Kubernetes Rolling Updates: implementing a Readiness Probe with Spring Boot Actuator

During a Rolling Update on Kubernetes, if a service has a Readiness Probe defined, Kubernetes will use the results of calling this heathcheck to determine when updated pods are ready to start accepting traffic.

Kubernetes supports two probes to determine the health of a pod:

• the Readiness Probe: used to determine if a pod is able to accept traffic
• the Liveliness Probe: used to determine if a pod is appropriately responding, and if not, it will be killed and a new pod restarted

Spring Boot’s Actuator default healthcheck to indicate if a service is up and ready for traffic can be used for a Kubernetes Readiness Probe. To include in an existing Spring Boot service, add the Actuator maven dependency:

<dependency>
  <groupId>org.springframework.boot</groupId
  <artifactId>spring-boot-starter-actuator</artifactId
</dependency>

This adds the default healthcheck accessible by /actuator/health, and returns a 200 (and json response { “status” : “up”} ) if the service is up and running. 

To configure Kubernetes to call this Actuator healthcheck to determine the health of a pod, add a readinessProbe section to the container spec section for your deployment.yaml:

spec:
  containers:
  - name: exampleservice-b 
    image: kevinhooke/examplespringboot-b:latest
    imagePullPolicy: Always
    ports:
    - containerPort: 8080
    readinessProbe:
      httpGet:
        path: /example-b/v1/actuator/health
        port: 8080
      initialDelaySeconds: 5
      timeoutSeconds: 5

Kubernetes will call this endpoint to check when the pod is deployed and ready for traffic. During a rolling update, as new pods are created with an updated image, you’ll see their status go from 0/1 available to 1/1 available as soon as the Spring Boot service has completed startup and the healthcheck is responding.

The gif below shows deployment of an update image to a pod. Notice how as new pods are created, they move from 0/1 to 1/1 and then when they are ready, the old pods are destroyed: