Spring Cloud Kubernetes

Table of Contents

• 1. Why do you need Spring Cloud Kubernetes?
• 2. Starters
• 3. DiscoveryClient for Kubernetes
• 4. Kubernetes native service discovery
• 5. Kubernetes PropertySource implementations
  • 5.1. Using a ConfigMap PropertySource
  • 5.2. Secrets PropertySource
  • 5.3. PropertySource Reload
• 6. Kubernetes Ecosystem Awareness
  • 6.1. Kubernetes Profile Autoconfiguration
  • 6.2. Istio Awareness
• 7. Pod Health Indicator
• 8. Info Contributor
• 9. Leader Election
• 10. LoadBalancer for Kubernetes
• 11. Security Configurations Inside Kubernetes
  • 11.1. Namespace
  • 11.2. Service Account
• 12. Service Registry Implementation
• 13. Spring Cloud Kubernetes Configuration Watcher
  • 13.1. Deployment YAML
  • 13.2. Monitoring ConfigMaps and Secrets
  • 13.3. HTTP Implementation
    • 13.3.1. Non-Default Management Port and Actuator Path
  • 13.4. Messaging Implementation
  • 13.5. Configuring RabbitMQ
  • 13.6. Configuring Kafka
• 14. Examples
• 15. Other Resources
• 16. Configuration properties
• 17. Building
  • 17.1. Basic Compile and Test
  • 17.2. Documentation
  • 17.3. Working with the code
    • 17.3.1. Activate the Spring Maven profile
    • 17.3.2. Importing into eclipse with m2eclipse
    • 17.3.3. Importing into eclipse without m2eclipse
• 18. Contributing
  • 18.1. Sign the Contributor License Agreement
  • 18.2. Code of Conduct
  • 18.3. Code Conventions and Housekeeping
  • 18.4. Checkstyle
    • 18.4.1. Checkstyle configuration
  • 18.5. IDE setup
    • 18.5.1. Intellij IDEA

This reference guide covers how to use Spring Cloud Kubernetes.

1. Why do you need Spring Cloud Kubernetes?

Spring Cloud Kubernetes provides implementations of well known Spring Cloud interfaces allowing developers to build and run Spring Cloud applications on Kubernetes. While this project may be useful to you when building a cloud native application, it is also not a requirement in order to deploy a Spring Boot app on Kubernetes. If you are just getting started in your journey to running your Spring Boot app on Kubernetes you can accomplish a lot with nothing more than a basic Spring Boot app and Kubernetes itself. To learn more, you can get started by reading the Spring Boot reference documentation for deploying to Kubernetes and also working through the workshop material Spring and Kubernetes.

2. Starters

Starters are convenient dependency descriptors you can include in your application. Include a starter to get the dependencies and Spring Boot auto-configuration for a feature set. Starters that begin with spring-cloud-starter-kubernetes-fabric8 provide implementations using the Fabric8 Kubernetes Java Client. Starters that begin with spring-cloud-starter-kubernetes-client provide implementations using the Kubernetes Java Client.

Starter	Features
Fabric8 Dependency <dependency> <groupId>org.springframework.cloud</groupId> <artifactId>spring-cloud-starter-kubernetes-fabric8</artifactId> </dependency> Kubernetes Client Dependency <dependency> <groupId>org.springframework.cloud</groupId> <artifactId>spring-cloud-starter-kubernetes-client</artifactId> </dependency>	Discovery Client implementation that resolves service names to Kubernetes Services.
Fabric8 Dependency <dependency> <groupId>org.springframework.cloud</groupId> <artifactId>spring-cloud-starter-kubernetes-fabric8-config</artifactId> </dependency> Kubernetes Client Dependency <dependency> <groupId>org.springframework.cloud</groupId> <artifactId>spring-cloud-starter-kubernetes-client-config</artifactId> </dependency>	Load application properties from Kubernetes ConfigMaps and Secrets. Reload application properties when a ConfigMap or Secret changes.
Fabric8 Dependency <dependency> <groupId>org.springframework.cloud</groupId> <artifactId>spring-cloud-starter-kubernetes-fabric8-all</artifactId> </dependency> Kubernetes Client Dependency <dependency> <groupId>org.springframework.cloud</groupId> <artifactId>spring-cloud-starter-kubernetes-client-all</artifactId> </dependency>	All Spring Cloud Kubernetes features.

3. DiscoveryClient for Kubernetes

This project provides an implementation of Discovery Client for Kubernetes. This client lets you query Kubernetes endpoints (see services) by name. A service is typically exposed by the Kubernetes API server as a collection of endpoints that represent http and https addresses and that a client can access from a Spring Boot application running as a pod.

This is something that you get for free by adding the following dependency inside your project:

Fabric8 Kubernetes Client

<dependency>
    <groupId>org.springframework.cloud</groupId>
    <artifactId>spring-cloud-starter-kubernetes-fabric8</artifactId>
</dependency>

Kubernetes Java Client

<dependency>
    <groupId>org.springframework.cloud</groupId>
    <artifactId>spring-cloud-starter-kubernetes-client</artifactId>
</dependency>

To enable loading of the DiscoveryClient, add @EnableDiscoveryClient to the according configuration or application class, as the following example shows:

@SpringBootApplication
@EnableDiscoveryClient
public class Application {
  public static void main(String[] args) {
    SpringApplication.run(Application.class, args);
  }
}

Then you can inject the client in your code simply by autowiring it, as the following example shows:

@Autowired
private DiscoveryClient discoveryClient;

You can choose to enable DiscoveryClient from all namespaces by setting the following property in application.properties:

spring.cloud.kubernetes.discovery.all-namespaces=true

To discover service endpoint addresses that are not marked as "ready" by the kubernetes api server, you can set the following property in application.properties (default: false):

spring.cloud.kubernetes.discovery.include-not-ready-addresses=true

Note	This might be useful when discovering services for monitoring purposes, and would enable inspecting the /health endpoint of not-ready service instances.

