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Defining Service Interfaces

Recalling from the previous chapter that components interact through services, we start by defining the service interfaces for the LoanComponent and CreditComponent components. Because both components are implemented in Java, we use Java to define their service interfaces. The LoanService interface is shown in Listing 2.1.

Listing 2.1. The LoanService Interface

@Remotable

public interface LoanService  {


      LoanResult apply(LoanRequest request);

}

The CreditService interface is presented in Listing 2.2.

Listing 2.2. The CreditService Interface

@Remotable

public interface CreditService {


      int checkCredit(String id);
}

LoanService defines one operation, apply(..), which takes a loan application as a parameter. CreditService defines one operation, checkCredit(..), which takes a customer ID and returns a numerical credit score. Both interfaces are marked with an SCA annotation, @Remotable, which specifies that both services may be invoked by remote clients (as opposed to clients in the same process). Other than the @Remotable annotations, the two service contracts adhere to basic Java.

Using Web Services Description Language (WSDL)

In the previous example, we chose Java to define the service contracts for LoanService and CreditService because it is easy to develop in, particularly when an application is mostly implemented in Java. There are other times, however, when it is more appropriate to use a language-neutral mechanism for defining service contracts. There are a number of interface definition languages, or IDLs, for doing so, but Web Services Description Language (WSDL) is the most accepted for writing new distributed applications. Although labeled as a “web services” technology, WSDL is in fact an XML-based way of describing any service—whether it is exposed to clients as web services—that can be used by most modern programming languages. To understand why WSDL would be used with SCA, we briefly touch on the role it plays in defining service interfaces.

WSDL serves as the lingua franca for code written in one language to invoke code written in another language. It does this by defining a common way to represent operations (what can be invoked), message types (the input and output to operations), and bindings to a protocol or transport (how operations must be invoked). WSDL uses other technologies such as XML Schema to define message types and SOAP for how invocations are sent over a transport layer (for example, HTTP). Programming languages define mappings to WSDL, making it possible for languages with little in common to communicate, as represented in Figure 2.3.

Figure 2.3

Figure 2.3 WSDL is used to map operations and data types.

Writing WSDL by hand is generally not a pleasant experience; for anything but trivial interfaces, it is a tedious process. Briefly compare the LoanService interface previously defined using Java to its WSDL counterpart (see Listing 2.3).

Listing 2.3. The LoanService WSDL

<?xml version="1.0" encoding="utf-8"?>
<wsdl:definitionsxmlns:ns1="http://loanservice.loanapp/"
xmlns:soap="http://schemas.xmlsoap.org/wsdl/soap/"
xmlns:wsdl="http://schemas.xmlsoap.org/wsdl/"
xmlns:xsd="http://www.w3.org/2001/XMLSchema"
                  name="LoanService" targetNamespace="http://loanservice.loanapp/">
    <wsdl:message name="applyResponse">
        <wsdl:part element="ns1:applyResponse" name="parameters">
        </wsdl:part>
    </wsdl:message>
    <wsdl:message name="apply">
        <wsdl:part element="ns1:apply" name="parameters">
        </wsdl:part>
    </wsdl:message>
    <wsdl:portType name="LoanServicePortType">
        <wsdl:operation name="apply">
            <wsdl:input message="ns1:apply" name="apply">
            </wsdl:input>
            <wsdl:output message="ns1:applyResponse"
name="applyResponse">
            </wsdl:output>
        </wsdl:operation>
    </wsdl:portType>
</wsdl:definitions>

Fortunately, SCA does not require WSDL to define service interfaces. Why, then, would someone choose to use WSDL? One scenario where WSDL is used is in top-down development. This style of development entails starting by defining an overall system design, including subsystems and the services they offer, in a way that is independent of the implementation technologies used. WSDL is a natural fit for this approach as it defines service interfaces without specifying how they are to be implemented. In this scenario, an architect could define all service interfaces upfront and provide developers with the WSDLs to implement them.

Few development organizations follow this top-down approach. Typically, service development is iterative. A more practical reason for starting with WSDL is to guarantee interoperability. If a service is created using language-specific means such as a Java interface, even if it is translated into WSDL by tooling, it may not be compatible with a client written in a different language. Using carefully hand-crafted WSDL can reduce this risk.

