ConfigMaps is the Kubernetes counterpart of the Spring Boot externalized configuration. ConfigMaps is a simple key/value store, which can store simple values to files. In this post "Configuring Spring Boot on Kubernetes with ConfigMap", we will see how to use ConfigMaps to externalize the application configuration.
One of the ways configuring the spring boot application on kubernetes is to use ConfigMaps. ConfigMaps is a way to decouple the application specific artifacts from the container image, thereby enabling better portability and externalization.
The sources of this blog post are available in my github repo. In this blog post, we will build simple GreeterApplication, which exposes a REST API to greet the user. The GreeterApplication will use ConfigMaps to externalize the application properties.
Setup
You might need access to Kubernetes Cluster to play with this application. The easiest way to get local Kubernetes cluster up and running is using minikube. The rest of the blog assumes you have minikube up and running.
There are two ways to use ConfigMaps,
ConfigMaps as Environment variables
Assuming you have cloned my github repo, let's refer to the cloned location of the source code as $PROJECT_HOME throughout this document.
You will notice that com.redhat.developers.GreeterController has a code to look up an environment variable GREETER_PREFIX.
package com.redhat.developers;
import lombok.extern.slf4j.Slf4j;
import org.springframework.beans.factory.annotation.Value;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.RestController;
@RestController
@Slf4j
public class GreeterController {
@Value("${greeter.message}")
private String greeterMessageFormat;
@GetMapping("/greet/{user}")
public String greet(@PathVariable("user") String user) {
String prefix = System.getenv().getOrDefault("GREETING_PREFIX", "Hi");
log.info("Prefix :{} and User:{}", prefix, user);
if (prefix == null) {
prefix = "Hello!";
}
return String.format(greeterMessageFormat, prefix, user);
}
}
By convention Spring Boot applications rather any Java applications passes these kind values via system properties. Let us now see how we can do the same with Kubernetes deployment.
- Let's create a Kubernetes ConfigMaps to hold the property called greeter.prefix, which will then be injected into the Kubernetes deployment via an environment variable called GREETER_PREFIX.
Create ConfigMap
kubectl create configmap spring-boot-configmaps-demo --from-literal=greeter.prefix="Hello"
- You can see the contents of the ConfigMap using the command
kubectl get configmap spring-boot-configmaps-demo-oyaml
Create Fragment deployment.yaml
Once we have the Kubernetes ConfigMaps created, we then need to inject the GREETER_PREFIX as an environment variable into the Kubernetes deployment. The following code snippet shows how to define an environment variable in a kubernetes deployment.yaml.
spec:
template:
spec:
containers:
- env:
- name: GREETING_PREFIX
valueFrom:
configMapKeyRef:
name: spring-boot-configmaps-demo
key: greeter.prefix
- The above snippet defines an environment variable called GREETING_PREFIX which will have its value set from ConfigMap spring-boot-configmaps-demo key greeter.prefix.
NOTE:
As the application is configured to use fabric8-maven-plugin, we can create Kubernetes deployment and service as fragments in '$PROJECT_HOME/src/main/fabric8'. The fabric8-maven-plugin takes care of building the complete Kubernetes manifests by merging the contents of the fragment(s) from '$PROJECT_HOME/src/main/fabric8' during deployment.
Deploy Application
To deploy the application, execute the following command from the $PROJECT_HOME ./mvnw clean fabric8:deploy.
Access Application
The application status can be checked with command, kubectl get pods -w once the application is deployed, let's do a simple curl like curl $(minikube service spring-boot-configmaps-demo --url)/greet/jerry; echo ""; command will return a message Hello jerry! Welcome to Configuring Spring Boot on Kubernetes! The return message has a prefix called "Hello", which we had injected via the environment variable GREETING_PREFIX with the value from the ConfigMap property "greeter.prefix".
Mounting ConfigMaps as files
Kubernetes ConfigMaps also allows us to load a file as a ConfigMap property. That gives us an interesting option of loading the Spring Boot application.properties via Kubernetes ConfigMaps.
To be able to load application.properties via ConfigMaps, we need to mount the ConfigMaps as the volume inside the Spring Boot application container.
Update application.properties
greeter.message=%s %s! Spring Boot application.properties has been mounted as volume on Kubernetes!
Create ConfigMap from file
kubectl create configmap spring-app-config --from-file=src/main/resources/application.properties
The command above will create a configmap called spring-app-config with the application.properties file stored as one of the properties.
The sample output ofkubectl get configmap spring-app-config -o yaml is shown below.
apiVersion: v1
data:
application.properties: greeter.message=%s %s! Spring Boot application.properties has been mounted as volume on Kubernetes!
on Kubernetes!
kind: ConfigMap
metadata:
creationTimestamp: 2017-09-19T04:45:27Z
name: spring-app-config
namespace: default
resourceVersion: "53471"
selfLink: /api/v1/namespaces/default/configmaps/spring-app-config
uid: 5bac774a-9cf5-11e7-9b8d-080027da6995
Modifying GreeterController
package com.redhat.developers;
import lombok.extern.slf4j.Slf4j;
import org.springframework.beans.factory.annotation.Value;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.RestController;
@RestController
@Slf4j
public class GreeterController {
@Value("${greeter.message}")
private String greeterMessageFormat;
@GetMapping("/greet/{user}")
public String greet(@PathVariable("user") String user) {
String prefix = System.getenv().getOrDefault("GREETING_PREFIX", "Hi");
log.info("Prefix :{} and User:{}", prefix, user);
if (prefix == null) {
prefix = "Hello!";
}
return String.format(greeterMessageFormat, prefix, user);
}
}
Update Fragment deployment.yaml
Update the deployment.yaml to add the volume mounts that will allow us to mount the application.properties under /deployments/config.
spec:
template:
spec:
containers:
- env:
- name: GREETING_PREFIX
valueFrom:
configMapKeyRef:
name: spring-boot-configmaps-demo
key: greeter.prefix
volumeMounts:
- name: application-config
mountPath: "/deployments/config"
readOnly: true
volumes:
- name: application-config
configMap:
name: spring-app-config
items:
- key: application.properties
path: application.properties
Let's deploy and access the application like how we did earlier, but this time the response will be using the application.properties from ConfigMaps.
The Spring Boot and Kubernetes Series
How to Configure the Spring Boot Application on Kubernetes
Part I: Configuring Spring Boot on Kubernetes with ConfigMap
Last updated: November 4, 2021