What are web services?

Database and Network Journal, June, 2002 by David A. Chappel, Tyler Jewell

A web service is a piece of business logic, located somewhere on the Internet, that is accessible through standard-based interact protocols such as HTTP or SMTP. Using a web service can be as simple as logging into a site or as complex as facilitating a multi-organization business negotiation.

Given this definition, several technologies used in recent years could have been classified as web service technology, but were not. These technologies include win32 technologies, J2EE, CORBA, and CGI scripting. The major difference between these technologies and the new breed of technology that are labelled as web services is their standardization. This new breed of technology is based on standardized XML (as opposed to a proprietary binary standard) and supported globally by most major technology firms. XML provides a language-neutral way for representing data, and the global corporate support ensures that every major new software technology will have a web services strategy within the next couple years. When combined, the software integration and interoperability possibilities for software programs using the web services model are staggering.

A web service has special behavioral characteristics: XML-based

By using XML as the data representation layer for all web services protocols and technologies that are created, these technologies can be interoperable at their core level. As a data transport, XML eliminates any networking, operating system, or platform binding that a protocol has.

Loosely coupled

A consumer of a web service is not tied to that web service directly; the web service interface can change over time without compromising the client's ability to interact with the service. A tightly coupled system implies that the client and server logic are closely tied to one another, implying that if one interface changes, the other must also be updated. Adopting a loosely coupled architecture tends to make software systems more manageable and allows simpler integration between different systems.

Coarse-grained

Object-oriented technologies such as Java expose their services through individual methods. An individual method is too fine an operation to proved any useful capability at a corporate level. Building a Java program from scratch requires the creation of several fine-grained methods that are then composed into a coarse-grained service that is consumed by either a client or another service. Businesses and the interfaces that they expose should be coarse-grained. Web services technology provides a natural way of defining coarse-grained services that access the right amount of business logic.

Ability to be synchronous or asynchronous

Synchronicity refers to the binding of the client to the execution of the service. In synchronous invocations, the client blocks and waits for the service to complete its operation before continuing. Asynchronous operations allow a client to invoke a service and then execute other functions. Asynchronous clients retrieve their result at a later point in time, while synchronous clients receive their result when the service has completed. Asynchronous capability is a key factor in enabling loosely coupled systems.

Supports Remote Procedure Calls (RPCS)

Web services allow clients to invoke procedures, functions, and methods on remote objects using an XML-based protocol. Remote procedures expose input and output parameters that a web service must support. Component development through Enterprise javabeans (EJBS) and .NET Components has increasingly become a part of architectures and enterprise deployments over the past couple of years. Both technologies are distributed and accessible through a variety of RPC mechanisms. A web service supports RPC by providing services of its own, equivalent to those of a traditional component, or by translating incoming invocations into an invocation of an EJB or a .NET component.

Supports document exchange

One of the key advantages of XML is its generic way of representing not only data, but also complex documents. These documents can be simple, such as when representing a current address, or they can be complex, representing an entire book or FRQ. Web services support the transparent exchange of documents to facilitate business integration.

The Major Web Services Technologies

Several technologies have been introduced under the web service rubric and many more will be introduced in coming years. In fact, the web service paradigm has grown so quickly that several competing technologies are attempting to provide the same capability. However, the web service vision of seamless worldwide business integration is not be feasible unless the core technologies are supported by every major software company in the world.

Over the past two years, three primary technologies have emerged as worldwide standards that make up the core of today's web services technology. These technologies are:

Simple Object Access Protocol (SOAP)

SOAP provides a standard packaging structure for transporting XML documents over a variety of standard Internet technologies, including SMTP, HTTP, and FTP. It also defines encoding and binding standards for encoding non-XML RPC invocations in XML for transport. SOAP provides a simple structure for doing RPC document exchange. By having a standard transport mechanism, heterogeneous clients and servers can suddenly become interoperable. .NET clients can invoke EJBs exposed through SOAP, and Java clients can invoke .NET Components exposed through SOAP.