A third reason to use hand-crafted WSDL is to better accommodate service versioning. Services exposed to remote clients should be designed for loose-coupling. An important characteristic of loose-coupling is that those services should work in a world of mismatched versions where a new version of a service will be backward compatible with old clients. Because WSDL uses XML Schema to define operation parameters, maintaining backward compatibility requires that the parameter-type schemas be designed to handle versioning. This is difficult to do directly in schema but even more difficult using Java classes. In cases where support for versioning is paramount, working directly with WSDL may be the least complex alternative.

One question people typically raise is if SCA does not mandate the use of WSDL, how can it ensure that two components written in different languages are able to communicate? SCA solves this problem by requiring that all interfaces exposed to remote clients be translatable into WSDL. For example, if a service interface is defined using Java, it must be written in such a way that it is possible to represent it in WSDL. This enables a runtime to match a client and service provider by mapping each side to WSDL behind the scenes, saving developers the task of doing this manually.

Given that SCA services available to remote clients must be translatable into WSDL, it is important to note that the latter imposes several restrictions on interface definitions. WSDL stipulates that service interfaces must not make use of operator overloading; in other words, they must not have multiple operations with the same name but different message types. WSDL also requires operation parameters to be expressible using XML Schema. The latter restriction is, in practice, not overly burdensome. Although it might disallow certain data types (for example, Java’s InputStream), virtually all data types suitable for loosely coupled service interactions can be accommodated by XML Schema. The next chapter will discuss service contract design in detail; for now, it is important to remember these two constraints for services exposed to remote clients.

Remotable Versus Local Services

Returning to the LoanService and CreditService interfaces, both are annotated with @Remotable, which indicates that a service may, but need not be, accessed remotely. For contracts defined using Java, SCA requires that any service exposed across a process boundary be explicitly marked as remotable. Services not marked as remotable—the default case—are local services: They are callable only from clients hosted in the same process. In contrast, service interfaces defined by WSDL are remotable by default. This makes sense given that most contracts defined by WSDL are likely to be intended for remote access.

Requiring service contracts to be explicitly marked as remotable indicates which services are designed to be accessible across process boundaries. The distinction is necessary because local and remotable services have different behavior. The next chapter covers these differences at length, which we briefly describe here.

Remotable Services Must Account for Network Latency

Clients of remotable services must accommodate network latency. This means that remotable services should be coarse-grained—that is, they should contain few operations that are passed larger data sets, as opposed to a number of individual operations that take a small number of parameters. This reduces the degree of network traffic and latency experienced by clients. In addition, remotable services often define asynchronous operations as a way to handle network latency and service interruptions. Local services are not subject to these demands as calls occur in the same process. Therefore, they tend to be finer-grained and use synchronous operations.

Clients of Remotable Services May Experience Communications Failures

Because invocations on remotable services generally travel over a network, there is a possibility communications may be interrupted. In SCA, the unchecked org.osoa.sca.ServiceUnavailableException exception will be thrown if a communication error occurs. Clients need to handle such exceptions, potentially by retrying or reporting an error.

Remotable Services Parameters Are Passed by Value

Parameters associated with remotable service operations behave differently than those of operations on local services. When remotable invocations are made, parameters are marshaled to a protocol format such as XML and passed over a network connection. This results in a copy of the parameters being made as the invocation is received by the service provider. Consequently, modifications made by the service provider will not be seen by the client. This behavior is termed “pass-by-value.” In contrast, because invocations on local services are made in the same process, operation parameters are not copied. Any changes made by the service provider will be visible to the client. This behavior is known as “pass-by-reference.” Marking a service as remotable signals to clients whether pass-by-value or pass-by-reference semantics will be in effect.

Table 2.1 summarizes the differences between remotable and local services.

Table 2.1. Remotable Versus Local Services

Remotable Services

Local Services

Are invoked in-process and remotely.

Are always invoked in-process.

Parameters are pass-by-value.

Parameters are pass-by-reference.

Are coarse-grained.

Tend to be fine-grained.

Are loosely coupled and favor asynchronous operations.

Commonly use synchronous operations.

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