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Configuring Liveness and Readiness Probes

This page shows how to configure liveness and readiness probes for Containers.

The kubelet uses liveness probes to know when to restart a Container. For example, liveness probes could catch a deadlock, where an application is running, but unable to make progress. Restarting a Container in such a state can help to make the application more available despite bugs.

The kubelet uses readiness probes to know when a Container is ready to start accepting traffic. A Pod is considered ready when all of its Containers are ready. One use of this signal is to control which Pods are used as backends for Services. When a Pod is not ready, it is removed from Service load balancers.

Before you begin

You need to have a Kubernetes cluster, and the kubectl command-line tool must be configured to communicate with your cluster. If you do not already have a cluster, you can create one by using Minikube.

Defining a liveness command

Many applications running for long periods of time eventually transition to broken states, and cannot recover except by being restarted. Kubernetes provides liveness probes to detect and remedy such situations.

In this exercise, you create a Pod that runs a Container based on the gcr.io/google_containers/busybox image. Here is the configuration file for the Pod:

exec-liveness.yaml 
apiVersion: v1
kind: Pod

metadata:
  labels:
    test: liveness
  name: liveness-exec
spec:
  containers:

  - name: liveness

    args:
    - /bin/sh
    - -c
    - touch /tmp/healthy; sleep 30; rm -rf /tmp/healthy; sleep 600

    image: gcr.io/google_containers/busybox

    livenessProbe:
      exec:
        command:
        - cat
        - /tmp/healthy
      initialDelaySeconds: 5
      periodSeconds: 5

In the configuration file, you can see that the Pod has a single Container. The periodSeconds field specifies that the kubelet should perform a liveness probe every 5 seconds. The initialDelaySeconds field tells the kubelet that it should wait 5 second before performing the first probe. To perform a probe, the kubelet executes the command cat /tmp/healthy in the Container. If the command succeeds, it returns 0, and the kubelet considers the Container to be alive and healthy. If the command returns a non-zero value, the kubelet kills the Container and restarts it.

When the Container starts, it executes this command:

/bin/sh -c "touch /tmp/healthy; sleep 30; rm -rf /tmp/healthy; sleep 600"

For the first 30 seconds of the Container’s life, there is a /tmp/healthy file. So during the first 30 seconds, the command cat /tmp/healthy returns a success code. After 30 seconds, cat /tmp/healthy returns a failure code.

Create the Pod:

kubectl create -f http://k8s.io/docs/tasks/configure-pod-container/exec-liveness.yaml

Within 30 seconds, view the Pod events:

kubectl describe pod liveness-exec

The output indicates that no liveness probes have failed yet:

FirstSeen    LastSeen    Count   From            SubobjectPath           Type        Reason      Message
--------- --------    -----   ----            -------------           --------    ------      -------
24s       24s     1   {default-scheduler }                    Normal      Scheduled   Successfully assigned liveness-exec to worker0
23s       23s     1   {kubelet worker0}   spec.containers{liveness}   Normal      Pulling     pulling image "gcr.io/google_containers/busybox"
23s       23s     1   {kubelet worker0}   spec.containers{liveness}   Normal      Pulled      Successfully pulled image "gcr.io/google_containers/busybox"
23s       23s     1   {kubelet worker0}   spec.containers{liveness}   Normal      Created     Created container with docker id 86849c15382e; Security:[seccomp=unconfined]
23s       23s     1   {kubelet worker0}   spec.containers{liveness}   Normal      Started     Started container with docker id 86849c15382e

After 35 seconds, view the Pod events again:

kubectl describe pod liveness-exec

At the bottom of the output, there are messages indicating that the liveness probes have failed, and the containers have been killed and recreated.

