Spring Cloud Kubernetes Configuration Watcher

Kubernetes provides the ability to mount a ConfigMap or Secret as a volume in the container of your application. When the contents of the ConfigMap or Secret changes, the mounted volume will be updated with those changes.

However, Spring Boot will not automatically update those changes unless you restart the application. Spring Cloud provides the ability refresh the application context without restarting the application by either hitting the actuator endpoint /refresh or via publishing a RefreshRemoteApplicationEvent using Spring Cloud Bus.

To achieve this configuration refresh of a Spring Cloud app running on Kubernetes, you can deploy the Spring Cloud Kubernetes Configuration Watcher controller into your Kubernetes cluster.

The application is published as a container and is available on Docker Hub. However, if you need to customize the config watcher behavior or prefer to build the image yourself you can easily build your own image from the source code on GitHub and use that.

Another option to configure it is to provide some environment variables in the deployment.yaml used to deploy configuration watcher. Here are some important ones:

env:
  - name: LOGGING_LEVEL_ORG_SPRINGFRAMEWORK_CLOUD_KUBERNETES_CONFIGURATION_WATCHER
    value: DEBUG
  - name: LOGGING_LEVEL_ORG_SPRINGFRAMEWORK_CLOUD_KUBERNETES_CLIENT_CONFIG_RELOAD
    value: DEBUG
  - name: LOGGING_LEVEL_ORG_SPRINGFRAMEWORK_CLOUD_KUBERNETES_COMMONS_CONFIG_RELOAD
    value: DEBUG

These enable debug logging on the configuration watcher and are particular useful on the initial set-up, to be able to diagnose potential miss-configurations.

env:
  - name: SPRING_CLOUD_KUBERNETES_RELOAD_NAMESPACES_0
    value: "namespace-a"

This one lets watcher know where to search for secrets and configmaps. You have two options here: selective namespaces (the setting above) and a namespace chosen by Namespace Resolution (this is the default option). Keep in mind that all these options require proper RBAC rules.

Changes from configmaps/secrets will only trigger an event being fired from configuration watcher if that particular change came from a source that has a label: spring.cloud.kubernetes.config=true or spring.cloud.kubernetes.secret=true.

To put it simpler, if you change a configmap (or secret), that does not have the label above, configuration watcher will skip firing an event for it (if you enabled debug logging, this will be visible in logs).

By default, configuration watcher will monitor all configmaps/secrets in the configured namespace(s). If you want to filter to watch only particular sources, you can do that by setting:

SPRING_CLOUD_KUBERNETES_CONFIG_INFORMER_ENABLED=TRUE

This will tell watcher to only monitor sources that have a label: spring.cloud.kubernetes.config.informer.enabled=true.

One more important configuration, especially for configmaps and secrets that are mounted as volumes (via spring.cloud.kubernetes.config.paths/spring.cloud.kubernetes.secrets.paths or using spring.config.import) is:

- name: SPRING_CLOUD_KUBERNETES_CONFIGURATION_WATCHER_REFRESHDELAY
  value: "10000"

This tells how many milliseconds should we wait before firing the event from configuration watcher. This is important because kubernetes documentation says:

When a ConfigMap currently consumed in a volume is updated, projected keys are eventually updated as well.

You need to "match" this eventually part to that value in milliseconds on your cluster.

Spring Cloud Kubernetes Configuration Watcher can send refresh notifications to applications in two ways.

Over HTTP, in which case the application being notified, must have the /refresh actuator endpoint exposed and accessible from within the cluster
Using Spring Cloud Bus, in which case you will need a message broker deployed to your custer for the application to use.

Deployment YAML

Below is a sample deployment YAML you can use to deploy the Kubernetes Configuration Watcher to Kubernetes.

---
apiVersion: v1
kind: List
items:
  - apiVersion: v1
    kind: Service
    metadata:
      labels:
        app: spring-cloud-kubernetes-configuration-watcher
      name: spring-cloud-kubernetes-configuration-watcher
    spec:
      ports:
        - name: http
          port: 8888
          targetPort: 8888
      selector:
        app: spring-cloud-kubernetes-configuration-watcher
      type: ClusterIP
  - apiVersion: v1
    kind: ServiceAccount
    metadata:
      labels:
        app: spring-cloud-kubernetes-configuration-watcher
      name: spring-cloud-kubernetes-configuration-watcher
  - apiVersion: rbac.authorization.k8s.io/v1
    kind: RoleBinding
    metadata:
      labels:
        app: spring-cloud-kubernetes-configuration-watcher
      name: spring-cloud-kubernetes-configuration-watcher:view
    roleRef:
      kind: Role
      apiGroup: rbac.authorization.k8s.io
      name: namespace-reader
    subjects:
      - kind: ServiceAccount
        name: spring-cloud-kubernetes-configuration-watcher
  - apiVersion: rbac.authorization.k8s.io/v1
    kind: Role
    metadata:
      namespace: default
      name: namespace-reader
    rules:
      - apiGroups: ["", "extensions", "apps"]
        resources: ["configmaps", "pods", "services", "endpoints", "secrets"]
        verbs: ["get", "list", "watch"]
  - apiVersion: apps/v1
    kind: Deployment
    metadata:
      name: spring-cloud-kubernetes-configuration-watcher-deployment
    spec:
      selector:
        matchLabels:
          app: spring-cloud-kubernetes-configuration-watcher
      template:
        metadata:
          labels:
            app: spring-cloud-kubernetes-configuration-watcher
        spec:
          serviceAccount: spring-cloud-kubernetes-configuration-watcher
          containers:
          - name: spring-cloud-kubernetes-configuration-watcher
            image: springcloud/spring-cloud-kubernetes-configuration-watcher:3.2.0
            imagePullPolicy: IfNotPresent
            readinessProbe:
              httpGet:
                port: 8888
                path: /actuator/health/readiness
            livenessProbe:
              httpGet:
                port: 8888
                path: /actuator/health/liveness
            ports:
            - containerPort: 8888