If your service exposes multiple ports, you will need to specify which port the DiscoveryClient should use. The DiscoveryClient will choose the port using the following logic.

1. If the service has a label primary-port-name it will use the port with the name specified in the label’s value.

2. If no label is present, then the port name specified in spring.cloud.kubernetes.discovery.primary-port-name will be used.

3. If neither of the above are specified it will use the port named https.

4. If none of the above conditions are met it will use the port named http.

5. As a last resort it wil pick the first port in the list of ports.

Warning

The last option may result in non-deterministic behaviour. Please make sure to configure your service and/or application accordingly.

By default all of the ports and their names will be added to the metadata of the ServiceInstance.

If, for any reason, you need to disable the DiscoveryClient, you can set the following property in application.properties:

spring.cloud.kubernetes.discovery.enabled=false

Some Spring Cloud components use the DiscoveryClient in order to obtain information about the local service instance. For this to work, you need to align the Kubernetes service name with the spring.application.name property.

Note	spring.application.name has no effect as far as the name registered for the application within Kubernetes

Spring Cloud Kubernetes can also watch the Kubernetes service catalog for changes and update the DiscoveryClient implementation accordingly. In order to enable this functionality you need to add @EnableScheduling on a configuration class in your application.

4. Kubernetes native service discovery

Kubernetes itself is capable of (server side) service discovery (see: kubernetes.io/docs/concepts/services-networking/service/#discovering-services). Using native kubernetes service discovery ensures compatibility with additional tooling, such as Istio (istio.io), a service mesh that is capable of load balancing, circuit breaker, failover, and much more.

The caller service then need only refer to names resolvable in a particular Kubernetes cluster. A simple implementation might use a spring RestTemplate that refers to a fully qualified domain name (FQDN), such as {service-name}.{namespace}.svc.{cluster}.local:{service-port}.

Additionally, you can use Hystrix for:

• Circuit breaker implementation on the caller side, by annotating the spring boot application class with @EnableCircuitBreaker

• Fallback functionality, by annotating the respective method with @HystrixCommand(fallbackMethod=

5. Kubernetes PropertySource implementations

The most common approach to configuring your Spring Boot application is to create an application.properties or application.yaml or an application-profile.properties or application-profile.yaml file that contains key-value pairs that provide customization values to your application or Spring Boot starters. You can override these properties by specifying system properties or environment variables.

5.1. Using a ConfigMap PropertySource

Kubernetes provides a resource named ConfigMap to externalize the parameters to pass to your application in the form of key-value pairs or embedded application.properties or application.yaml files. The Spring Cloud Kubernetes Config project makes Kubernetes ConfigMap instances available during application bootstrapping and triggers hot reloading of beans or Spring context when changes are detected on observed ConfigMap instances.

The default behavior is to create a Fabric8ConfigMapPropertySource based on a Kubernetes ConfigMap that has a metadata.name value of either the name of your Spring application (as defined by its spring.application.name property) or a custom name defined within the bootstrap.properties file under the following key: spring.cloud.kubernetes.config.name.

However, more advanced configuration is possible where you can use multiple ConfigMap instances. The spring.cloud.kubernetes.config.sources list makes this possible. For example, you could define the following ConfigMap instances:

spring:
  application:
    name: cloud-k8s-app
  cloud:
    kubernetes:
      config:
        name: default-name
        namespace: default-namespace
        sources:
         # Spring Cloud Kubernetes looks up a ConfigMap named c1 in namespace default-namespace
         - name: c1
         # Spring Cloud Kubernetes looks up a ConfigMap named default-name in whatever namespace n2
         - namespace: n2
         # Spring Cloud Kubernetes looks up a ConfigMap named c3 in namespace n3
         - namespace: n3
           name: c3

In the preceding example, if spring.cloud.kubernetes.config.namespace had not been set, the ConfigMap named c1 would be looked up in the namespace that the application runs.

Any matching ConfigMap that is found is processed as follows:

• Apply individual configuration properties.

• Apply as yaml the content of any property named application.yaml.

• Apply as a properties file the content of any property named application.properties.

The single exception to the aforementioned flow is when the ConfigMap contains a single key that indicates the file is a YAML or properties file. In that case, the name of the key does NOT have to be application.yaml or application.properties (it can be anything) and the value of the property is treated correctly. This features facilitates the use case where the ConfigMap was created by using something like the following:

kubectl create configmap game-config --from-file=/path/to/app-config.yaml

Assume that we have a Spring Boot application named demo that uses the following properties to read its thread pool configuration.

• pool.size.core

• pool.size.maximum

This can be externalized to config map in yaml format as follows:

kind: ConfigMap
apiVersion: v1
metadata:
  name: demo
data:
  pool.size.core: 1
  pool.size.max: 16

Individual properties work fine for most cases. However, sometimes, embedded yaml is more convenient. In this case, we use a single property named application.yaml to embed our yaml, as follows:

kind: ConfigMap
apiVersion: v1
metadata:
  name: demo
data:
  application.yaml: |-
    pool:
      size:
        core: 1
        max:16

The following example also works:

kind: ConfigMap
apiVersion: v1
metadata:
  name: demo
data:
  custom-name.yaml: |-
    pool:
      size:
        core: 1
        max:16

You can also configure Spring Boot applications differently depending on active profiles that are merged together when the ConfigMap is read. You can provide different property values for different profiles by using an application.properties or application.yaml property, specifying profile-specific values, each in their own document (indicated by the --- sequence), as follows:

kind: ConfigMap
apiVersion: v1
metadata:
  name: demo
data:
  application.yml: |-
    greeting:
      message: Say Hello to the World
    farewell:
      message: Say Goodbye
    ---
    spring:
      profiles: development
    greeting:
      message: Say Hello to the Developers
    farewell:
      message: Say Goodbye to the Developers
    ---
    spring:
      profiles: production
    greeting:
      message: Say Hello to the Ops

In the preceding case, the configuration loaded into your Spring Application with the development profile is as follows:

  greeting:
    message: Say Hello to the Developers
  farewell:
    message: Say Goodbye to the Developers

However, if the production profile is active, the configuration becomes:

  greeting:
    message: Say Hello to the Ops
  farewell:
    message: Say Goodbye

If both profiles are active, the property that appears last within the ConfigMap overwrites any preceding values.

Another option is to create a different config map per profile and spring boot will automatically fetch it based on active profiles

kind: ConfigMap
apiVersion: v1
metadata:
  name: demo
data:
  application.yml: |-
    greeting:
      message: Say Hello to the World
    farewell:
      message: Say Goodbye

kind: ConfigMap
apiVersion: v1
metadata:
  name: demo-development
data:
  application.yml: |-
    spring:
      profiles: development
    greeting:
      message: Say Hello to the Developers
    farewell:
      message: Say Goodbye to the Developers

kind: ConfigMap
apiVersion: v1
metadata:
  name: demo-production
data:
  application.yml: |-
    spring:
      profiles: production
    greeting:
      message: Say Hello to the Ops
    farewell:
      message: Say Goodbye

To tell Spring Boot which profile should be enabled at bootstrap, you can pass SPRING_PROFILES_ACTIVE environment variable. To do so, you can launch your Spring Boot application with an environment variable that you can define it in the PodSpec at the container specification. Deployment resource file, as follows:

apiVersion: apps/v1
kind: Deployment
metadata:
  name: deployment-name
  labels:
    app: deployment-name
spec:
  replicas: 1
  selector:
    matchLabels:
      app: deployment-name
  template:
    metadata:
      labels:
        app: deployment-name
    spec:
        containers:
        - name: container-name
          image: your-image
          env:
          - name: SPRING_PROFILES_ACTIVE
            value: "development"

Note	You should check the security configuration section. To access config maps from inside a pod you need to have the correct Kubernetes service accounts, roles and role bindings.

Another option for using ConfigMap instances is to mount them into the Pod by running the Spring Cloud Kubernetes application and having Spring Cloud Kubernetes read them from the file system. This behavior is controlled by the spring.cloud.kubernetes.config.paths property. You can use it in addition to or instead of the mechanism described earlier. You can specify multiple (exact) file paths in spring.cloud.kubernetes.config.paths by using the , delimiter.

Note	You have to provide the full exact path to each property file, because directories are not being recursively parsed.

Note	If you use spring.cloud.kubernetes.config.paths or spring.cloud.kubernetes.secrets.path the automatic reload functionality will not work. You will need to make a POST request to the /actuator/refresh endpoint or restart/redeploy the application.

Table 1. Properties:

Name	Type	Default	Description
spring.cloud.kubernetes.config.enabled	Boolean	true	Enable ConfigMaps PropertySource
spring.cloud.kubernetes.config.name	String	${spring.application.name}	Sets the name of ConfigMap to look up
spring.cloud.kubernetes.config.namespace	String	Client namespace	Sets the Kubernetes namespace where to lookup
spring.cloud.kubernetes.config.paths	List	null	Sets the paths where ConfigMap instances are mounted
spring.cloud.kubernetes.config.enableApi	Boolean	true	Enable or disable consuming ConfigMap instances through APIs

5.2. Secrets PropertySource

Kubernetes has the notion of Secrets for storing sensitive data such as passwords, OAuth tokens, and so on. This project provides integration with Secrets to make secrets accessible by Spring Boot applications. You can explicitly enable or disable This feature by setting the spring.cloud.kubernetes.secrets.enabled property.

When enabled, the Fabric8SecretsPropertySource looks up Kubernetes for Secrets from the following sources:

1. Reading recursively from secrets mounts

2. Named after the application (as defined by spring.application.name)

3. Matching some labels

Note:

By default, consuming Secrets through the API (points 2 and 3 above) is not enabled for security reasons. The permission 'list' on secrets allows clients to inspect secrets values in the specified namespace. Further, we recommend that containers share secrets through mounted volumes.

If you enable consuming Secrets through the API, we recommend that you limit access to Secrets by using an authorization policy, such as RBAC. For more information about risks and best practices when consuming Secrets through the API refer to this doc.

If the secrets are found, their data is made available to the application.

Assume that we have a spring boot application named demo that uses properties to read its database configuration. We can create a Kubernetes secret by using the following command:

kubectl create secret generic db-secret --from-literal=username=user --from-literal=password=p455w0rd

The preceding command would create the following secret (which you can see by using kubectl get secrets db-secret -o yaml):

apiVersion: v1
data:
  password: cDQ1NXcwcmQ=
  username: dXNlcg==
kind: Secret
metadata:
  creationTimestamp: 2017-07-04T09:15:57Z
  name: db-secret
  namespace: default
  resourceVersion: "357496"
  selfLink: /api/v1/namespaces/default/secrets/db-secret
  uid: 63c89263-6099-11e7-b3da-76d6186905a8
type: Opaque

Note that the data contains Base64-encoded versions of the literal provided by the create command.

