2 The OSGi infrastructure

Over the last decade , the concept of the global network has increased in significance with the widespread deployment of the Internet. At present, it is not the monopoly of big companies, but it is deployed, too, at the residential level. At the same time, major evolutions have occurred within equipment and terminals, using more improved processors and electronic chips both in terms of execution speed and memory size . Since these devices have become more smart, and are able to communicate and interact together, new prospects have arisen concerning interoperability between remote software/hardware environments. Thanks to these network-connected devices, it has become possible to read e-mail on the TV or on a cellular phone, or again to control the state of home equipment.

Figure 8.1: Interconnection of a home network to the internet network through OSGi gateway

2.1 The OSGi framework

An industrial consortium of telecom operators, telecom equipment and home equipment manufacturers was created in 1999 in order to define and promote an open standard (framework) allowing the development and deployment of e-services in the home, accessible and remotely controlled on the Internet. This standard, written in the Java language, is named OSGi ( Open Source Gateway initiative) gateway. Presently, there are more than 5 gateway products on the market, and some of them are less than 300Kbytes in size and can be concurrent candidates for embedded systems.

2.2 History and evolution

In the 1.0 OSGi specification release, dated May 2000, the emphasis was put on the definition of basic APIs, intended for implementation in the framework and available to service developers. These APIs were written in Java. This language was chosen for portability reasons, its downloading capacity and its security properties. In the 2.0 release of the OSGi APIs, dated October 2000, security was strengthened and a special management bundle was added in order to fully define and control the security aspects of a bundle in real time. The minimal infrastructure of the gateway deals with:

The Java environment: the set of packages and classes required for the framework work.
The Framework which defines APIs that load, create and launch the services.
The Log service that collects information concerning the framework during execution.
The Http service which defines the APIs that allow the launch and use of an http server.

This minimal infrastructure can be grown (enhanced) by more services; both by infrastructure services like (Jini, Corba, uPnP, Havi etc) and by application services (Figure 8.2).

Figure 8.2: OSGi framework architecture

2.3 Functional description and properties

The OSGI framework architecture relies on some concepts that we describe below. For a given application, installed in the framework, it is assumed that it offers one or more services.

These are encapsulated in a " bundle " that is a .jar file in which we put together the service and other files such as pictures, sounds, movies and all the resources necessary for the working service(s). It also needs to contain a manifest file, which describes the name and the contents of the bundle and an activator which manages the "starting" and "stopping" states of a bundle and/or a service.

Once a bundle is in the "installed" state, the framework is able to manage the services in the bundle. These are registered with their properties within the framework, and bundle states are dynamically managed (lifecycle management). The framework is informed about service dependencies and sends an event notification after each state change.

Service launching is performed by a class that encapsulates the service and that manages the starting and the stopping states by two methods Start() and Stop(), defined and implemented in the service activator.

Figure 8.3: The lifecycle of a bundle