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Monitoring application health by using health checks

You can monitor application health on your OKD cluster by configuring readiness, liveness, and startup probes for your application containers. You can also observe probe failure events so you can address issues before they affect users.

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

Health checks

You can configure health checks by understanding the differences between readiness, liveness, and startup probes.

A health check periodically performs diagnostics on a running container using any combination of the readiness, liveness, and startup health checks.

You can include one or more probes in the specification for the pod that contains the container which you want to perform the health checks.

If you want to add or edit health checks in an existing pod, you must edit the pod DeploymentConfig object or use the web console. You cannot use the CLI to add or edit health checks for an existing pod.
Readiness probe

A readiness probe determines if a container is ready to accept service requests. If the readiness probe fails for a container, the kubelet removes the pod from the list of available service endpoints.

After a failure, the probe continues to examine the pod. If the pod becomes available, the kubelet adds the pod to the list of available service endpoints.

Liveness health check

A liveness probe determines if a container is still running. If the liveness probe fails due to a condition such as a deadlock, the kubelet kills the container. The pod then responds based on the pod restart policy.

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

Startup probe

A startup probe indicates whether the application within a container is started. All other probes are disabled until the startup succeeds. If the startup probe does not succeed within a specified time period, the kubelet kills the container, and the container is subject to the pod restartPolicy.

Some applications can require additional startup time on their first initialization. You can use a startup probe with a liveness or readiness probe to delay that probe long enough to handle lengthy start-up time using the failureThreshold and periodSeconds parameters.

For example, you can add a startup probe to a liveness probe. Use a failureThreshold of 30 failures and a periodSeconds of 10 seconds. This combination (30 × 10s = 300s) gives a maximum startup window of 5 minutes. After the startup probe succeeds the first time, the liveness probe takes over.

You can configure liveness, readiness, and startup probes with any of the following types of tests:

  • HTTP GET: When using an HTTP GET test, the test determines the healthiness of the container by using a webhook. The test is successful if the HTTP response code is between 200 and 399.

    You can use an HTTP GET test with applications that return HTTP status codes when completely initialized.

  • Container Command: When using a container command test, the probe executes a command inside the container. The probe is successful if the test exits with a 0 status.

  • Transmission Control Protocol (TCP) socket: When using a TCP socket test, the probe attempts to open a socket to the container. The container is considered healthy only if the probe can establish a connection. You can use a TCP socket test with applications that do not start listening until initialization is complete.

You can configure several fields to control the behavior of a probe:

  • initialDelaySeconds: The time, in seconds, after the container starts before the probe can be scheduled. The default is 0.

  • periodSeconds: The delay, in seconds, between performing probes. The default is 10. This value must be greater than timeoutSeconds.

  • timeoutSeconds: The number of seconds of inactivity after which the probe times out and the container is assumed to have failed. The default is 1. This value must be lower than periodSeconds.

  • successThreshold: The number of times that the probe must report success after a failure to reset the container status to successful. The value must be 1 for a liveness probe. The default is 1.

  • failureThreshold: The number of times that the probe is allowed to fail. The default is 3. When the threshold is reached:

    • for a liveness probe, the container is restarted

    • for a readiness probe, the pod is marked Unready

    • for a startup probe, the container is killed and is subject to the restartPolicy of the pod

Example probes

The following are samples of different probes as they appear in an object specification.

Sample readiness probe with a container command readiness probe in a pod spec
apiVersion: v1
kind: Pod
metadata:
  labels:
    test: health-check
  name: my-application
# ...
spec:
  containers:
  - name: goproxy-app
    args:
    image: registry.k8s.io/goproxy:0.1
    readinessProbe:
      exec:
        command:
        - cat
        - /tmp/healthy
# ...

where:

spec.containers.name

Specifies the container name.

spec.containers.image

Specifies the container image to deploy.

spec.containers.readinessProbe

Specifies a readiness probe.

spec.containers.readinessProbe.exec

Specifies a container command test.

spec.containers.readinessProbe.exec.command

Specifies the commands to execute on the container.