Your application can then use this secret — for example, by exporting the secret’s value as environment variables:

apiVersion: v1
kind: Deployment
metadata:
  name: ${project.artifactId}
spec:
   template:
     spec:
       containers:
         - env:
            - name: DB_USERNAME
              valueFrom:
                 secretKeyRef:
                   name: db-secret
                   key: username
            - name: DB_PASSWORD
              valueFrom:
                 secretKeyRef:
                   name: db-secret
                   key: password

You can select the Secrets to consume in a number of ways:

1. By listing the directories where secrets are mapped:

   -Dspring.cloud.kubernetes.secrets.paths=/etc/secrets/db-secret,etc/secrets/postgresql

   If you have all the secrets mapped to a common root, you can set them like:

   -Dspring.cloud.kubernetes.secrets.paths=/etc/secrets

2. By setting a named secret:

   -Dspring.cloud.kubernetes.secrets.name=db-secret

3. By defining a list of labels:

   -Dspring.cloud.kubernetes.secrets.labels.broker=activemq
   -Dspring.cloud.kubernetes.secrets.labels.db=postgresql

As the case with ConfigMap, more advanced configuration is also possible where you can use multiple Secret instances. The spring.cloud.kubernetes.secrets.sources list makes this possible. For example, you could define the following Secret instances:

spring:
  application:
    name: cloud-k8s-app
  cloud:
    kubernetes:
      secrets:
        name: default-name
        namespace: default-namespace
        sources:
         # Spring Cloud Kubernetes looks up a Secret named s1 in namespace default-namespace
         - name: s1
         # Spring Cloud Kubernetes looks up a Secret named default-name in whatever namespace n2
         - namespace: n2
         # Spring Cloud Kubernetes looks up a Secret named s3 in namespace n3
         - namespace: n3
           name: s3

In the preceding example, if spring.cloud.kubernetes.secrets.namespace had not been set, the Secret named s1 would be looked up in the namespace that the application runs.

Table 2. Properties:

Name	Type	Default	Description
spring.cloud.kubernetes.secrets.enabled	Boolean	true	Enable Secrets PropertySource
spring.cloud.kubernetes.secrets.name	String	${spring.application.name}	Sets the name of the secret to look up
spring.cloud.kubernetes.secrets.namespace	String	Client namespace	Sets the Kubernetes namespace where to look up
spring.cloud.kubernetes.secrets.labels	Map	null	Sets the labels used to lookup secrets
spring.cloud.kubernetes.secrets.paths	List	null	Sets the paths where secrets are mounted (example 1)
spring.cloud.kubernetes.secrets.enableApi	Boolean	false	Enables or disables consuming secrets through APIs (examples 2 and 3)

Notes:

• The spring.cloud.kubernetes.secrets.labels property behaves as defined by Map-based binding.

• The spring.cloud.kubernetes.secrets.paths property behaves as defined by Collection-based binding.

• Access to secrets through the API may be restricted for security reasons. The preferred way is to mount secrets to the Pod.

You can find an example of an application that uses secrets (though it has not been updated to use the new spring-cloud-kubernetes project) at spring-boot-camel-config

5.3. PropertySource Reload

Warning

This functionality has been deprecated in the 2020.0 release. Please see the Spring Cloud Kubernetes Configuration Watcher controller for an alternative way to achieve the same functionality.

Some applications may need to detect changes on external property sources and update their internal status to reflect the new configuration. The reload feature of Spring Cloud Kubernetes is able to trigger an application reload when a related ConfigMap or Secret changes.

By default, this feature is disabled. You can enable it by using the spring.cloud.kubernetes.reload.enabled=true configuration property (for example, in the application.properties file).

The following levels of reload are supported (by setting the spring.cloud.kubernetes.reload.strategy property):

• refresh (default): Only configuration beans annotated with @ConfigurationProperties or @RefreshScope are reloaded. This reload level leverages the refresh feature of Spring Cloud Context.

• restart_context: the whole Spring ApplicationContext is gracefully restarted. Beans are recreated with the new configuration. In order for the restart context functionality to work properly you must enable and expose the restart actuator endpoint

management:
  endpoint:
    restart:
      enabled: true
  endpoints:
    web:
      exposure:
        include: restart

• shutdown: the Spring ApplicationContext is shut down to activate a restart of the container. When you use this level, make sure that the lifecycle of all non-daemon threads is bound to the ApplicationContext and that a replication controller or replica set is configured to restart the pod.

Assuming that the reload feature is enabled with default settings (refresh mode), the following bean is refreshed when the config map changes:

@Configuration
@ConfigurationProperties(prefix = "bean")
public class MyConfig {

    private String message = "a message that can be changed live";

    // getter and setters

}

To see that changes effectively happen, you can create another bean that prints the message periodically, as follows

@Component
public class MyBean {

    @Autowired
    private MyConfig config;

    @Scheduled(fixedDelay = 5000)
    public void hello() {
        System.out.println("The message is: " + config.getMessage());
    }
}

You can change the message printed by the application by using a ConfigMap, as follows:

apiVersion: v1
kind: ConfigMap
metadata:
  name: reload-example
data:
  application.properties: |-
    bean.message=Hello World!

