Life cycle hooks or graceful shutdown

As you run into failures in real-life scenarios, you may find that you want to take additional action before containers are shutdown or right after they are started. Kubernetes actually provides life cycle hooks for just this kind of use case.

The following example controller definition defines both a postStart action and a preStop action to take place before Kubernetes moves the container into the next stage of its life cycle (you can refer to more details about this in point 1 in the References section at the end of the chapter):

apiVersion: v1 
kind: ReplicationController 
metadata: 
  name: apache-hook 
  labels: 
    name: apache-hook 
spec: 
  replicas: 3 
  selector: 
    name: apache-hook 
  template: 
    metadata: 
      labels: 
        name: apache-hook 
    spec: 
      containers: 
      - name: apache-hook 
        image: bitnami/apache:latest 
        ports: 
        - containerPort: 80 
        lifecycle: 
          postStart: 
            httpGet: 
              path: http://my.registration-server.com/register/ 
              port: 80 
          preStop: 
            exec: 
              command: ["/usr/local/bin/apachectl","-k","graceful-
              stop"]

Listing 2-11: apache-hooks-controller.yaml

You'll note for the postStart hook, we define an httpGet action, but for the preStop hook, I define an exec action. Just as with our health checks, the httpGet action attempts to make an HTTP call to the specific endpoint and port combination, while the exec action runs a local command in the container.

The httpGet and exec actions are both supported for the postStart and preStop hooks. In the case of preStop, a parameter named reason will be sent to the handler as a parameter. See the following table for valid values:

It's important to note that hook calls are delivered at least once. Therefore, any logic in the action should gracefully handle multiple calls. Another important note is that postStart runs before a pod enters its ready state. If the hook itself fails, the pod will be considered unhealthy.