Sample container command startup probe and liveness probe with container command tests in a pod spec
apiVersion: v1
kind: Pod
metadata:
  labels:
    test: health-check
  name: my-application
# ...
spec:
  containers:
  - name: goproxy-app
    args:
    image: registry.k8s.io/goproxy:0.1
    livenessProbe:
      httpGet:
        scheme: HTTPS
        path: /healthz
        port: 8080
        httpHeaders:
        - name: X-Custom-Header
          value: Awesome
    startupProbe:
      httpGet:
        path: /healthz
        port: 8080
      failureThreshold: 30
      periodSeconds: 10
# ...

where:

spec.containers.name

Specifies the container name.

spec.containers.image

Specifies the container image to deploy.

spec.containers.livenessProbe

Specifies a liveness probe.

spec.containers.livenessProbe.httpGet

Specifies an HTTP GET test.

spec.containers.livenessProbe.httpGet.scheme

Specifies the internet scheme: HTTP or HTTPS. The default value is HTTP.

spec.containers.livenessProbe.httpGet.port

Specifies the port on which the container is listening.

spec.containers.startupProbe

Specifies a startup probe.

spec.containers.startupProbe.httpGet

Specifies an HTTP GET test.

spec.containers.startupProbe.httpGet.port

Specifies the port on which the container is listening.

spec.containers.startupProbe.failureThreshold

Specifies the number of times to try the probe after a failure.

spec.containers.startupProbe.periodSeconds

Specifies the number of seconds to perform the probe.

Sample liveness probe with a container command test that uses a timeout in a pod spec
apiVersion: v1
kind: Pod
metadata:
  labels:
    test: health-check
  name: my-application
# ...
spec:
  containers:
  - name: goproxy-app
    args:
    image: registry.k8s.io/goproxy:0.1
    livenessProbe:
      exec:
        command:
        - /bin/bash
        - '-c'
        - timeout 60 /opt/eap/bin/livenessProbe.sh
      periodSeconds: 10
      successThreshold: 1
      failureThreshold: 3
# ...

where:

spec.containers.name

Specifies the container name.

spec.containers.image

Specifies the container image to deploy.

spec.containers.livenessProbe

Specifies the liveness probe.

spec.containers.livenessProbe.exec

Specifies the type of probe, here a container command probe.

spec.containers.livenessProbe.exec.command

Specifies the command line to execute inside the container.

spec.containers.livenessProbe.periodSeconds

Specifies how often in seconds to perform the probe.

spec.containers.livenessProbe.successThreshold

Specifies the number of consecutive successes needed to show success after a failure.

spec.containers.livenessProbe.failureThreshold

Specifies the number of times to try the probe after a failure.

Sample readiness probe and liveness probe with a TCP socket test in a deployment
kind: Deployment
apiVersion: apps/v1
metadata:
  labels:
    test: health-check
  name: my-application
spec:
# ...
  template:
    spec:
      containers:
        - resources: {}
          readinessProbe:
            tcpSocket:
              port: 8080
            timeoutSeconds: 1
            periodSeconds: 10
            successThreshold: 1
            failureThreshold: 3
          terminationMessagePath: /dev/termination-log
          name: ruby-ex
          livenessProbe:
            tcpSocket:
              port: 8080
            initialDelaySeconds: 15
            timeoutSeconds: 1
            periodSeconds: 10
            successThreshold: 1
            failureThreshold: 3
# ...

where:

spec.template.spec.containers.readinessProbe

Specifies the readiness probe.

spec.template.spec.containers.livenessProbe

Specifies the liveness probe.

Configuring health checks using the CLI

To configure readiness, liveness, and startup probes, add one or more probes to the specification for the pod that contains the container on which you want to perform the health checks. Probes let the cluster detect unhealthy containers and respond before failures affect application availability.

