Poweerr Combiinaattiion::SCA,, OSGii aand Sprriing

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Advantages of combining SCA with OSGi
SCA and OSGi are complementary.
SCA provides a definition for components with services and references. It describes a separation between service
implementation and service usage and promotes a model where complex detail about communication methods is
kept out of the business code within the implementation. SCA describes service integration through composites and
provides a declarative way to apply policies for infrastructure capabilities such as security. SCA supports a
distributed environment potentially using a heterogeneous mixture of component technologies.

The OSGi container is able to address a wide range of systems – from embed devices up to enterprise servers.
Dependency handling, multi-version support and remote management are integral parts of OSGi. Software
components and services can be installed, updated and removed on the fly.
The combination of SCA and OSGi provides the best of both worlds including:
• A common Service Component model
• A standardized way to integrate a variety of communication mechanisms in a runtime container
• An OSGi based SCA container can run on a wide range of system and has a clear and simple extension
mechanism for supporting new implementation types and new communication bindings
• Integration of different implementation types allows the use of the technology best suited to the job
The following figure shows the integration of a variety of communication mechanisms in an combined OSGi/SCA
container. For every supported transport/serialization protocol an SCA binding is provided, e.g. JMS binding, SOAP
binding, CORBA binding.

The Application developer implements application components the most suitable programming language for the
business task in hand, e.g. in Java for complex data manipulation or in BPEL for describing a business process.
Services and references are configured with bindings and endpoints in the SCA composite file. The SCA container
provides the required bindings for the references and services. Via the bindings the SCA components access
services running in other (remote) containers, e.g. in an EJB Container or .NET container. A performance optimized
protocol might be used for the communication between SCA containers.
SCA brings distributed computing support into OSGi. It allows OSGi applications to integrate in heterogeneous
environments, e.g. with .NET and EJB applications.

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Combining SCA with OSGi and Spring
The preceding section describes the advantages of combining SCA with OSGi. This section explains the
advantages of an additional integration of these technologies with Spring.
A way of providing the combination is to take the SCA/OSGi container shown in figure 5 and add in support for the
SCA Spring implementation type described in the section Combining SCA with Spring. This combined container
allows for the creation of components as Spring application contexts, which is a great way of creating a service
implementation from simple Java Beans.

Spring implementation type:
Spring is used inside the Spring implementation type and the component is built as a Spring application context.
Injection is used to supply beans which represent references and properties of the implementation. SCA is used to
provide the wiring and other configuration of the services and references.
OSGi implementation type:
As explained previously, Spring can also be used as the implementation technology for an enhanced OSGi
Declarative Services (DS). The DS is responsible for the dependency management inside the OSGi implementation
type. For the distributed communication via services and references, SCA is used.
Java implementation type:
This is the pure POJO implementation type described in the SCA Java Component Implementation specification.
SCA is used for the dependency management inside of the Java implementation type as well as for the services
and references to the outside.

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Inside the OSGi implementation type the OSGi binding as described previously is not required, because the DS can
be used for dependencies inside the OSGi implementation type as well as for dependencies to Services running
natively in the OSGi container (e.g. Service D), while SCA references can be used for services accessed through
any other communication methods and SCA services used to expose services offered by OSGi implementations. In
the Java implementation type the OSGi Binding would still be required to get access to any native OSGi services
such as Service D.

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Summary
SCA, OSGi and Spring are all useful and powerful facilities for the Java programmer to use. In the new serviceoriented
world that we are entering, using SCA, OSGi and Spring together provide powerful capabilities for building
service implementations from sets of simple Java Beans using few APIs, with managed dependencies, version
control and dynamic update capabilities, allied to the capability to compose those implementations with other
service components written in Java or in other languages and existing in a distributed network of systems using a
range of communication methods.
Simplicity, flexibility, manageability, testability, reusability. A key combination for enterprise developers.

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