Service Component Architecture (SCA) is a simple model for creating service-oriented applications. This article highlights the benefits of SCA and introduces SCA concepts by walking through an example. The example has been developed using the Apache Tuscany open source project (http://incubator.apache.org/tuscany/). All the sample code in this article is licensed under the Apache License 2.0 (www.apache.org/licenses/LICENSE-2.0) and the resources with the article gives a link to the sample files. Both the Apache Tuscany and PHP SCA_SDO (http://pecl.php.net/package/sca_sdo) projects provide a free service oriented infrastructure for creating, packaging, deploying, and managing applications built with the SCA programming model.

The SCA programming model itself is described by a set of specifications that are being developed by many vendors and individuals contributing to the Open Service Oriented Architecture collaboration (www.osoa.org).

SCA
Service Oriented Architecture (SOA) is an architectural approach driven by the need to overcome the challenges of tightly coupled and department-specific applications. SOA promises benefits such as improved business agility, improved flexibility, cost reduction, and the easy sharing of information in heterogeneous and distributed environments.

SOA provides a blueprint but implementing an SOA remains a challenge. The choice of technology available to the implementer is bewildering and skills in a variety of technologies are required to be successful. Service Component Architecture (SCA) addresses the complexity of developing an SOA solution through its simple model for creating service-oriented applications for the whole enterprise - from the client to the back-end. Businesses using SCA can benefit from the following:

• Rapid development and increase in productivity: SCA views an application as a set of connected components. It provides a simple language-neutral component model for implementing new components or reusing existing components. A component can be implemented in any language supported by an SCA runtime. SCA promotes true loose coupling by separating component implementation from the details of component composition. This bottom-up development style allows the developer to focus on developing business-related code with-out worrying about how this will fit into the overall solution.
• Higher organizational agility and flexibility: SCA also supports a top-down development approach of creating business solutions with its flexible service assembly model. SCA components can be wired together in a composition. A component can be replaced with another component in the composition as long as they share the same contract. The composition can be adjusted to IT infrastructure requirements such as service connections, transport protocols, transactions, security, and reliable messaging. Selectable transport bindings make solutions available in the widest possible set of deployment situations.
• Return on Investment through reuse: The SCA component model makes it very easy to leverage investments made in existing applications and services. Its standardized approach to encapsulation and interface abstraction enables service reuse through wiring and rewiring to construct new applications. SCA itself is technology-neutral and isn't intended to replace existing technology. It simply provides a component composition model that describes how new and existing services are assembled.

The dark blue boxes (Component A and Component B) show components. Components are at the heart of SCA as they encapsulate business logic. Depending on runtime support, components implemented using any programming technique can be included. For example, Apache Tuscany currently supports the Java language, JavaScript, Ruby, Python, and C++ component types and provides an extension API for building new extensions.

SCA components can have properties (the yellow boxes shown at the top of components A and B). Properties control the behavior of the component and can be changed at deployment time. For example, a stock quote application might have a property that indicates the currency that stock values will be quoted in.

SCA components describe the interfaces that they expose for other components to call, shown as the green arrows on the left-hand side of the component boxes and called "services" in SCA. Components also describe the interfaces of other components that they expect to call as the business logic executes, shown as the pink arrows on the right-hand side of the component boxes and called "references" in SCA. These exposed services and references can be "wired" together to describe a working system.

The diagram shows two components, A and B, assembled together within the bounds of a larger "composite," called composite A. The SCA composite describes a collection of wired components and, as you can see, the composite also echoes those services and references that must be exposed beyond the bounds of the composite. Wiring together components within a composite is akin to building a tightly coupled application that may run in a single process. Wiring together the services and references exposed by a composite represents a more loosely coupled system where each composite may run in a separate process or processor and is connected over a network with various protocol/transport bindings. This way SCA provides a consistent model for describing standalone and distributed applications.