If you want to add or edit health checks in an existing pod, you must edit the pod DeploymentConfig object or use the web console. You cannot use the CLI to add or edit health checks for an existing pod.
Procedure

  1. Create a YAML file that defines a Pod object with one or more probes:

    apiVersion: v1
kind: Pod
metadata:
  labels:
    test: health-check
  name: my-application
spec:
  containers:
  - name: my-container
    args:
    image: registry.k8s.io/goproxy:0.1
    livenessProbe:
      tcpSocket:
        port: 8080
      initialDelaySeconds: 15
      periodSeconds: 20
      timeoutSeconds: 10
    readinessProbe:
      httpGet:
        host: my-host
        scheme: HTTPS
        path: /healthz
        port: 8080
    startupProbe:
      exec:
        command:
        - cat
        - /tmp/healthy
      failureThreshold: 30
      periodSeconds: 20
      timeoutSeconds: 10

    Include only the probe types your application needs. The example shows liveness, readiness, and startup probes together for reference.

    where:

    spec.containers.name

    Specifies the container name.

    spec.containers.image

    Specifies the container image to deploy.

    spec.containers.livenessProbe

    Specifies a liveness probe. This value is optional.

    spec.containers.livenessProbe.tcpSocket

    Specifies a test to perform, here a Transmission Control Protocol (TCP) socket test.

    spec.containers.livenessProbe.tcpSocket.port

    Specifies the port on which the container is listening.

    spec.containers.livenessProbe.initialDelaySeconds

    Specifies the time, in seconds, after the container starts before the probe can be scheduled.

    spec.containers.livenessProbe.periodSeconds

    Specifies the number of seconds to perform the probe. The default is 10. This value must be greater than timeoutSeconds.

    spec.containers.livenessProbe.timeoutSeconds

    Specifies the number of seconds of inactivity after which the probe is assumed to have failed. The default is 1. This value must be lower than periodSeconds.

    spec.containers.readinessProbe

    Specifies a readiness probe. This value is optional.

    spec.containers.readinessProbe.httpGet

    Specifies the type of test to perform, here an HTTP test.

    spec.containers.readinessProbe.httpGet.host

    Specifies a host IP address. When host is not defined, the PodIP is used.

    spec.containers.readinessProbe.httpGet.scheme

    Specifies HTTP or HTTPS. When scheme is not defined, the HTTP scheme is used.

    spec.containers.readinessProbe.httpGet.port

    Specifies the port on which the container is listening.

    spec.containers.startupProbe

    Specifies a startup probe. This value is optional.

    spec.containers.startupProbe.exec

    Specifies the type of test to perform, here a container execution probe.

    spec.containers.startupProbe.exec.command

    Specifies the commands to execute on the container.

    spec.containers.startupProbe.failureThreshold

    Specifies the number of times to try the probe after a failure.

    spec.containers.startupProbe.periodSeconds

    Specifies the number of seconds to perform the probe. The default is 10. This value must be greater than timeoutSeconds.

    spec.containers.startupProbe.timeoutSeconds

    Specifies the number of seconds of inactivity after which the probe is assumed to have failed. The default is 1. This value must be lower than periodSeconds.

    If the initialDelaySeconds value is lower than the periodSeconds value, the first readiness probe occurs at some point between the two periods due to an issue with timers.

    The timeoutSeconds value must be lower than the periodSeconds value.

  2. Apply the YAML file by running the following command:

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

  3. Verify the state of the health check pod by running the following command:

    $ oc describe pod my-application
    Example output
    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 "registry.k8s.io/liveness"
  Normal  Pulled     1s    kubelet, ip-10-0-143-40.ec2.internal  Successfully pulled image "registry.k8s.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

    The following example shows output when a liveness probe fails and the container is restarted:

    ....

