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Chapter 6. Monitoring application health

In software systems, components can become unhealthy due to transient issues such as temporary connectivity loss, configuration errors, or problems with external dependencies. OpenShift Container Platform applications have a number of options to detect and handle unhealthy containers.

6.1. Understanding health checks

A probe is a Kubernetes action that periodically performs diagnostics on a running container. Currently, two types of probes exist, each serving a different purpose.

Readiness Probe
A Readiness check determines if the container in which it is scheduled is ready to service requests. If the readiness probe fails a container, the endpoints controller ensures the container has its IP address removed from the endpoints of all services. A readiness probe can be used to signal to the endpoints controller that even though a container is running, it should not receive any traffic from a proxy.

For example, a Readiness check can control which Pods are used. When a Pod is not ready, it is removed.

Liveness Probe
A Liveness checks determines if the container in which it is scheduled is still running. If the liveness probe fails due to a condition such as a deadlock, the kubelet kills the container The container then responds based on its restart policy.

For example, a liveness probe on a node with a restartPolicy of Always or OnFailure kills and restarts the Container on the node.

Sample Liveness Check

apiVersion: v1
kind: Pod
metadata:
  labels:
    test: liveness
  name: liveness-http
spec:
  containers:
  - name: liveness-http
    image: k8s.gcr.io/liveness 1
    args:
    - /server
    livenessProbe: 2
      httpGet:   3
        # host: my-host
        # scheme: HTTPS
        path: /healthz
        port: 8080
        httpHeaders:
        - name: X-Custom-Header
          value: Awesome
      initialDelaySeconds: 15  4
      timeoutSeconds: 1   5
    name: liveness   6

1
Specifies the image to use for the liveness probe.
2
Specifies the type of heath check.
3
Specifies the type of Liveness check:
  • HTTP Checks. Specify httpGet.
  • Container Execution Checks. Specify exec.
  • TCP Socket Check. Specify tcpSocket.
4
Specifies the number of seconds before performing the first probe after the container starts.
5
Specifies the number of seconds between probes.

Sample Liveness check output wth unhealthy container

$ oc describe pod pod1

....

FirstSeen LastSeen    Count   From            SubobjectPath           Type        Reason      Message
--------- --------    -----   ----            -------------           --------    ------      -------
37s       37s     1   {default-scheduler }                            Normal      Scheduled   Successfully assigned liveness-exec to worker0
36s       36s     1   {kubelet worker0}   spec.containers{liveness}   Normal      Pulling     pulling image "k8s.gcr.io/busybox"
36s       36s     1   {kubelet worker0}   spec.containers{liveness}   Normal      Pulled      Successfully pulled image "k8s.gcr.io/busybox"
36s       36s     1   {kubelet worker0}   spec.containers{liveness}   Normal      Created     Created container with docker id 86849c15382e; Security:[seccomp=unconfined]
36s       36s     1   {kubelet worker0}   spec.containers{liveness}   Normal      Started     Started container with docker id 86849c15382e
2s        2s      1   {kubelet worker0}   spec.containers{liveness}   Warning     Unhealthy   Liveness probe failed: cat: can't open '/tmp/healthy': No such file or directory

6.1.1. Understanding the types of health checks

Liveness checks and Readiness checks can be configured in three ways:

HTTP Checks
The kubelet uses a web hook to determine the healthiness of the container. The check is deemed successful if the HTTP response code is between 200 and 399.

A HTTP check is ideal for applications that return HTTP status codes when completely initialized.

Container Execution Checks
The kubelet executes a command inside the container. Exiting the check with status 0 is considered a success.
TCP Socket Checks
The kubelet attempts to open a socket to the container. The container is only considered healthy if the check can establish a connection. A TCP socket check is ideal for applications that do not start listening until initialization is complete.

6.2. Configuring health checks

To configure health checks, create a pod for each type of check you want.