Any change to the property named bean.message in the ConfigMap associated with the pod is reflected in the output. More generally speaking, changes associated to properties prefixed with the value defined by the prefix field of the @ConfigurationProperties annotation are detected and reflected in the application. Associating a ConfigMap with a pod is explained earlier in this chapter.

The full example is available in spring-cloud-kubernetes-reload-example.

The reload feature supports two operating modes: * Event (default): Watches for changes in config maps or secrets by using the Kubernetes API (web socket). Any event produces a re-check on the configuration and, in case of changes, a reload. The view role on the service account is required in order to listen for config map changes. A higher level role (such as edit) is required for secrets (by default, secrets are not monitored). * Polling: Periodically re-creates the configuration from config maps and secrets to see if it has changed. You can configure the polling period by using the spring.cloud.kubernetes.reload.period property and defaults to 15 seconds. It requires the same role as the monitored property source. This means, for example, that using polling on file-mounted secret sources does not require particular privileges.

Table 3. Properties:

Name	Type	Default	Description
spring.cloud.kubernetes.reload.enabled	Boolean	false	Enables monitoring of property sources and configuration reload
spring.cloud.kubernetes.reload.monitoring-config-maps	Boolean	true	Allow monitoring changes in config maps
spring.cloud.kubernetes.reload.monitoring-secrets	Boolean	false	Allow monitoring changes in secrets
spring.cloud.kubernetes.reload.strategy	Enum	refresh	The strategy to use when firing a reload (refresh, restart_context, or shutdown)
spring.cloud.kubernetes.reload.mode	Enum	event	Specifies how to listen for changes in property sources (event or polling)
spring.cloud.kubernetes.reload.period	Duration	15s	The period for verifying changes when using the polling strategy

Notes: * You should not use properties under spring.cloud.kubernetes.reload in config maps or secrets. Changing such properties at runtime may lead to unexpected results. * Deleting a property or the whole config map does not restore the original state of the beans when you use the refresh level.

6. Kubernetes Ecosystem Awareness

All of the features described earlier in this guide work equally well, regardless of whether your application is running inside Kubernetes. This is really helpful for development and troubleshooting. From a development point of view, this lets you start your Spring Boot application and debug one of the modules that is part of this project. You need not deploy it in Kubernetes, as the code of the project relies on the Fabric8 Kubernetes Java client, which is a fluent DSL that can communicate by using http protocol to the REST API of the Kubernetes Server.

To disable the integration with Kubernetes you can set spring.cloud.kubernetes.enabled to false. Please be aware that when spring-cloud-kubernetes-config is on the classpath, spring.cloud.kubernetes.enabled should be set in bootstrap.{properties|yml} (or the profile specific one) otherwise it should be in application.{properties|yml} (or the profile specific one). Also note that these properties: spring.cloud.kubernetes.config.enabled and spring.cloud.kubernetes.secrets.enabled only take effect when set in bootstrap.{properties|yml}

6.1. Kubernetes Profile Autoconfiguration

When the application runs as a pod inside Kubernetes, a Spring profile named kubernetes automatically gets activated. This lets you customize the configuration, to define beans that are applied when the Spring Boot application is deployed within the Kubernetes platform (for example, different development and production configuration).

6.2. Istio Awareness

When you include the spring-cloud-kubernetes-fabric8-istio module in the application classpath, a new profile is added to the application, provided the application is running inside a Kubernetes Cluster with Istio installed. You can then use spring @Profile("istio") annotations in your Beans and @Configuration classes.

The Istio awareness module uses me.snowdrop:istio-client to interact with Istio APIs, letting us discover traffic rules, circuit breakers, and so on, making it easy for our Spring Boot applications to consume this data to dynamically configure themselves according to the environment.

7. Pod Health Indicator

Spring Boot uses HealthIndicator to expose info about the health of an application. That makes it really useful for exposing health-related information to the user and makes it a good fit for use as readiness probes.

The Kubernetes health indicator (which is part of the core module) exposes the following info:

• Pod name, IP address, namespace, service account, node name, and its IP address

• A flag that indicates whether the Spring Boot application is internal or external to Kubernetes

8. Info Contributor

Spring Cloud Kubernetes includes an InfoContributor which adds Pod information to Spring Boot’s /info Acturator endpoint.

You can disable this InfoContributor by setting management.info.kubernetes.enabled to false in bootstrap.[properties | yaml].

9. Leader Election

The Spring Cloud Kubernetes leader election mechanism implements the leader election API of Spring Integration using a Kubernetes ConfigMap.

Multiple application instances compete for leadership, but leadership will only be granted to one. When granted leadership, a leader application receives an OnGrantedEvent application event with leadership Context. Applications periodically attempt to gain leadership, with leadership granted to the first caller. A leader will remain a leader until either it is removed from the cluster, or it yields its leadership. When leadership removal occurs, the previous leader receives OnRevokedEvent application event. After removal, any instances in the cluster may become the new leader, including the old leader.

To include it in your project, add the following dependency.

Fabric8 Leader Implementation

<dependency>
    <groupId>org.springframework.cloud</groupId>
    <artifactId>spring-cloud-kubernetes-fabric8-leader</artifactId>
</dependency>

To specify the name of the configmap used for leader election use the following property.

spring.cloud.kubernetes.leader.config-map-name=leader

10. LoadBalancer for Kubernetes

This project includes Spring Cloud Load Balancer for load balancing based on Kubernetes Endpoints and provides implementation of load balancer based on Kubernetes Service. To include it to your project add the following dependency.