An Example Scenario
We'll use the fictional MostMortgage company's mortgage loan approval application to introduce SCA in more detail. The loan approval application accepts a mortgage request including the customer's details and the requested loan amount. It first checks the customer's credit to make sure the credit score meets the minimum requirement. The interest rate is determined based on the principal requested, the term of the loan, and the customer's home state. It then uses a mortgage calculator to calculate the ratio by dividing the potential monthly payment by the customer's income. The ratio and credit score are passed to do a risk assessment that makes the final decision (see Figure 2).

Using SCA To Implement the Mortgage Loan Approval Application
In the next sections we'll implement the loan approval application using SCA and walk through the creation of individual SCA artifacts. At a high level the loan approval application can be broken down into a number of SCA components that are assembled together into a composite. The components in this composite consist of Loan Approval, Credit Check, Interest Rate, Mortgage Calculator, and Risk Assessment components. The entire composite is deployed in a SCA system (see Figure 3).

SCA Components
An SCA component is the basic building block for creating SOA applications and is characterized by three distinct and yet related pieces of information: a) The program logic that provides the function of the building block (referred to as implementation), b) The definition of how this building block might interact with other components (referred to as component type) c) The concrete description of how this building block fits with all the other blocks to build a solution (referred to as assembly or composition). We'll explain each in more detail in the following sections and give examples but here's an overview.

• Component Type: The component implementation is provided using any programming language that's supported by an SCA runtime. Component implementers are free to write in any style they're comfortable with but are bound by the services, references and properties, as defined by the component type, in the way that they interact with other SCA components. The SCA specifications describe how each programming language maps to SCA.
• Component Type: Component type describes the shape of a component in terms of the services it exposes, the references it depends on and the properties that control the component's behavior. Component-type information can be found either in a file where, by convention, the name is ImplementationFileName.componentType and/or by introspection of the component implementation.
• Component Composition/Assembly: Once a component's implementation and its component type are defined it's ready to be assembled into a network of services that together provide an SOA solution. The assembly is defined in an SCA composite file. The SCA runtime uses the information in this file to instantiate an SCA application.

Component Type
The MortgageCalculator component-type file (MortgageCalculator.componentType) describes the single service that components of this type provide. The MortgageCalculator component doesn't reference other components and doesn't provide any settable properties so <reference> and <property> elements don't appear.

<componentType>
    <service name="MortgageCalculatorService">
       <interface.java interface="mortgage.MortgageCalculator"/>
    </service>
</componentType>

Component Implementation
The class "mortgage.MortgageCalculatorImpl" (MortgageCalculatorImpl.java) contains the business logic for this component.

public class MortgageCalculatorImpl implements MortgageCalculator {
    public double getMonthlyPayment(double principal, int years, float interestRate) {
       double monthlyRate = interestRate / 12.0 / 100.0;
       double p = Math.pow(1 + monthlyRate, years * 12);
       double q = p / (p - 1);
       double monthlyPayment = principal * monthlyRate * q;
       return monthlyPayment;
    }
}

In the next section (Component Services) we show that if we chose to use annotations, as we can in the Java language, the component-type information can be included in the implementation file. Most of the code snippets in this paper use annotations to provide component-type information instead of using a component-type file.

Component Services
Let's take a look at how a component offers a service to others. In the following example MortgageCalculator exposes a service that contains one method, called getMonthlyPayment, by using a @Service annotation. As the Java language runtime supports annotations our method can be exposed as a service interface by simply annotating the class.

@Service(MortgageCalculator.class)
public class MortageCalculatorImpl implements MortageCalculator {

    public double getMonthlyPayment(double principal, float interestRate) {
       ...
    }
}

The @Service annotation tells the SCA runtime that the MortgageCalculatorImpl class instances are exposed as services with an interface defined by the MortgageCalculator interface.

