at the same level as "name":

readinessProbe:

  initialDelaySeconds: 5
  timeoutSeconds: 1
  successThreshold: 1
  failureThreshold: 3
  periodSeconds: 10

  exec:
   command:
    - /bin/sh
    - -i
    - -c
    - psql -h 127.0.0.1 -U $POSTGRESQL_USER -q -d $POSTGRESQL_DATABASE -c 'SELECT 1'

Probe Operations

After the container is started, Kubernetes waits for initialDelaySeconds, specified in seconds, then it triggers the execution of the probe specified by "exec", "httpGet", "tcpSocket", etc. Once the probe execution is started, Kubernetes waits for timeoutSeconds (default 1 second) for the probe execution to complete.

If the probe execution is successful, the success counts towards the successThreshold_initialDelaySeconds. A total number of consecutive successful execution specified in successThreshold must be counted after a failure, for the container to be considered as passing the probe. For liveness probes, this value must be 1. The default value is 1.

If the probe does not complete within timeoutSeconds seconds or it explicitly fails, the failure counts towards the failureThreshold. A total number of successive failed execution specified in failureThreshold must be counted before the container to be considered as failing the probe.

The probe is executed periodically with a periodicity of periodSeconds.

Liveness Probe Failure

If the liveness probe fails, Kubernetes kills the container and the container is subjected to its restart policy. A liveness probe that fails occasionally is indicated by the number of restarts:

NAME                   READY     STATUS    RESTARTS   AGE 
rest-service-1-9p9hj   1/1       Running   3          1m

Note that a pod will maintain its name after a restart.

If the liveness probe fails consistently, the pod enters a crash loop backoff state What is exactly the condition that makes it go from "Running" to "CrashLoopBackOff"?:

NAME                   READY     STATUS             RESTARTS   AGE
rest-service-1-9p9hj   0/1       CrashLoopBackOff   5          3m

Readiness Probe Failure

If the readiness probe fails, the EndpointsController removes the Pod’s IP address from the endpoints of all Services that match the Pod. The service will still exist, but it'll list less endpoints. If the service is backed by one-replica pod, it'll have zero endpoints.

The container will still show in a Running phase (status), but it will not be "READY".

NAME                       READY     STATUS    RESTARTS   AGE
po/rest-service-3-bm1t9    0/1       Running   0          2m

Note that if the pod "heals" - the readiness probe starts passing after the configured number of successful run reaches successThreshold, the endpoint is re-attached to the service, automatically.

Probe Types

Container Execution Checks

Kubernetes executes the command specified by "exec" inside the container. If the command exists with 0, the probe execution is considered a success, anything else is a failure.

Local Manifest Pod

https://docs.openshift.com/container-platform/latest/install_config/master_node_configuration.html#node-configuration-files

Bare Pod

https://kubernetes.io/docs/concepts/workloads/controllers/daemonset/#bare-pods

A pod that is not backed by a replication controller. Bare pods cannot be evacuated from nodes.

Static Pod

https://kubernetes.io/docs/concepts/workloads/controllers/daemonset/#static-pods

Pod Presets

https://docs.openshift.com/container-platform/latest/dev_guide/pod_preset.html