Fabric8 Implementation

<dependency>
    <groupId>org.springframework.cloud</groupId>
    <artifactId>spring-cloud-starter-kubernetes-fabric8-loadbalancer</artifactId>
</dependency>

Kubernetes Java Client Implementation

<dependency>
    <groupId>org.springframework.cloud</groupId>
    <artifactId>spring-cloud-starter-kubernetes-client-loadbalancer</artifactId>
</dependency>

To enable load balancing based on Kubernetes Service name use the following property. Then load balancer would try to call application using address, for example service-a.default.svc.cluster.local

spring.cloud.kubernetes.loadbalancer.mode=SERVICE

To enabled load balancing across all namespaces use the following property. Property from spring-cloud-kubernetes-discovery module is respected.

spring.cloud.kubernetes.discovery.all-namespaces=true

11. Security Configurations Inside Kubernetes

11.1. Namespace

Most of the components provided in this project need to know the namespace. For Kubernetes (1.3+), the namespace is made available to the pod as part of the service account secret and is automatically detected by the client. For earlier versions, it needs to be specified as an environment variable to the pod. A quick way to do this is as follows:

      env:
      - name: "KUBERNETES_NAMESPACE"
        valueFrom:
          fieldRef:
            fieldPath: "metadata.namespace"

11.2. Service Account

For distributions of Kubernetes that support more fine-grained role-based access within the cluster, you need to make sure a pod that runs with spring-cloud-kubernetes has access to the Kubernetes API. For any service accounts you assign to a deployment or pod, you need to make sure they have the correct roles.

Depending on the requirements, you’ll need get, list and watch permission on the following resources:

Table 4. Kubernetes Resource Permissions

Dependency	Resources
spring-cloud-starter-kubernetes-fabric8	pods, services, endpoints
spring-cloud-starter-kubernetes-fabric8-config	configmaps, secrets
spring-cloud-starter-kubernetes-client	pods, services, endpoints
spring-cloud-starter-kubernetes-client-config	configmaps, secrets

For development purposes, you can add cluster-reader permissions to your default service account. On a production system you’ll likely want to provide more granular permissions.

The following Role and RoleBinding are an example for namespaced permissions for the default account:

kind: Role
apiVersion: rbac.authorization.k8s.io/v1
metadata:
  namespace: YOUR-NAME-SPACE
  name: namespace-reader
rules:
  - apiGroups: ["", "extensions", "apps"]
    resources: ["configmaps", "pods", "services", "endpoints", "secrets"]
    verbs: ["get", "list", "watch"]

---

kind: RoleBinding
apiVersion: rbac.authorization.k8s.io/v1
metadata:
  name: namespace-reader-binding
  namespace: YOUR-NAME-SPACE
subjects:
- kind: ServiceAccount
  name: default
  apiGroup: ""
roleRef:
  kind: Role
  name: namespace-reader
  apiGroup: ""

12. Service Registry Implementation

In Kubernetes service registration is controlled by the platform, the application itself does not control registration as it may do in other platforms. For this reason using spring.cloud.service-registry.auto-registration.enabled or setting @EnableDiscoveryClient(autoRegister=false) will have no effect in Spring Cloud Kubernetes.

13. 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.

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

1. Over HTTP in which case the application being notified must of the /refresh actuator endpoint exposed and accessible from within the cluster

2. Using Spring Cloud Bus, in which case you will need a message broker deployed to your custer for the application to use.

13.1. 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:2.0.1-SNAPSHOT
            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.

13.2. Monitoring ConfigMaps and Secrets

Spring Cloud Kubernetes Configuration Watcher will react to changes in ConfigMaps with a label of spring.cloud.kubernetes.config with the value true or any Secret with a label of spring.cloud.kubernetes.secret with the value true. If the ConfigMap or Secret does not have either of those labels or the values of those labels is not true then any changes will be ignored.

The labels Spring Cloud Kubernetes Configuration Watcher looks for on ConfigMaps and Secrets can be changed by setting spring.cloud.kubernetes.configuration.watcher.configLabel and spring.cloud.kubernetes.configuration.watcher.secretLabel respectively.

If a change is made to a ConfigMap or Secret with valid labels 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.

13.3. 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.

13.3.1. 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.

13.4. 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.

13.5. 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

13.6. 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

14. Examples

Spring Cloud Kubernetes tries to make it transparent for your applications to consume Kubernetes Native Services by following the Spring Cloud interfaces.

In your applications, you need to add the spring-cloud-kubernetes-discovery dependency to your classpath and remove any other dependency that contains a DiscoveryClient implementation (that is, a Eureka discovery client). The same applies for PropertySourceLocator, where you need to add to the classpath the spring-cloud-kubernetes-config and remove any other dependency that contains a PropertySourceLocator implementation (that is, a configuration server client).

The following projects highlight the usage of these dependencies and demonstrate how you can use these libraries from any Spring Boot application:

• Spring Cloud Kubernetes Examples: the ones located inside this repository.

• Spring Cloud Kubernetes Full Example: Minions and Boss

  • Minion

  • Boss

• Spring Cloud Kubernetes Full Example: SpringOne Platform Tickets Service

• Spring Cloud Gateway with Spring Cloud Kubernetes Discovery and Config

• Spring Boot Admin with Spring Cloud Kubernetes Discovery and Config

15. Other Resources

This section lists other resources, such as presentations (slides) and videos about Spring Cloud Kubernetes.

• S1P Spring Cloud on PKS

• Spring Cloud, Docker, Kubernetes → London Java Community July 2018

Please feel free to submit other resources through pull requests to this repository.

