OSGi Getting Started

By Michael N. Lipp

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Introduction

Part 1: The container

Part 2: Simplest Bundle

Part 3: Simple Bundle

Part 4: Eclipse (OSGi) Plugin

Part 5: Bndtools

Part 6: Shift in Perspective

Part 7: Modules and Services

Part 8: Accessing a Service

Part 9: Tracking a Service

Part 10: Using a Service

Part 11: Providing a Service

Interlude: Cleaning up

Part 12: Service Component

Part 13: Configuration Admin

Part 14: Repositories

Part 15: Versions

The Configuration Admin Service

Work in progress

An important issue in a component framework is the configuration of the components. Components might be usable out-of-the-box, but usually they must be adapted to the context. A common simple example is some component that listens on a network port which must, of course, be configurable.

The configuration information should be persistent, i. e. it must be preserved across restarts. It should also be modifiable during runtime and modifications should be effective without restart. OSGi defines such a service for maintaining and distributing configuration information as the “Configuration Admin” service1.

Configuration Admin in Action

Let’s have a look at this service in action. Go back to the project from the part “Using a Service” and remove the run property org.osgi.service.log.admin.loglevel, added as last step in order to actually get logging output. Then add org.apache.felix.configadmin and de.dentrassi.osgi.net.luminis.cmc to the run bundles (or checkout and import the prepared project). The former bundle adds an implementation of the Configuration Admin service to our run environment, the latter adds some commands to the Felix GoGo shell2. Unfortunately, the bundle with the GoGo shell extensions is implemented using Declarative Services (as presented in the previous part), so we have to also add org.apache.felix.scr. And because the current implementation (2.1.16 as of this writing) uses OSGi promises and functions, we additionally have to add bundles org.osgi.util.promise and org.osgi.util.function3. Now we can start the framework with our “Hello World” application.

As you know from “Using a Service” we do not see any log messages from our bundle’s classes because they are logged with level INFO and the default threshold of OSGi loggers is WARNING. In “Using a Service” we fixed this with a bundle property that configured the OSGI log service. However, if a Configuration Admin service is available, OSGi loggers can also be configured using this service. Enter the following commands in the Felix console:

g! cm:create org.osgi.service.log.admin
g! cm:put org.osgi.service.log.admin ROOT INFO
Aug 13, 2019 6:39:10 PM LogService
INFORMATION: Hello Word sleeping
Aug 13, 2019 6:39:15 PM LogService
INFORMATION: Hello Word sleeping

And the log messages are back again. Before you restart the framework in order to check the persistence of the configuration, make sure to un-check the “Clean storage area before launch” flag in the Eclipse run configuration generated by Bndtools4:

Uncheck clear storage area

Of course you can also query the information just entered:

g! cm:list
Configuration list:
----------------------------
org.osgi.service.log.admin                                    ?

g! cm:get org.osgi.service.log.admin
Configuration for service (pid) "org.osgi.service.log.admin" (bundle location = ?)
key           type               value
------        ------             ------
ROOT          java.lang.String   INFO
service.pid   java.lang.String   org.osgi.service.log.admin

The question mark that you see in the outputs is the value of the “bundle location” associated with a configuration object. A large part of the Configuration Admin specification is about this bundle location and its handling. Dont’t read it. More or less by chance I found this5:

Locations were a mistake in the Configuration Admin API. Ok, I’ve said it. They were a failed attempt to provide security at an unsuitable place. Mea culpa … The intention was that we could restrict configurations to specific bundles, this restriction was actually automatic when the location was set to null and a bundle used it. Countless hours have been lost figuring out why Configuration Admin did not call update only to discover that the location was wrong. Alas, the sins we commit when we try to specify.

So what should you do with the location? Well, just set it always to “?”. This is a recent addition to the specification that basically removes the awkward location check.6

Concepts

The Configuration Admin service maintains a database of Configuration Objects. The main purpose of configuration objects is to store key/value pairs7. Each configuration object has a unique PID (Persistent IDentity). The first command (cm:create org.osgi.service.log.admin) creates a configuration object with PID org.osgi.service.log.admin. The command cm:put org.osgi.service.log.admin ROOT INFO adds (to the just created configuration object with PID org.osgi.service.log.admin) the key ROOT with value INFO.

Parties interested in configuration changes of a specific configuration object register services of type ManagedService. If a party is interested in changes of lots of configuration objects (e. g. objects with PIDs starting with a given prefix) is should register a service of type ConfigurationListener instead.

The log service registers a ConfigurationListener because you cannot only configure the default log level as shown. There is a PID for each LoggerContext and the key/value pairs in each configuration object correspond to logger names and log levels. The details can be found in the log specification.

You can check the registration of a ConfigurationListener by the log service in the GoGo shell:

g! each (allServiceReferences org.osgi.service.cm.ConfigurationListener null) { $it bundle }
    1|Active     |    1|org.apache.felix.log (1.2.0)
    8|Active     |    1|org.apache.felix.configadmin (1.9.16)

The system that we have configured looks like this:

Component relationships

As you may have guessed, the GoGo shell commands make use of the ConfigurationAdmin service provided by the Config Admin implementation. This is the interface that you’d also use to manipulate configurations in your program.

A Configurable Component

The obvious next step is to make our own component configurable in some way, i. e. it has to register an implementation of ManagedService. The Javadoc describes the basic usage pattern pretty well. I’ve applied it to the “Hello World” sample application and made the message frequency configurable. You can find the resulting code in the companion project.

Note that the update method is always called at least once after registering a managed service. We can make use of this by moving the actual start of our application to the update method. This results in the following startup sequence:

Application Startup

The application structure now includes an additional relationship between the ManagedService exposed by the “Hello World” application and the Configuration Admin service.

Application Structure

You have probably already figured it out, try:

g! cm:create io.github.mnl.osgiGettingStarted.loggingBundle.HelloWorld
g! cm:put -t l io.github.mnl.osgiGettingStarted.loggingBundle.HelloWorld waitTime 10000

To be continued


  1. The name has always seemed a bit strange to me. The major task of this service is to maintain and spread configuration information. Of course, the service also provides an API for administering the information, but I’d never have focused on that aspect in its name. 

  2. The commands that this GoGo extension provides aren’t strictly necessary. The Configuration Admin implementation adds the ConfigurationAdmin service to the GoGo shell (with scope “cm”, just like the extension). We could therefore execute all actions using the service as a starting point. However, the extension simplifies things. 

  3. The complete list of bundles in the “Source” tab of bnd.bnd looks like this:

    -runbundles: \
    	org.apache.felix.log,\
    	org.apache.felix.gogo.command,\
    	org.apache.felix.gogo.runtime,\
    	org.apache.felix.gogo.shell,\
    	de.mnl.osgi.osgi2jul;version=1.1.3,\
    	de.mnl.osgi.coreutils;version=1.0.3,\
    	de.dentrassi.osgi.net.luminis.cmc;version=0.6.0,\
    	org.apache.felix.configadmin,\
    	org.apache.felix.scr,\
    	org.osgi.util.promise,\
    	org.osgi.util.function
    

  4. To be honest—I didn’t test this systematically—it probably is too late. My impression is that the flag must be unchecked before you start the program. It’s real meaning seems to be “use a temporary storage location for the next launch”. 

  5. It’s one of those strange things in the OSGi context that they didn’t bother to put this information in the latest version of the specification—at least not at a prominent enough place for me to find it. 

  6. See https://web.archive.org/web/20151103190624/http://enroute.osgi.org:80/services/org.osgi.service.cm.html 

  7. There’s a bit more, configuration objects implement this interface