Chapter 12. Developing an application for the Karaf image

To use the features, add fabric8-karaf-features dependency to the project POM file.

<dependency>
  <groupId>io.fabric8</groupId>
  <artifactId>fabric8-karaf-features</artifactId>
  <version>${fabric8.version}</version>
  <classifier>features</classifier>
  <type>xml</type>
</dependency>

The fabric8 karaf features will be installed into the Karaf server.

The bundle fabric8-karaf-core provides the functionalities used by Blueprint and ConfigAdmin extensions.

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

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.

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:

You can add Fabric8 Karaf Config admin support to your custom Karaf distribution.

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

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.

By default, the fabric8-karaf-checks endpoints are registered into the built-in HTTP server engine (Undertow) running on port 8181. To avoid the health and readiness check requests being blocked by other long running HTTP processes in the container, the endpoints can be registered into a separate Undertow container.

These checks can be configured in the etc/io.fabric8.checks.cfg file by setting the following properties:

The following example illustrates the configuration of the health and readiness properties in the etc/io.fabric8.checks.cfg file:

httpPort = 8182
readinessCheckPath = /readiness-check
healthCheckPath = /health-check