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3.3 Simple Object Access Protocol (SOAP)

Although originally conceived as a technology to bridge the gap between disparate RPC-based communication platforms, SOAP has evolved into the most widely supported messaging format and protocol for use with XML Web services. Hence the SOAP acronym is frequently referred to as the Service-Oriented Architecture (or Application) Protocol, instead of the Simple Object Access Protocol.

The SOAP specification establishes a standard message format that consists of an XML document capable of hosting RPC and document-centric data (see Figure 3.19). This facilitates synchronous (request and response) as well as asynchronous (process-driven) data exchange models. With WSDL establishing a standard endpoint description format for applications, the document-centric message format is much more common.

03fig19.gifFigure 3.19 SOAP establishes two primary standard message formats.

The architectures we explore throughout this book make reference to both types of message formats using the standard diagram symbols provided in Figures 3.20 and 3.21.

03fig20.gifFigure 3.20 The symbol used to represent a SOAP message with a document-centric payload.

03fig21.gifFigure 3.21 The symbol used to represent a SOAP message with an RPC-centric payload.

Additionally, we discuss the use of SOAP message attachments, which are described in separate second-generation Web services specifications. (Specifically, the WS-Attachments and SOAP Messages with Attachments (SwA) standards, are covered in Chapter 4.) SOAP messages containing attachments are represented with the symbol shown in Figure 3.22.

03fig22.gifFigure 3.22 The symbol used to represent a SOAP message delivering its data as an attachment.

3.3.1 SOAP messaging framework

SOAP institutes a method of communication based on a processing model that is in relative alignment with the overall Web services framework described at the beginning of this chapter. It differs only in that it introduces terms and concepts that relate specifically to the manner in which SOAP messages need to be handled (within a technical implementation of this framework).

Note that the diagram symbols used to identify SOAP nodes in the following sections are not displayed in other chapters. Their existence is implied in subsequent architectural diagrams that include Web services.

SOAP nodes

A SOAP node represents the processing logic responsible for transmitting, receiving, and performing a variety of processing tasks on SOAP messages. An implementation of a SOAP node is typically platform specific, and is commonly labeled as a SOAP server or a SOAP listener. Specialized variations also exist, such as SOAP routers. Conceptually, all are considered SOAP nodes. Figure 3.23 establishes the SOAP node as the underlying transport mechanism for a Web service.

03fig23.gifFigure 3.23 A SOAP node.

SOAP nodes are best viewed as the technical infrastructure that powers all of the communication scenarios explored in the earlier section, “Web service interaction.” When discussing the involvement of SOAP nodes, however, a slight departure from the terms and concepts established by the Web services framework needs to be incorporated.

SOAP node types

Like Web services, SOAP nodes can exist as initial senders, intermediaries, and ultimate receivers. Whereas Web services are also classified as requestors and providers, SOAP nodes performing the equivalent tasks (sending, receiving) are referred to as SOAP senders and SOAP receivers (see Figure 3.24). The SOAP specification, however, does not classify these as roles. For the purpose of this book, we'll refer to them as “types” of SOAP nodes.

03fig24.gifFigure 3.24 Fundamental SOAP node types along a message path.

As illustrated in Figure 3.25, a node type of SOAP initial sender is also a SOAP sender, and the SOAP ultimate receiver is also a SOAP receiver.

03fig25.gifFigure 3.25 SOAP nodes with multiple types.

Figure 3.26 shows how a SOAP node, acting as an intermediary, transitions through both SOAP sender and receiver types during the processing of a SOAP message.

03fig26.gifFigure 3.26 A SOAP node going through the transition of being a SOAP receiver and sender during the processing of a SOAP message.

As with Web service roles, the same SOAP node can act as different types depending on its position within the message path and the state of the current business activity. For instance, a SOAP node transmitting a message as the initial sender can later receive a response as the ultimate receiver.

Roles for SOAP nodes also exist, but are described separately at the end of this section.

3.3.2 SOAP message structure

The container of SOAP message information is referred to as a SOAP envelope. Let's open it and take a brief look at the underlying structure of a typical SOAP message.

The root Envelope element that frames the message document consists of a mandatory body section and an optional header area.

Example 3.9. A skeleton envelope construct

<env:Envelope xmlns:env="http://www.w3.org/2003/05/soap-envelope">

The SOAP header is expressed using the Header construct, which can contain one or more sections or blocks.

Example 3.10. The Header construct with a header block

<env:Envelope xmlns:env="http://www.w3.org/2003/05/soap-envelope">
    <n:shipping >

Common uses of header blocks include:

  • implementation of (predefined or application-specific) SOAP extensions, such as those introduced by second-generation specifications

  • identification of target SOAP intermediaries

  • providing supplementary meta information about the SOAP message

While a SOAP message progresses along a message path, intermediaries may add, remove, or process information in SOAP header blocks. Although an optional part of a SOAP message, the use of the header section to carry header blocks is commonplace when working with second-generation Web services specifications.

The one part of a SOAP message that is not optional is the body. As represented by the Body construct, this section acts as a container for the data being delivered by the SOAP message. Data within the SOAP body is often referred to as the payload or payload data.

Example 3.11. The Body construct

<env:Envelope xmlns:env="http://www.w3.org/2003/05/soap-envelope">
    <x:Book xmlns:x="http://www.examples.ws/">
        Service-Oriented Architecture
        A Field Guide to Integrating XML
        and Web services

The Body construct can also be used to host exception information within nested Fault elements. Although fault sections can reside alongside standard data payloads, this type of information is often sent separately in response messages that communicate error conditions.

The Fault construct consists of a series of system elements used to identify characteristics of the exception.

Example 3.12. A sample fault construct providing error information

<env:Envelope xmlns:env="http://www.w3.org/2003/05/soap-envelope">
        <env:Text xml:lang="en">
          versions do not match

SOAP node roles

Now that you've had a look at the internal structure and syntax of a SOAP message, let's finish by briefly introducing SOAP node roles. When discussing SOAP nodes, roles relate to an optional env:role3 attribute that a SOAP message can use to identify header blocks intended for specific types of SOAP receivers. Therefore, SOAP roles are associated only to types of SOAP nodes that perform a receiving function. In other words, intermediaries and ultimate receivers (see Figure 3.27).

03fig27.gifFigure 3.27 Roles that can be assumed by receiving SOAP nodes

The two most common env:role attribute values are next and ultimateReceiver. An intermediary node will process only header blocks identified with the next role, whereas a node acting as the ultimate receiver will process both.

To learn more about SOAP, header blocks, and how they relate to second-generation Web services, visit www.specifications.ws.


  • Implementations of the SOAP messaging framework can be collectively conceptualized as an end-to-end messaging engine that drives communication throughout contemporary service-oriented architectures.

  • A SOAP message consists of a simple XML document structure. The parent envelope construct houses an optional header and a required body construct. Exception information can be placed in a special Fault element that is nested within the message body.

  • The utilization of SOAP header blocks by second-generation Web services specifications is an important aspect of this framework that vastly increases its power and complexity.

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