Quick Guide to SCA

What is SCA?

SCA is a standard programming model for abstracting business functions as components and using them as building blocks to assemble business solutions. An SCA component offers services and depends on functions that are called references. It also has an implementation associated it with it which is the business logic that can be implemented in any technology.

SCA provides a declarative way to describe how the services in an assembly interact with one another and what quality of services (security, transaction, etc) is applied to the interaction. Since service interaction and quality of service is declarative, solution developers remain focus on business logic and therefore development cycle is simplified and shortened. This also promotes the development of reusable services that can be used in different contexts. For example, a shopping cart service can be used in a retail application or a travel application without changing. Services can interact with one another synchronously or asynchronously and can be implemented in any technology.

SCA also brings flexibility to deployment. A solution assembled with SCA is deployed as a unit and can be distributed over one or more nodes in the network and can be reconfigured without programming changes.

Applications that adopt SCA programming model can interact with non-SCA applications. Meaning non-SCA application can call into SCA enabled applications and SCA enabled applications can call out into non-SCA enabled applications.

Now let's talk about SCA building blocks.

SCA Component

The basic building block for SCA is a component. It is the abstraction of a given business function. A component is described with the following attributes:

• Service: Describes the functions that this type of component provides. A component can offer one ore more services. A service is an interface.
• Reference: This describes the dependencies this type of component has in order to function. A reference is an interface.
• Property: This defines configuration parameters that can controls how the business function can behave. For example, what currency to use for an account component.
• Intent policies: This describes assumptions on how the component will behave. There are two types of policies.
  • Implementation policy- Impact the behavior of an implementation. For example, transaction, monitor and logging
  • Interaction policy - defines how the components behave with one another. For example, security.
• Implementation: Every component has some implementation associated with it. This can be a new business logic or an existing one that is now being used in the assembly. A business logic can handle different operations and some of which are exposed externally as callable services. Component implementation can be in any technology, for example for example BPEL for business processes or XSL-T for transformations or Ruby for scripting or pure Java. How the services, references, properties and intents are defined for an implementation is specific to that particular implementation type.

This is demonstrated below.

The implementation of a component can be in any language that is suitable for the user, for example BPEL for business processes or XSL-T for transformations or Ruby for scripting or pure Java. How the services, references, properties and intents are defined for an implementation is specific to that particular implementation type.

SCA Wire

As mentioned above, an SCA component may have zero or more references. Refrences in SCA define how SCA components invoke services. The relationship between a reference and a service is typically demonstrated through a line in a SCA diagrams and is referred to as a wire.

The term wire can at the begining seem confusing because you may ask how a wire is realized. There is no physical definition for a wire, it is really derived from the relationship between a Service and its refrence(s) at runtime. This is realized through dependency injection in Tuscany.

SCA Composite

Individual components like those described above can be used on their own, or they can be assembled together in a composite. A composite can be viewed as a component whose implementation is not code but an aggregation of one or more components co-operating to provide higher level services. Think of composite as a solution, for example credit check composite. A composite can also be used within a larger solution, for example credit check can be part of a order processing composite. A composite has the same charactersitics as a component. It provides Services, has References to other dependencies, and can be configured using Properties and can have intent policies in just the same way as an individual components can. In thise case, attributes of some of the components that are embedded in the composite get 'promoted' and becom the attribute of the composite. In the example below, you see a calculator composite which consists of 5 components, a calculator service has references to four components:Add, Subtract, Multiply and Divide.

The assembly or wiring is defined in .composite file through Service Component Definition Language (SCDL). For example, calculator.composite would define that calculator component references the other four components.

SCA Domain

The artifacts that make up a solution get packaged into what is called a contribution. A contribution can take a number of different forms. For example, it could be a zip or jar file, or it could be a directory tree on the file system. A contribution can contain composites, java classes, BPEL processes, XSD files, wsdl files, etc. An SCA application can be divided into multiple contributions with dependencies between them. In general, some services depend closely on other services and it makes sense to package them together. If services are more independent it is best to package them separately so that they can be reused in different contexts. A contribution is a deployable unit. A solution may require multiple contributions that share artifacts and artifacts can be shared between (imported) between contributions.

Contribution packages get contributed to what is called SCA domain which is the scope of adminstration at runtime. An SCA Domain represents a complete runtime configuration, potentially distributed over a
series of interconnected runtime nodes. A domain is a logical view of the running applications or a coherent grouping of components that are working together. A composite gets instantiated when it is actually used in an SCA environment.

SCA Domains can vary in size from the very small to the very large:

• a very small domain could be one within a test environment inside an IDE
• a medium sized domain could be a single server or small cluster supporting a single application
• a large domain could describe all the services within a department or company

In a large domain there may be all sorts of policies about where components can run and how they connect to each other or to external services. However, during development one is not concerned with all this. The code is packaged and made available for deployment. Tuscany SCA Java supports contributions in the form of JAR or filesystem.

Below is an example of domain with two contributions.