Procedure

To create health checks:

  1. Create a Liveness Container Execution Check:

    1. Create a YAML file similar to the following: 

      apiVersion: v1
kind: Pod
metadata:
  labels:
    test: liveness
  name: liveness-exec
spec:
  containers:
  - args:
    image: k8s.gcr.io/liveness
    livenessProbe:
      exec:  1
        command: 2
        - cat
        - /tmp/health
      initialDelaySeconds: 15 3
...
      1
      Specify a Liveness check and the type of Liveness check.
      2
      Specify the commands to use in the container.
      3
      Specify the number of seconds before performing the first probe after the container starts.

    2. Verify the state of the health check pod: 

      $ oc describe pod liveness-exec

Events:
  Type    Reason     Age   From                                  Message
  ----    ------     ----  ----                                  -------
  Normal  Scheduled  9s    default-scheduler                     Successfully assigned openshift-logging/liveness-exec to ip-10-0-143-40.ec2.internal
  Normal  Pulling    2s    kubelet, ip-10-0-143-40.ec2.internal  pulling image "k8s.gcr.io/liveness"
  Normal  Pulled     1s    kubelet, ip-10-0-143-40.ec2.internal  Successfully pulled image "k8s.gcr.io/liveness"
  Normal  Created    1s    kubelet, ip-10-0-143-40.ec2.internal  Created container
  Normal  Started    1s    kubelet, ip-10-0-143-40.ec2.internal  Started container
      Note

      The timeoutSeconds parameter has no effect on the Readiness and Liveness probes for Container Execution Checks. You can implement a timeout inside the probe itself, as OpenShift Container Platform cannot time out on an exec call into the container. One way to implement a timeout in a probe is by using the timeout parameter to run your liveness or readiness probe: 

      spec:
  containers:
    livenessProbe:
      exec:
        command:
          - /bin/bash
          - '-c'
          - timeout 60 /opt/eap/bin/livenessProbe.sh 1
      timeoutSeconds: 1
      periodSeconds: 10
      successThreshold: 1
      failureThreshold: 3
      1
      Timeout value and path to the probe script.

    3. Create the check: 

      $ oc create -f <file-name>.yaml

  2. Create a Liveness TCP Socket Check:

    1. Create a YAML file similar to the following: 

      apiVersion: v1
kind: Pod
metadata:
  labels:
    test: liveness
  name: liveness-tcp
spec:
  containers:
  - name: contaier1 1
    image: k8s.gcr.io/liveness
    ports:
    - containerPort: 8080 2
    livenessProbe:  3
      tcpSocket:
        port: 8080
      initialDelaySeconds: 15 4
      timeoutSeconds: 1  5
      1 2
      Specify the container name and port for the check to connect to.
      3
      Specify the Liveness heath check and the type of Liveness check.
      4
      Specify the number of seconds before performing the first probe after the container starts.
      5
      Specify the number of seconds between probes.

    2. Create the check: 

      $ oc create -f <file-name>.yaml

  3. Create an Readiness HTTP Check:

    1. Create a YAML file similar to the following: 

      apiVersion: v1
kind: Pod
metadata:
  labels:
    test: readiness
  name: readiness-http
spec:
  containers:
  - args:
    image: k8s.gcr.io/readiness 1
    readinessProbe: 2
    httpGet:
    # host: my-host 3
    # scheme: HTTPS 4
      path: /healthz
      port: 8080
    initialDelaySeconds: 15  5
    timeoutSeconds: 1  6
      1
      Specify the image to use for the liveness probe.
      2
      Specify the Readiness heath check and the type of Readiness check.
      3
      Specify a host IP address. When host is not defined, the PodIP is used.
      4
      Specify HTTP or HTTPS. When scheme is not defined, the HTTP scheme is used.
      5
      Specify the number of seconds before performing the first probe after the container starts.
      6
      Specify the number of seconds between probes.

    2. Create the check: 

      $ oc create -f <file-name>.yaml
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