Events:
  Type     Reason          Age                From                                               Message
  ----     ------          ----               ----                                               -------
  Normal   Scheduled       <unknown>                                                             Successfully assigned aaa/liveness-http to ci-ln-37hz77b-f76d1-wdpjv-worker-b-snzrj
  Normal   AddedInterface  47s                multus                                             Add eth0 [10.129.2.11/23]
  Normal   Pulled          46s                kubelet, ci-ln-37hz77b-f76d1-wdpjv-worker-b-snzrj  Successfully pulled image "registry.k8s.io/liveness" in 773.406244ms
  Normal   Pulled          28s                kubelet, ci-ln-37hz77b-f76d1-wdpjv-worker-b-snzrj  Successfully pulled image "registry.k8s.io/liveness" in 233.328564ms
  Normal   Created         10s (x3 over 46s)  kubelet, ci-ln-37hz77b-f76d1-wdpjv-worker-b-snzrj  Created container liveness
  Normal   Started         10s (x3 over 46s)  kubelet, ci-ln-37hz77b-f76d1-wdpjv-worker-b-snzrj  Started container liveness
  Warning  Unhealthy       10s (x6 over 34s)  kubelet, ci-ln-37hz77b-f76d1-wdpjv-worker-b-snzrj  Liveness probe failed: HTTP probe failed with statuscode: 500
  Normal   Killing         10s (x2 over 28s)  kubelet, ci-ln-37hz77b-f76d1-wdpjv-worker-b-snzrj  Container liveness failed liveness probe, will be restarted
  Normal   Pulling         10s (x3 over 47s)  kubelet, ci-ln-37hz77b-f76d1-wdpjv-worker-b-snzrj  Pulling image "registry.k8s.io/liveness"
  Normal   Pulled          10s                kubelet, ci-ln-37hz77b-f76d1-wdpjv-worker-b-snzrj  Successfully pulled image "registry.k8s.io/liveness" in 244.116568ms

Application health monitoring by using the Developer perspective

You can monitor application health by adding readiness, liveness, and startup probes from the Developer perspective, either when you deploy an application or on a deployed application. Probes detect unhealthy containers and keep applications available before failures affect users.

You can use the Developer perspective to add three types of health probes to your container to ensure that your application is healthy:

  • Use a readiness probe to check if the container is ready to handle requests.

  • Use a liveness probe to check if the container is running.

  • Use a startup probe to check if the application within the container has started.

You can add health checks either while creating and deploying an application, or after you have deployed an application.

Editing health checks using the Developer perspective

You can edit, remove, or add readiness, liveness, and startup probes on a deployed application from the Topology view in the Developer perspective. Use the Edit Health Checks page to update probe parameters, remove probes, or add new probe types.

Prerequisites

  • You have switched to the Developer perspective in the web console.

  • You have created and deployed an application on OKD using the Developer perspective.

  • You have added health checks to your application.

Procedure

  1. In the Topology view, right-click your application and select Edit Health Checks. Alternatively, in the side panel, click the Actions drop-down list and select Edit Health Checks.

  2. To remove a previously added health probe, click the Remove icon adjoining it.

  3. To edit the parameters of an existing probe:

    1. Click the Edit Probe link next to a previously added probe to see the parameters for the probe.

    2. Modify the parameters as required, and click the check mark to save your changes.

  4. To add a new health probe, click the add probe links. For example, to add a liveness probe that checks if your container is running:

    1. Click Add Liveness Probe to see a form containing the parameters for the probe.

    2. Edit the probe parameters as required.

      The Timeout value must be lower than the Period value. The Timeout default value is 1. The Period default value is 10.

    3. Click the check mark at the bottom of the form. The Liveness Probe Added message is displayed.

  5. Click Save to save your modifications and add the additional probes to your container. You are redirected to the Topology view.

Verification

  1. In the side panel, verify that the probes have been added by clicking on the deployed pod under the Pods section.

  2. In the Pod Details page, click the listed container in the Containers section.

  3. In the Container Details page, verify that the Liveness probe - HTTP Get 10.129.4.65:8080/ has been added to the container, in addition to the earlier existing probes.

Monitoring health check failures using the Developer perspective

You can monitor health check failures for a deployed application from the Topology view in the Developer perspective. Use the Observe tab to view events that report probe failures and show when containers need attention before users are affected.

Prerequisites

  • You have switched to the Developer perspective in the web console.

  • You have created and deployed an application on OKD using the Developer perspective.

  • You have added health checks to your application.

Procedure

  1. In the Topology view, click the application node to see the side panel.

  2. Click the Observe tab to see health check failure events in the Events (Warning) section.

  3. Click the down arrow adjoining Events (Warning) to see the details of the health check failure.

Additional resources