The Service Account and associated Role Binding is important for Spring Cloud Kubernetes Configuration to work properly. The controller needs access to read data about ConfigMaps, Pods, Services, Endpoints and Secrets in the Kubernetes cluster.

Monitoring ConfigMaps and Secrets

If a change is made to a ConfigMap or Secret with valid labels (as detailed above), then Spring Cloud Kubernetes Configuration Watcher will take the name of the ConfigMap or Secret and send a notification to the application with that name. This might not be enough for your use-case though, you could for example want to:

bind a config-map to multiple applications, so that a change inside a single configmap triggers a refresh for many services
have profile based sources trigger events for your application

For that reasons there is an addition annotation you could specify:

spring.cloud.kubernetes.configmap.apps or spring.cloud.kubernetes.secret.apps. It takes a String of apps separated by comma, that specifies the names of applications that will receive a notification when changes happen in this secret/configmap.

For example:

kind: ConfigMap
apiVersion: v1
metadata:
  name: example-configmap
  labels:
    spring.cloud.kubernetes.config: "true"
  annotations:
    spring.cloud.kubernetes.configmap.apps: "app-a, app-b"

HTTP Implementation

The HTTP implementation is what is used by default. When this implementation is used, Spring Cloud Kubernetes Configuration Watcher and a change to a ConfigMap or Secret occurs then the HTTP implementation will use the Spring Cloud Kubernetes Discovery Client to fetch all instances of the application which match the name of the ConfigMap or Secret and send an HTTP POST request to the application’s actuator /refresh endpoint. By default, it will send the post request to /actuator/refresh using the port registered in the discovery client.

You can also configure the configuration watcher to call the instances shutdown actuator endpoint. To do this you can set spring.cloud.kubernetes.configuration.watcher.refresh-strategy=shutdown.

Non-Default Management Port and Actuator Path

If the application is using a non-default actuator path and/or using a different port for the management endpoints, the Kubernetes service for the application can add an annotation called boot.spring.io/actuator and set its value to the path and port used by the application. For example

apiVersion: v1
kind: Service
metadata:
  labels:
    app: config-map-demo
  name: config-map-demo
  annotations:
    boot.spring.io/actuator: http://:9090/myactuator/home
spec:
  ports:
    - name: http
      port: 8080
      targetPort: 8080
  selector:
    app: config-map-demo

Another way you can choose to configure the actuator path and/or management port is by setting spring.cloud.kubernetes.configuration.watcher.actuatorPath and spring.cloud.kubernetes.configuration.watcher.actuatorPort.

Messaging Implementation

The messaging implementation can be enabled by setting profile to either bus-amqp (RabbitMQ) or bus-kafka (Kafka) when the Spring Cloud Kubernetes Configuration Watcher application is deployed to Kubernetes. By default, when using the messaging implementation the configuration watcher will send a RefreshRemoteApplicationEvent using Spring Cloud Bus to all application instances. This will cause the application instances to refresh the application’s configuration properties without restarting the instance.

You can also configure the configuration to shut down the application instances in order to refresh the application’s configuration properties. When the application shuts down, Kubernetes will restart the application instance and the new configuration properties will be loaded. To use this strategy set spring.cloud.kubernetes.configuration.watcher.refresh-strategy=shutdown.

Configuring RabbitMQ

When the bus-amqp profile is enabled you will need to configure Spring RabbitMQ to point it to the location of the RabbitMQ instance you would like to use as well as any credentials necessary to authenticate. This can be done by setting the standard Spring RabbitMQ properties, for example

spring:
  rabbitmq:
    username: user
    password: password
    host: rabbitmq

Configuring Kafka

When the bus-kafka profile is enabled you will need to configure Spring Kafka to point it to the location of the Kafka Broker instance you would like to use. This can be done by setting the standard Spring Kafka properties, for example

spring:
  kafka:
    producer:
      bootstrap-servers: localhost:9092