Chapter 9. Develop an Application for the Karaf Image

The bundle fabric8-karaf-core exports a service PlaceholderResolver with the following interface:

public interface PlaceholderResolver {
    /**
     * Resolve a placeholder using the strategy indicated by the prefix
     *
     * @param value the placeholder to resolve
     * @return the resolved value or null if not resolved
     */
    String resolve(String value);

    /**
     * Replaces all the occurrences of variables with their matching values from the resolver using the given source string as a template.
     *
     * @param source the string to replace in
     * @return the result of the replace operation
     */
    String replace(String value);

    /**
     * Replaces all the occurrences of variables within the given source builder with their matching values from the resolver.
     *
     * @param value the builder to replace in
     * @rerurn true if altered
     */
    boolean replaceIn(StringBuilder value);

    /**
     * Replaces all the occurrences of variables within the given dictionary
     *
     * @param dictionary the dictionary to replace in
     * @rerurn true if altered
     */
    boolean replaceAll(Dictionary<String, Object> dictionary);

    /**
     * Replaces all the occurrences of variables within the given dictionary
     *
     * @param dictionary the dictionary to replace in
     * @rerurn true if altered
     */
    boolean replaceAll(Map<String, Object> dictionary);
}

The PlaceholderResolver service acts as a collector for different property placeholder resolution strategies. The resolution strategies it provides by default are listed in the table.

The property placeholder service supports the following options:

To set the property placeholder service options you can use system properties or environment variables or both.

containers:
  -
   env:
   - name: FABRIC8_K8S_SECRETS_PATH
     value: /etc/secrets
     volumeMounts:
   - name: activemq-secret-volume
     mountPath: /etc/secrets/activemq
     readOnly: true
   - name: postgres-secret-volume
     mountPath: /etc/secrets/postgres
     readOnly: true

volumes:
  - name: activemq-secret-volume
  secret:
  secretName: activemq
  - name: postgres-secret-volume
   secret:
  secretName: postgres

You can add a custom placeholder resolver to support a specific need, such as custom encryption. You can also use the PlaceholderResolver service to make the resolvers available to Blueprint and ConfigAdmin.

To add a custom property placeholders resolvers, follow these steps:

my.property = $[myResolver:value-to-resolve]

The fabric8-karaf-cm provides a ConfigAdmin bridge that inject ConfigMap values in Karaf’s ConfigAdmin.

To be added by the ConfigAdmin bridge, a ConfigMap has to be labeled with karaf.pid, where its values corresponds to the pid of your component.

kind: ConfigMap
apiVersion: v1
metadata:
  name: myconfig
  labels:
    karaf.pid: com.mycompany.bundle
data:
  example.property.1: my property one
  example.property.2: my property two

Individual properties work for most cases. But to define your configuration, you can use a single property names. It is same as the pid file in karaf/etc. For example,

kind: ConfigMap
apiVersion: v1
metadata:
  name: myconfig
  labels:
    karaf.pid: com.mycompany.bundle
data:
  com.mycompany.bundle.cfg: |
    example.property.1: my property one
    example.property.2: my property two

The fabric8-karaf-cm provides a ConfigurationPlugin which resolves configuration property placeholders.

To enable property substitution with the fabric8-karaf-cm plug-in, you must set the Java property, fabric8.config.plugin.enabled to true. For example, you can set this property using the JAVA_OPTIONS environment variable in the Karaf image, as follows:

JAVA_OPTIONS=-Dfabric8.config.plugin.enabled=true

An example of configuration property placeholders is shown below.

    amq.usr = $[k8s:secret:$[env:ACTIVEMQ_SERVICE_NAME]/username]
    amq.pwd = $[k8s:secret:$[env:ACTIVEMQ_SERVICE_NAME]/password]
    amq.url = tcp://$[env+service:ACTIVEMQ_SERVICE_NAME]

    <?xml version="1.0" encoding="UTF-8"?>

    <blueprint xmlns="http://www.osgi.org/xmlns/blueprint/v1.0.0"
               xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
               xmlns:cm="http://aries.apache.org/blueprint/xmlns/blueprint-cm/v1.1.0"
               xsi:schemaLocation="
                 http://www.osgi.org/xmlns/blueprint/v1.0.0
                 https://www.osgi.org/xmlns/blueprint/v1.0.0/blueprint.xsd
                 http://camel.apache.org/schema/blueprint
                 http://camel.apache.org/schema/blueprint/camel-blueprint.xsd">

      <cm:property-placeholder persistent-id="my.service" id="my.service" update-strategy="reload"/>

      <bean id="activemq" class="org.apache.activemq.camel.component.ActiveMQComponent">
         <property name="userName"  value="${amq.usr}"/>
         <property name="password"  value="${amq.pwd}"/>
         <property name="brokerURL" value="${amq.url}"/>
      </bean>
    </blueprint>

Fabric8 Karaf Config Admin supports the following options.

You can provide additional custom heath checks to prevent the Karaf server from receiving user traffic before it is ready to process the requests. TO enable custom health checks you need to implement the io.fabric8.karaf.checks.HealthChecker or io.fabric8.karaf.checks.ReadinessChecker interfaces and register those objects in the OSGi registry.

Your project will need to add the following mvn dependency to the project pom.xml file.

<dependency>
  <groupId>io.fabric8</groupId>
  <artifactId>fabric8-karaf-checks</artifactId>
</dependency>

An example that performs a health check to make sure you have some free disk space, is shown below:

import io.fabric8.karaf.checks.*;
import org.apache.felix.scr.annotations.*;
import org.apache.commons.io.FileSystemUtils;
import java.util.Collections;
import java.util.List;

@Component(
    name = "example.DiskChecker",
    immediate = true,
    enabled = true,
    policy = ConfigurationPolicy.IGNORE,
    createPid = false
)
@Service({HealthChecker.class, ReadinessChecker.class})
public class DiskChecker implements HealthChecker, ReadinessChecker {

    public List<Check> getFailingReadinessChecks() {
        // lets just use the same checks for both readiness and health
        return getFailingHeathChecks();
    }

    public List<Check> getFailingHealthChecks() {
        long free = FileSystemUtils.freeSpaceKb("/");
        if (free < 1024 * 500) {
            return Collections.singletonList(new Check("disk-space-low", "Only " + free + "kb of disk space left."));
        }
        return Collections.emptyList();
    }
}