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 "gcr.io/google_containers/busybox"
36s       36s     1   {kubelet worker0}   spec.containers{liveness}   Normal      Pulled      Successfully pulled image "gcr.io/google_containers/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

Wait another 30 seconds, and verify that the Container has been restarted:

kubectl get pod liveness-exec

The output shows that RESTARTS has been incremented:

NAME            READY     STATUS    RESTARTS   AGE
liveness-exec   1/1       Running   1          1m

Defining a liveness HTTP request

Another kind of liveness probe uses an HTTP GET request. Here is the configuration file for a Pod that runs a container based on the gcr.io/google_containers/liveness image.

http-liveness.yaml 
apiVersion: v1
kind: Pod
metadata:
  labels:
    test: liveness
  name: liveness-http
spec:
  containers:

  - name: liveness

    args:
    - /server

    image: gcr.io/google_containers/liveness

    livenessProbe:
      httpGet:
        path: /healthz
        port: 8080
        httpHeaders:
          - name: X-Custom-Header
            value: Awesome
      initialDelaySeconds: 3
      periodSeconds: 3

In the configuration file, you can see that the Pod has a single Container. The livenessProbe field specifies that the kubelet should perform a liveness probe every 3 seconds. The initialDelaySeconds field tells the kubelet that it should wait 3 seconds before performing the first probe. To perform a probe, the kubelet sends an HTTP GET request to the server that is running in the Container and listening on port 8080. If the handler for the server’s /healthz path returns a success code, the kubelet considers the Container to be alive and healthy. If the handler returns a failure code, the kubelet kills the Container and restarts it.

Any code greater than or equal to 200 and less than 400 indicates success. Any other code indicates failure.

You can see the source code for the server in server.go.

For the first 10 seconds that the Container is alive, the /healthz handler returns a status of 200. After that, the handler returns a status of 500.

http.HandleFunc("/healthz", func(w http.ResponseWriter, r *http.Request) {
    duration := time.Now().Sub(started)
    if duration.Seconds() > 10 {
        w.WriteHeader(500)
        w.Write([]byte(fmt.Sprintf("error: %v", duration.Seconds())))
    } else {
        w.WriteHeader(200)
        w.Write([]byte("ok"))
    }

The kubelet starts performing health checks 3 seconds after the Container starts. So the first couple of health checks will succeed. But after 10 seconds, the health checks will fail, and the kubelet will kill and restart the Container.

To try the HTTP liveness check, create a Pod:

kubectl create -f http://k8s.io/docs/tasks/configure-pod-container/http-liveness.yaml

After 10 seconds, view Pod events to verify that liveness probes have failed and the Container has been restarted:

kubectl describe pod liveness-http

Using a named port

You can use a named ContainerPort for HTTP liveness checks:

ports:
- name: liveness-port
  containerPort: 8080
  hostPort: 8080

livenessProbe:
  httpGet:
  path: /healthz
  port: liveness-port

Defining readiness probes

Sometimes, applications are temporarily unable to serve traffic. For example, an application might need to load large data or configuration files during startup. In such cases, you don’t want to kill the application, but you don’t want to send it requests either. Kubernetes provides readiness probes to detect and mitigate these situations. A pod with containers reporting that they are not ready does not receive traffic through Kubernetes Services.

Readiness probes are configured similarly to liveness probes. The only difference is that you use the readinessProbe field instead of the livenessProbe field.

readinessProbe:
  exec:
    command:
    - cat
    - /tmp/healthy
  initialDelaySeconds: 5
  periodSeconds: 5

Discussion

Probes have these additional fields that you can use to more precisely control the behavior of liveness and readiness checks:

HTTP probes have these additional fields:

For an HTTP probe, the kubelet sends an HTTP request to the specified path and port to perform the check. The kubelet sends the probe to the container’s IP address, unless the address is overridden by the optional host field in httpGet. In most scenarios, you do not want to set the host field. Here’s one scenario where you would set it. Suppose the Container listens on 127.0.0.1 and the Pod’s hostNetwork field is true. Then host, under httpGet, should be set to 127.0.0.1. If your pod relies on virtual hosts, which is probably the more common case, you should not use host, but rather set the Host header in httpHeaders.

In addition to command probes and HTTP probes, Kubernetes supports TCP probes.

What’s next

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