SOAP was originally an acronym for Simple Object Access Protocol. (Now it's just a name.) SOAP 1.1 is the standard messaging protocol used by J2EE Web Services, and is the de facto standard for Web services in general. SOAP's primary application is Application-to-Application (A2A) communication. Specifically, it's used in Business-to-Business (B2B) and Enterprise Application Integration (EAI), which are two sides of the same coin: Both focus on integrating software applications and sharing data. To be truly effective in B2B and EAI, a protocol must be platform-independent, flexible, and based on standard, ubiquitous technologies. Unlike earlier B2B and EAI technologies, such as CORBA and EDI, SOAP meets these requirements, enjoys widespread use, and has been endorsed by most enterprise software vendors and major standards organizations (W3C, WS-I, OASIS, etc.).

Despite all the hoopla, however, SOAP is just another XML markup language accompanied by rules that dictate its use. SOAP has a clear purpose: exchanging data over networks. Specifically, it concerns itself with encapsulating and encoding XML data and defining the rules for transmitting and receiving that data. In a nutshell, SOAP is a network application protocol.

A SOAP XML document instance, which is called a SOAP message,[1] is usually carried as the payload of some other network protocol. For example, the most common way to exchange SOAP messages is via HTTP (HyperText Transfer Protocol), used by Web browsers to access HTML Web pages. The big difference is that you don't view SOAP messages with a browser as you do HTML. SOAP messages are exchanged between applications on a network and are not meant for human consumption. HTTP is just a convenient way of sending and receiving SOAP messages.

[1] The SOAP XML document is also called the SOAP envelope.

SOAP messages can also be carried by e-mail using SMTP (Simple Mail Transfer Protocol) and by other network protocols, such as FTP (File Transfer Protocol) and raw TCP/IP (Transmission Control Protocol/Internet Protocol). At this time, however, the WS-I Basic Profile 1.0 sanctions the use of SOAP only over HTTP. Figure illustrates how SOAP can be carried by various protocols between software applications on a network.

SOAP over HTTP, SMTP, and Raw TCP/IP


Web services can use One-Way messaging or Request/Response messaging. In the former, SOAP messages travel in only one direction, from a sender to a receiver. In the latter, a SOAP message travels from the sender to the receiver, which is expected to send a reply back to the sender. Figure illustrates these two forms of messaging.

One-Way versus Request/Response Messaging


SOAP defines how messages can be structured and processed by software in a way that is independent of any programming language or platform, and thus facilitates interoperability between applications written in different programming languages and running on different operating systems. Of course, this is nothing new: CORBA IIOP and DCE RPC also focused on cross-platform interoperability. These legacy protocols were never embraced by the software industry as a whole, however, so they never became pervasive technologies. SOAP, on the other hand, has enjoyed unprecedented acceptance, and adoption by virtually all the players in distributed computing, including Microsoft, IBM, Sun Microsystems, BEA, HP, Oracle, and SAP, to name a few.

The tidal wave of support behind SOAP is interesting. One of the main reasons is probably its grounding in XML. The SOAP message format is defined by an XML schema, which exploits XML namespaces to make SOAP very extensible. Another advantage of SOAP is its explicit definition of an HTTP binding, a standard method for HTTP tunneling. HTTP tunneling is the process of hiding another protocol inside HTTP messages in order to pass through a firewall unimpeded. Firewalls will usually allow HTTP traffic through port 80, but will restrict or prohibit the use of other protocols and ports.

A port is a communication address on a computer that complements the Internet address. Each network application on a computer uses a different port to communicate. By convention, Web servers use port 80 for HTTP requests, but application servers can use any one of thousands of other ports.

The power that comes from XML's extensibility and the convenience of using the ubiquitous, firewall-immune HTTP protocol partly explain SOAP's success. It's difficult to justify SOAP's success purely on its technical merits, which are good but less than perfect. Another factor in SOAP's success is the stature of its patrons. SOAP is the brainchild of Dave Winner, Don Box, and Bob Atkinson. Microsoft and IBM supported it early, which sent a strong signal to everyone else in the industry: "If you want to compete in this arena, you better jump aboard SOAP." The event that secured industry-wide support for SOAP was its publication by the World Wide Web Consortium as a Note[2] in May of 2000, making it the de facto standard protocol for A2A messaging. Overnight, SOAP became the darling of distributed computing and started the biggest technology shift since the introduction of Java in 1995 and XML in 1998. SOAP is the cornerstone of what most people think of as Web services today, and will be for a long time to come.

[2] In the W3C standardization process, a Note does not represent commitment by the W3C to pursue work related to the technology it describes, but the W3C has taken responsibility for SOAP 1.2 and is working to make it an official recommendation, which is the highest level of endorsement offered by the W3C.

Recently, the W3C has defined a successor to SOAP 1.1. SOAP 1.2 does a decent job of tightening up the SOAP processing rules and makes a number of changes that will improve interoperability. SOAP 1.2 is very new and has not yet been widely adopted, however, so it's not included in the WS-I Basic Profile 1.0. This exclusion is bound to end when the BP is updated, but for now J2EE 1.4 Web Services, which adheres to the WS-I Basic Profile 1.0, does not support the use of SOAP 1.2.

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