Component References
Now let's look at how a component references other components. We'll use the Loan Approval component that references other components as our example here. Loan Approval is implemented using the Java language and will use annotations. It uses @Reference to indicate its dependency on RiskAssessment, CreditCheck, InterestRateQuote, and MortgageCalculator. The referenced components can be local or remote and the SCA runtime will ensure that these references are correctly set at runtime based on the wiring found in the completed application's SCDL files (shown later in this article). See Listing 1.

Component Interfaces
The business functions provided by a service or required by a reference are described using interfaces in SCA. The interfaces represent the contract for a service or reference. Java and WSDL are two typical interface definition languages.

@Remotable
public interface CreditCheck {
    int getCreditScore(String ssn);
}

Interfaces can be local or remotable. Local interfaces are the most optimized for local interactions between components in the same composite. In contrast, remotable interfaces can be used for loosely coupled remote interactions.

Some business services have peer-to-peer relationships that require a two-way dependency at the service level. In these cases, the business service represents both a consumer of a service provided by a partner business service and a provider of a service to the partner business service. This is especially the case when the interactions are based on asynchronous messaging rather than on remote procedure calls. SCA uses bi-directional interfaces to directly model peer-to-peer bi-directional business service relationships.

For some services a sequence of operations must be called to achieve some higher-level goal. The sequence of operations is referred to as conversation. If the service uses a bi-directional interface, the conversation may include both operations and callbacks. SCA allows interfaces to be marked as conversational to bracket the series of operations in the same conversation.

Component Properties
Component properties can be used to alter the behavior of a component at runtime without making code changes. Let's assume that the LoanApproval component has a component property called "minimumCreditScore," which can be set to different values based on company policy. Below is a code snippet from the LoanApproval component implementation that uses an @Property annotation to identify a property called minimumCreditScore. The property has a default value of 650:

private int minimumCreditScore = 650;

// Property declaration using a setter method
@Property(name = "minimumCreditScore", override = "may")
public void setMinimumCreditScore(int minimumCreditScore) {
    this.minimumCreditScore = minimumCreditScore;
}

The following illustrates customization of the component by setting the "minimumCreditScore" property to 600 in the composite SCDL file to override the default value (650) defined in the component type (remember that we're using Java language annotations to define the component type):

<component name="LoanApprovalComponent">
    <implementation.java class="mortgage.LoanApprovalImpl" />
    <property name="minimumCreditScore">600</property>
    ...
</component>

Composites - Composing Components
So far we've concentrated on developing individual components, making them available as services and defining their dependencies on other services. Now let's look at how the components can be assembled to provide a business solution. This is referred to as a composite, which is a logical concept. A composite contains one or more components (see Figure 5).

If we look at the composite file (default.scdl) for MortgageComposite we can see how this draws all of the components together.

The SCA runtime uses the information in this SCDL file to instantiate, assemble, and configure the components. As can be seen from the example each component is identified by a <component> element in the file and can have references to other components. In this example, LoanApprovalComponent has four <reference> elements that are wired to four other components in the composite. The wiring is depicted through arrows in the diagram. The interfaces on both sides of the wire have to be compatible.

A composite can be reused as a component in the assembly but we don't show an example in this article.

Local Services
The composite file we've just seen shows how component references are "wired" to other components. There's no information included in this composite file to describe what techniques should be used to pass messages between the components. In this case SCA assumes that the components will be local to one another, i.e., they'll be instantiated and run in the same process address space. As we're using the Java language in this example the component instances will run in the same Java VM. SCA is free in this case to use the most efficient mechanism for moving a message from one component to another. This is likely to be a direct component-to-component call with little or no mediation.

On the face of it composition of components using local wiring may not appear to be very useful. Why not simply code these components as normal Java classes and have them interact in the normal way? In the case of coarse-grain components SCA has a number of advantages.

• Components can easily be reused and reconfigured in other compositions
• Components that are local today can be made remote tomorrow
• Components implemented using different supported programming languages can easily be assembled