16. Configuration properties

To see the list of all Kubernetes related configuration properties please check the Appendix page.

17. Building

17.1. Basic Compile and Test

To build the source you will need to install JDK 1.8.

Spring Cloud uses Maven for most build-related activities, and you should be able to get off the ground quite quickly by cloning the project you are interested in and typing

$ ./mvnw install

Note	You can also install Maven (>=3.3.3) yourself and run the mvn command in place of ./mvnw in the examples below. If you do that you also might need to add -P spring if your local Maven settings do not contain repository declarations for spring pre-release artifacts.

Note

Be aware that you might need to increase the amount of memory available to Maven by setting a MAVEN_OPTS environment variable with a value like -Xmx512m -XX:MaxPermSize=128m. We try to cover this in the .mvn configuration, so if you find you have to do it to make a build succeed, please raise a ticket to get the settings added to source control.

For hints on how to build the project look in .travis.yml if there is one. There should be a "script" and maybe "install" command. Also look at the "services" section to see if any services need to be running locally (e.g. mongo or rabbit). Ignore the git-related bits that you might find in "before_install" since they’re related to setting git credentials and you already have those.

The projects that require middleware generally include a docker-compose.yml, so consider using Docker Compose to run the middeware servers in Docker containers. See the README in the scripts demo repository for specific instructions about the common cases of mongo, rabbit and redis.

Note	If all else fails, build with the command from .travis.yml (usually ./mvnw install).

17.2. Documentation

The spring-cloud-build module has a "docs" profile, and if you switch that on it will try to build asciidoc sources from src/main/asciidoc. As part of that process it will look for a README.adoc and process it by loading all the includes, but not parsing or rendering it, just copying it to ${main.basedir} (defaults to ${basedir}, i.e. the root of the project). If there are any changes in the README it will then show up after a Maven build as a modified file in the correct place. Just commit it and push the change.

17.3. Working with the code

If you don’t have an IDE preference we would recommend that you use Spring Tools Suite or Eclipse when working with the code. We use the m2eclipse eclipse plugin for maven support. Other IDEs and tools should also work without issue as long as they use Maven 3.3.3 or better.

17.3.1. Activate the Spring Maven profile

Spring Cloud projects require the 'spring' Maven profile to be activated to resolve the spring milestone and snapshot repositories. Use your preferred IDE to set this profile to be active, or you may experience build errors.

17.3.2. Importing into eclipse with m2eclipse

We recommend the m2eclipse eclipse plugin when working with eclipse. If you don’t already have m2eclipse installed it is available from the "eclipse marketplace".

Note

Older versions of m2e do not support Maven 3.3, so once the projects are imported into Eclipse you will also need to tell m2eclipse to use the right profile for the projects. If you see many different errors related to the POMs in the projects, check that you have an up to date installation. If you can’t upgrade m2e, add the "spring" profile to your settings.xml. Alternatively you can copy the repository settings from the "spring" profile of the parent pom into your settings.xml.

17.3.3. Importing into eclipse without m2eclipse

If you prefer not to use m2eclipse you can generate eclipse project metadata using the following command:

$ ./mvnw eclipse:eclipse

The generated eclipse projects can be imported by selecting import existing projects from the file menu.

18. Contributing

Spring Cloud is released under the non-restrictive Apache 2.0 license, and follows a very standard Github development process, using Github tracker for issues and merging pull requests into master. If you want to contribute even something trivial please do not hesitate, but follow the guidelines below.

18.1. Sign the Contributor License Agreement

Before we accept a non-trivial patch or pull request we will need you to sign the Contributor License Agreement. Signing the contributor’s agreement does not grant anyone commit rights to the main repository, but it does mean that we can accept your contributions, and you will get an author credit if we do. Active contributors might be asked to join the core team, and given the ability to merge pull requests.

18.2. Code of Conduct

This project adheres to the Contributor Covenant code of conduct. By participating, you are expected to uphold this code. Please report unacceptable behavior to spring-code-of-conduct@pivotal.io.

18.3. Code Conventions and Housekeeping

None of these is essential for a pull request, but they will all help. They can also be added after the original pull request but before a merge.

• Use the Spring Framework code format conventions. If you use Eclipse you can import formatter settings using the eclipse-code-formatter.xml file from the Spring Cloud Build project. If using IntelliJ, you can use the Eclipse Code Formatter Plugin to import the same file.

• Make sure all new .java files to have a simple Javadoc class comment with at least an @author tag identifying you, and preferably at least a paragraph on what the class is for.

• Add the ASF license header comment to all new .java files (copy from existing files in the project)

• Add yourself as an @author to the .java files that you modify substantially (more than cosmetic changes).

• Add some Javadocs and, if you change the namespace, some XSD doc elements.

• A few unit tests would help a lot as well — someone has to do it.

• If no-one else is using your branch, please rebase it against the current master (or other target branch in the main project).

• When writing a commit message please follow these conventions, if you are fixing an existing issue please add Fixes gh-XXXX at the end of the commit message (where XXXX is the issue number).

