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Understanding Service-Oriented Architecture

  • July 8, 2005
  • By Michael Stevens
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Service-oriented architecture stresses interoperability, the ability of systems using different platforms and languages to communicate with each other. Each service provides an interface that can be invoked through a connector type. An interoperable connector consists of a protocol and a data format that each of the potential clients of the service understands. Interoperability is achieved by supporting the protocol and data formats of the service's current and potential clients.

Techniques for supporting standard protocol and data formats consist of mapping each platform's characteristics and language to a mediating specification. The mediating specification maps between the formats of the interoperable data format to the platform-specific data formats. Sometimes this requires mapping character sets such as ASCII to EBCDIC as well as mapping data types. For instance, Web services is a mediating specification for communicating between systems. JAX-RPC and JAXM map Java data types to SOAP. Other platforms that support Web services mediate between Web service specifications and their own internal specifications for character sets and data types.

Loose Coupling

Coupling refers to the number of dependencies between modules. There are two types of coupling: loose and tight. Loosely coupled modules have a few well-known dependencies. Tightly coupled modules have many unknown dependencies. Every software architecture strives to achieve loose coupling between modules. Service-oriented architecture promotes loose coupling between service consumers and service providers and the idea of a few well-known dependencies between consumers and providers.

A system's degree of coupling directly affects its modifiability. The more tightly coupled a system is, the more a change in a service will require changes in service consumers. Coupling is increased when service consumers require a large amount of information about the service provider to use the service. In other words, if a service consumer knows the location and detailed data format for a service provider, the consumer and provider are more tightly coupled. If the consumer of the service does not need detailed knowledge of the service before invoking it, the consumer and provider are more loosely coupled.

SOA accomplishes loose coupling through the use of contracts and bindings. A consumer asks a third-party registry for information about the type of service it wishes to use. The registry returns all the services it has available that match the consumer's criteria. The consumer chooses which service to use, binds to it over a transport, and executes the method on it, based on the description of the service provided by the registry. The consumer does not depend directly on the service's implementation but only on the contract the service supports. Since a service may be both a consumer and a provider of some services, the dependency on only the contract enforces the notion of loose coupling in service-oriented architecture.

Although coupling between service consumers and service producers is loose, implementation of the service can be tightly coupled with implementation of other services. For instance, if a set of services shares a framework, a database, or otherwise has information about each other's implementation, they may be tightly coupled. In many instances, coupling cannot be avoided, and it sometimes contradicts the goal of code reusability.

Network-Addressable Interface

The role of the network is central to the concept of SOA. A service must have a network-addressable interface. A consumer on a network must be able to invoke a service across the network. The network allows services to be reused by any consumer at any time. The ability for an application to assemble a set of reusable services on different machines is possible only if the services support a network interface. The network also allows the service to be locationindependent, meaning that its physical location is irrelevant.

It is possible to access a service through a local interface and not through the network, but only if both the consumer and service provider are on the same machine. This is done mainly to enhance performance. Although a service may be configured for access from a consumer on the same machine, the service must also simultaneously support a request from across the network.

Because of this requirement, service interface design is focused to a large extent on performance. In a pure object-based system design, data and behavior are encapsulated into objects. This design works well for objects in the same machine. However, when those objects are distributed across a network, performance degrades quickly because of the "chatter" that occurs between fine-grained objects. Because we can assume that services will be distributed, it is possible to design service interfaces to be more coarse-grained and, as a result, enhance network performance.

Coarse-Grained Interfaces

The concept of granularity applies to services in two ways. First, it is applied to the scope of the domain the entire service implements. Second, it is applied to the scope of the domain that each method within the interface implements.

The levels of granularity are relative to each other. For instance, if a service implements all the functions of a banking system, then we consider it coarse-grained. If it supports just credit-card validation, we consider it fine-grained. In addition, if a method for inquiring about a customer returns all customer information, including address, this method would be coarser-grained than a method that does not return the customer's address.

The appropriate level of granularity for a service and its methods is relatively coarse. A service generally supports a single distinct business concept or process. It contains software that implements the business concept so that it can be reused in multiple large, distributed systems.

Before components and services, distributed systems were centered on the idea of distributed objects ( Object Management Group 2002). Distributed object-based systems consist of many fine-grained networked objects communicating with each other across a network. Each object has dependencies with many other objects in the system. Since accessing an object requires a network hop and thus does not perform well, the design principles for distributed object-based systems quickly moved toward coarser-grained interfaces.

Figure 6 illustrates a distributed object-based system. The number of connections between objects is great. As system size and complexity grows, these dependencies become difficult to manage. Performance suffers because of the large number of network hops. Maintainability also suffers because of the large number of dependencies between objects. Since any object can connect to and use any other object, it becomes difficult to know what dependencies exist. When the developer makes a necessary change to an interface, it might affect a large number of other distributed objects. The developer must then compile and deploy together all the changed objects and the objects that depend on them.

Click here for a larger image.

Figure 6 Fine-grained distributed objects.

A service-based system controls the network access to the objects within the service through a set of coarse-grained interfaces, as shown in Figure 7. A service may still be implemented as a set of fine-grained objects, but the objects themselves are not accessible over a network connection. A service implemented as objects has one or more coarse-grained objects that act as distributed façades. These objects are accessible over the network and provide access to the internal object state from external consumers of the service. However, objects internal to the service communicate directly with each other within a single machine, not across a network connection. All service interfaces are relatively coarsegrained compared with distributed object interfaces. However, within the range of coarse, there are options. It is important to understand these options for interface design.

Click here for a larger image.

Figure 7 Coarse-grained services.

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