OSGI Service Configuration and its resolution order

OSGI is a fundamental element in the technology stack of Adobe Experience Manager (AEM). It is used to control the bundles of AEM and their configuration.

OSGI bundles or components can be composed into an application and deployed and they can be stopped, installed, started individually. The interdependencies are handled automatically. Each OSGI Component is contained in one of the various bundles.

In the cases when you don't want to hardcode the values in the code eg. some host url or some parameter which is dynamic and can be changed frequently or the value which is different for each runmode of your aem instance, in those cases you use OSGI configuration.

By defining configuration values, you can define values used by an OSGI service and use these values while the service is running. Unlike hard-coding values in an AEM OSGI service, defining values in the AEM configMgr lets you change values without re-compiling the bundle.

You can manage the OSGI configuration settings for such bundles by :

Adobe CQ Web console - The Web Console is the standard interface for OSGI configuration. It provides a UI for editing the various properties, where possible values can be selected from predefined lists. Any configurations made with the Web Console are applied immediately and applicable to the current instance, irrespective of the current run mode.

Content-nodes (sling:osgiConfig) in the repository - This requires manual configuration using CRXDE Lite. You can create sling:OsgiConfig nodes according to a specific run mode in different config folders. You can even save configurations for more than one run mode in the same repository. Any appropriate configurations are applied immediately (dependent on the run mode).

Creating a OSGI Configuration service - 

Use the below Java class example to create an osgi config service:

package com.foo.community.core;

import java.util.Map;
import org.apache.felix.scr.annotations.Activate;
import org.apache.felix.scr.annotations.Component;
import org.apache.felix.scr.annotations.Deactivate;
import org.apache.felix.scr.annotations.Modified;
import org.apache.felix.scr.annotations.Property;
import org.apache.felix.scr.annotations.Service;
import org.apache.sling.commons.osgi.PropertiesUtil;
import org.osgi.service.component.ComponentContext;


@Component(immediate=true, metatype=true)
@Service({MyConfig.class})
public class MyConfig
{
  @Property(label="Sample Property", value={"property1"}, 
    description="Sample property to demonstrate osgi config")
  private static final String SAMPLE_PROPERTY = "sampleProperty";
  private String sampleProperty;
  
  public MyConfig() {}
  
  public String getSampleProperty()
  {
    return sampleProperty;
  }
  
  @Activate
  protected void activate(Map context)
  {
   sampleProperty = PropertiesUtil.toString(context
     .get("sampleProperty"), "property1");
  }
  
  @Deactivate
  protected void deactivate()
  {
   sampleProperty = null;
  }
  
  @Modified
  protected void modified(ComponentContext context)
  {
   sampleProperty = PropertiesUtil.toString(context
      .getProperties().get("sampleProperty"), "property1");
  }
}

Notice that in the above code I have used SCR annotations to create a service but those annotations are deprecated now, so you can use OSGI annotations instead.
To know more about scr annotations refer this page.

OSGi Configuration with the Web Console -

You can access the Configuration tab of the Web Console by:
http://localhost:4502/system/console/configMgr
then search for the MyConfig, you can see the config we created above as below:

OSGi Configuration with the Repository (CRX Nodes) -

To add the new configuration to the repository:

• Go to your project folder- /apps/<yourProject>, then create the config folder (sling:Folder):

            config - applicable to all run modes
            config.<run-mode> - specific to a particular run mode

• Under this folder create a node:

           Type: sling:OsgiConfig
           Name: the persistent identity (PID)
           for eg. in our case-  com.foo.community.core.MyConfig

• For each parameter that you want to configure, create a property on this node:

          Name: the parameter name(shown in brackets) in the Web console;
          Type: as appropriate.
          Value: as required.

Below is the screenshot of the config I created for global and author runmode-

It's xml representation (com.foo.community.core.MyConfig.xml) is shown below:

<?xml version="1.0" encoding="UTF-8"?>
<jcr:root xmlns:sling="http://sling.apache.org/jcr/sling/1.0"
    xmlns:jcr="http://www.jcp.org/jcr/1.0"
    jcr:primaryType="sling:OsgiConfig"
    sampleProperty="Value edited from crx author"/>

Reading config values through Javascript Use-API:

Use the below JS Use-API code snippet to use the config value and return to sightly-

var global = this;
use(function () {
    if (global.sling && global.Packages) {
       var myConfigService = global.sling.
           getService(global.Packages.com.foo.community.core.MyConfig);
    }
 var sampleProperty = myConfigService.getSampleProperty();

    return {
        sampleProperty : sampleProperty
    };
});

To read osgi config values through Java, refer this post.

Resolution Order of the configurations-

Resolution Order at Startup-

The following order of precedence is used:

• Repository nodes under /apps/*/config of type sling:OsgiConfig.
• Repository nodes with type sling:OsgiConfig under /libs/*/config.
• Any .config files from <cq-installation-dir>/crx-quickstart/launchpad/config/ on local file system.

Resolution Order at Runtime-

Configuration changes made while the system is running trigger a reload with the modified configuration.
Then the following order of precedence applies:

• Modifying a configuration in the Web console will take immediate effect as it takes precedence at runtime.
• Modifying a configuration in /apps will take immediate effect.
• Modifying a configuration in /libs will take immediate effect.

References- https://helpx.adobe.com/experience-manager/6-3/sites/deploying/using/configuring-osgi.html