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Simple Object Access Protocol (SOAP)

This section looks at the history, design center, and core capabilities of SOAP as a means for establishing the base on which to build our understanding of Web services.

The Making of SOAP

Microsoft started thinking about XML-based distributed computing in 1997. The goal was to enable applications to communicate via Remote Procedure Calls (RPCs) on top of HTTP. DevelopMentor and Userland joined the discussions. The name SOAP was coined in early 1998. Things moved forward, but as the group tried to involve wider circles at Microsoft, politics stepped in and the process was stalled. The DCOM camp at the company disliked the idea of SOAP and believed that Microsoft should use its dominant position in the market to push the DCOM wire protocol via some form of HTTP tunneling instead of pursuing XML. Some XML-focused folks at Microsoft believed that the SOAP idea was good but that it had come too early. Perhaps they were looking for some of the advanced facilities that could be provided by XML Schema and Namespaces. Frustrated by the deadlock, Userland went public with a cut of the spec published as XML-RPC in the summer of 1998.

In 1999, as Microsoft was working on its version of XML Schema (XML Data) and adding support for namespaces in its XML products, the idea of SOAP gained additional momentum. It was still an XML-based RPC mechanism, however. That's why it met with resistance from the BizTalk (http://www.biztalk.org) team. The BizTalk model was based more on messaging than RPCs. It took people a few months to resolve their differences. SOAP 0.9 appeared for public review on September 13, 1999. It was submitted to the IETF as an Internet public draft. With few changes, in December 1999, SOAP 1.0 came to life.

On May 8, 2000 SOAP 1.1 was submitted as a Note to the World Wide Web Consortium (W3C) with IBM as a co-author—an unexpected and refreshing change. In addition, the SOAP 1.1 spec was much more extensible, eliminating concerns that backing SOAP implied backing some Microsoft proprietary technology. This change, and the fact that IBM immediately released a Java SOAP implementation that was subsequently donated to the Apache XML Project (http://xml.apache.org) for open-source development, convinced even the greatest skeptics that SOAP is something to pay attention to. Sun voiced support for SOAP and started work on integrating Web services into the J2EE platform. Not long after, many vendors and open-source projects were working on Web service implementations.

Right before the XTech 2000 Conference, the W3C made an announcement that it was looking into starting an activity in the area of XML protocols: "We've been under pressure from many sources, including the advisory board, to address the threat of fragmentation of and investigate the exciting opportunities in the area of XML protocols. It makes sense to address this now because the technology is still early in its evolution..." (http://lists.w3.org/Archives/Public/xml-dist-app/2000Feb/0006.html). On September 13, 2000 the XML Protocol working group at the W3C was formed to design the core XML protocol that was to become the core of XML-based distributed computing in the years to come. The group started with SOAP 1.1 as a foundation and produced the first working draft of SOAP 1.2 on July 9, 2001.

What Should SOAP Do?

SOAP claims to be a specification for a ubiquitous XML distributed computing infrastructure. It's a nice buzzword-compliant phrase, but what does it mean? Let's parse it bit by bit to find out what SOAP should do.

XML means that, as a second-generation XML protocol, SOAP is based on XML 1.0, XML Schema, and XML Namespaces.

Distributed computing implies that SOAP can be used to enable the interoperability of remote applications (in a very broad sense of the phrase). Distributed computing is a fuzzy term and it means different things to different people and in different situations. Here are some "facets" you can use to think about a particular distributed computing scenario: the protocol stack used for communication, connection management, security, transaction support, marshalling and unmarshalling of data, protocol evolution and version management, error handling, audit trails, and so on. The requirements for different facets will vary between scenarios. For example, a stock ticker service that continuously distributes stock prices to a number of subscribers will have different needs than an e-commerce payment-processing service. The stock ticker service will probably need no support for transactions and only minimal, if any, security or audit trails (it distributes publicly available data). The e-commerce payment-processing service will require Cerberean security, heavy-duty transaction support, and full audit trails.