18.4. Checkstyle

Spring Cloud Build comes with a set of checkstyle rules. You can find them in the spring-cloud-build-tools module. The most notable files under the module are:

spring-cloud-build-tools/

└── src
    ├── checkstyle
    │   └── checkstyle-suppressions.xml (3)
    └── main
        └── resources
            ├── checkstyle-header.txt (2)
            └── checkstyle.xml (1)

1. Default Checkstyle rules

2. File header setup

3. Default suppression rules

18.4.1. Checkstyle configuration

Checkstyle rules are disabled by default. To add checkstyle to your project just define the following properties and plugins.

pom.xml

<properties>
<maven-checkstyle-plugin.failsOnError>true</maven-checkstyle-plugin.failsOnError> (1)
        <maven-checkstyle-plugin.failsOnViolation>true
        </maven-checkstyle-plugin.failsOnViolation> (2)
        <maven-checkstyle-plugin.includeTestSourceDirectory>true
        </maven-checkstyle-plugin.includeTestSourceDirectory> (3)
</properties>

<build>
        <plugins>
            <plugin> (4)
                <groupId>io.spring.javaformat</groupId>
                <artifactId>spring-javaformat-maven-plugin</artifactId>
            </plugin>
            <plugin> (5)
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-checkstyle-plugin</artifactId>
            </plugin>
        </plugins>

    <reporting>
        <plugins>
            <plugin> (5)
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-checkstyle-plugin</artifactId>
            </plugin>
        </plugins>
    </reporting>
</build>

1. Fails the build upon Checkstyle errors

2. Fails the build upon Checkstyle violations

3. Checkstyle analyzes also the test sources

4. Add the Spring Java Format plugin that will reformat your code to pass most of the Checkstyle formatting rules

5. Add checkstyle plugin to your build and reporting phases

If you need to suppress some rules (e.g. line length needs to be longer), then it’s enough for you to define a file under ${project.root}/src/checkstyle/checkstyle-suppressions.xml with your suppressions. Example:

projectRoot/src/checkstyle/checkstyle-suppresions.xml

<?xml version="1.0"?>
<!DOCTYPE suppressions PUBLIC
        "-//Puppy Crawl//DTD Suppressions 1.1//EN"
        "https://www.puppycrawl.com/dtds/suppressions_1_1.dtd">
<suppressions>
    <suppress files=".*ConfigServerApplication\.java" checks="HideUtilityClassConstructor"/>
    <suppress files=".*ConfigClientWatch\.java" checks="LineLengthCheck"/>
</suppressions>

It’s advisable to copy the ${spring-cloud-build.rootFolder}/.editorconfig and ${spring-cloud-build.rootFolder}/.springformat to your project. That way, some default formatting rules will be applied. You can do so by running this script:

$ curl https://raw.githubusercontent.com/spring-cloud/spring-cloud-build/master/.editorconfig -o .editorconfig
$ touch .springformat

18.5. IDE setup

18.5.1. Intellij IDEA

In order to setup Intellij you should import our coding conventions, inspection profiles and set up the checkstyle plugin. The following files can be found in the Spring Cloud Build project.

spring-cloud-build-tools/

└── src
    ├── checkstyle
    │   └── checkstyle-suppressions.xml (3)
    └── main
        └── resources
            ├── checkstyle-header.txt (2)
            ├── checkstyle.xml (1)
            └── intellij
                ├── Intellij_Project_Defaults.xml (4)
                └── Intellij_Spring_Boot_Java_Conventions.xml (5)

1. Default Checkstyle rules

2. File header setup

3. Default suppression rules

4. Project defaults for Intellij that apply most of Checkstyle rules

5. Project style conventions for Intellij that apply most of Checkstyle rules

Figure 1. Code style

Go to File → Settings → Editor → Code style. There click on the icon next to the Scheme section. There, click on the Import Scheme value and pick the Intellij IDEA code style XML option. Import the spring-cloud-build-tools/src/main/resources/intellij/Intellij_Spring_Boot_Java_Conventions.xml file.

Figure 2. Inspection profiles

Go to File → Settings → Editor → Inspections. There click on the icon next to the Profile section. There, click on the Import Profile and import the spring-cloud-build-tools/src/main/resources/intellij/Intellij_Project_Defaults.xml file.

Checkstyle

To have Intellij work with Checkstyle, you have to install the Checkstyle plugin. It’s advisable to also install the Assertions2Assertj to automatically convert the JUnit assertions

Go to File → Settings → Other settings → Checkstyle. There click on the + icon in the Configuration file section. There, you’ll have to define where the checkstyle rules should be picked from. In the image above, we’ve picked the rules from the cloned Spring Cloud Build repository. However, you can point to the Spring Cloud Build’s GitHub repository (e.g. for the checkstyle.xml : raw.githubusercontent.com/spring-cloud/spring-cloud-build/master/spring-cloud-build-tools/src/main/resources/checkstyle.xml). We need to provide the following variables:

• checkstyle.header.file - please point it to the Spring Cloud Build’s, spring-cloud-build-tools/src/main/resources/checkstyle-header.txt file either in your cloned repo or via the raw.githubusercontent.com/spring-cloud/spring-cloud-build/master/spring-cloud-build-tools/src/main/resources/checkstyle-header.txt URL.

• checkstyle.suppressions.file - default suppressions. Please point it to the Spring Cloud Build’s, spring-cloud-build-tools/src/checkstyle/checkstyle-suppressions.xml file either in your cloned repo or via the raw.githubusercontent.com/spring-cloud/spring-cloud-build/master/spring-cloud-build-tools/src/checkstyle/checkstyle-suppressions.xml URL.

• checkstyle.additional.suppressions.file - this variable corresponds to suppressions in your local project. E.g. you’re working on spring-cloud-contract. Then point to the project-root/src/checkstyle/checkstyle-suppressions.xml folder. Example for spring-cloud-contract would be: /home/username/spring-cloud-contract/src/checkstyle/checkstyle-suppressions.xml.

Important

Remember to set the Scan Scope to All sources since we apply checkstyle rules for production and test sources.