Infrastructure implies that SOAP is aimed at low-level distributed systems developers, not developers of application/business logic or business users. Infrastructure products such as application servers become "SOAP enabled" by including a Web service engine that understands SOAP. SOAP works behind the scenes making sure your applications can interoperate without your having to worry too much about it.

Ubiquitous means omnipresent, universal. On first look, it seems to be a meaningless term, thrown into the phrase to make it sound grander. It turns out, however, that this is the most important part. The ubiquity goal of SOAP is a blessing because, if SOAP-enabled systems are everywhere on the Internet, it should be easier to do distributed computing. After all, that's what SOAP is all about. However, the ubiquity of SOAP is also a curse, because one technology specification should be able to support many different types of distributed computing scenarios, from the stock ticker service to the e-commerce payment-processing service. To meet this goal, SOAP needs to be a highly abstract and flexible technology. However, the more abstract SOAP becomes, the less support it will provide for specific distributed computing scenarios. Furthermore, greater abstraction means more risk that different SOAP implementations will fail to interoperate. This is the eternal tug-of-war between generality and specificity.

What Is SOAP, Really?

Like most new technologies that change the rules of how applications are being developed, Web services and SOAP have sometimes been over-hyped. Despite the hype, however, SOAP is still of great importance because it is the industry's best effort to date to standardize on the infrastructure technology for cross-platform XML distributed computing.

Above all, SOAP is relatively simple. Historically, simplicity is a key feature of most successful architectures that have achieved mass adoption. The Web with HTTP and HTML at its core is a prime example. Simple systems are easier to describe, understand, implement, test, maintain, and evolve. At its heart, SOAP is a specification for a simple yet flexible second-generation XML protocol. SOAP 1.0 printed at about 40 pages. The text of the specification has grown since then (the authors have to make sure the specification is clear and has no holes), but the core concepts remain simple.

Because SOAP is focused on the common aspects of all distributed computing scenarios, it provides the following:

  • A mechanism for defining the unit of communication. In SOAP, all information is packaged in a clearly identifiable SOAP message. This is done via a SOAP envelope that encloses all other information. A message can have a body in which potentially arbitrary XML can be used. It can also have any number of headers that encapsulate information outside the body of the message.

  • A mechanism for error handling that can identify the source and cause of the error and allows for error-diagnostic information to be exchanged between participants of an interaction. This is done via the notion of a SOAP fault.

  • An extensibility mechanism so that evolution is not hindered and there is no lock-in. XML, schemas, and namespaces really shine here. The two key requirements on extensions are that they can be orthogonal to other extensions and they can be introduced and used without the need for centralized registration or coordination. Typically, extensions are introduced via SOAP headers. They can be used to build more complex protocols on top of SOAP.

  • A flexible mechanism for data representation that allows for the exchange of data already serialized in some format (text, XML, and so on) as well as a convention for representing abstract data structures such as programming language datatypes in an XML format.

  • A convention for representing Remote Procedure Calls (RPCs) and responses as SOAP messages, because RPCs are the most common type of distributed computing interaction and because they map so well to procedural programming language constructs.

  • A document-centric approach to reflect more natural document exchange models for business interactions. This is needed to support the cases in which RPCs result in interfaces that are too fine grained and, therefore, brittle.

  • A binding mechanism for SOAP messages to HTTP, because HTTP is the most common communication protocol on the Internet.

Although solid consensus exists in the industry about the core capabilities of SOAP, there is considerably less agreement on how higher-level issues such as security and transaction-management should be addressed. Nearly everyone agrees that to tackle the broad spectrum of interesting problems we are faced with, we need to work in parallel on a set of layered specifications for XML distributed computing. Indeed, many loosely coupled industry initiatives are developing standards and technologies around SOAP. Tracking these efforts is like trying to shoot at many moving targets. The authors of this book have tried our best to address the relevant efforts in this space and to provide you with up-to-date information. Chapter 1 showed how many of these efforts layered around the notion of the Web services interoperability stack. Chapter 5, "Using SOAP for e-Business," goes into more detail about the set of standards surrounding SOAP that enable secure, robust, and scalable enterprise-grade Web services.

Now, let's take a look at how SkatesTown is planning to use SOAP and Web services.

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