Chapter: XML and Web Services : Web Services Building Blocks


SOAP, originally defined as Simple Object Access Protocol, is a protocol specification for exchanging structured information in the implementation of Web Services in computer networks.



SOAP, originally defined as Simple Object Access Protocol, is a protocol specification for exchanging structured information in the implementation of Web Services in computer networks. It relies on Extensible Markup Language (XML) for its message format, and usually relies on other Application Layer protocols, most notably Hypertext Transfer Protocol (HTTP) and Simple Mail Transfer Protocol (SMTP), for message negotiation and transmission. SOAP can form the foundation layer of a web services protocol stack, providing a basic messaging framework upon which web services can be built. This XML based protocol consists of three parts: an envelope, which defines what is in the message and how to process it, a set of encoding rules for expressing instances of application-defined datatypes, and a convention for representing procedure calls and responses. SOAP has three major characteristics: Extensibility (security and WS-routing are among the extensions under development), Neutrality (SOAP can be used over any transport protocol such as HTTP, SMTP or even TCP), and Independence (SOAP allows for any programming


As an example of how SOAP procedures can be used, a SOAP message could be sent to a web-service-enabled web site such as a real-estate price database, with the parameters needed for a search. The site would then return an XML-formatted document with the resulting data, e.g., prices, location, features. With the data being returned in a standardized machine-parseable format, it can then be integrated directly into a third-party web site or application.

The SOAP specification defines the messaging framework which consists of:


·                    The SOAP processing model defining the rules for processing a SOAP message

·                    The SOAP extensibility model defining the concepts of SOAP features and SOAP modules

·                   The SOAP underlying protocol binding framework describing the rules for defining a binding to an underlying protocol that can be used for exchanging SOAP messages between SOAP nodes 

·                   The SOAP message construct defining the structure of a SOAP message

Processing model


The SOAP processing model describes a distributed processing model, its participants, the SOAP nodes and how a SOAP receiver processes a SOAP message. The following SOAP nodes are defined:


§    SOAP sender


A SOAP node that transmits a SOAP message.


§    SOAP receiver


A SOAP node that accepts a SOAP message.


§    SOAP message path


The set of SOAP nodes through which a single SOAP message passes.



§    Initial SOAP sender (Originator)


The SOAP sender that originates a SOAP message at the starting point of a SOAP message path.


§    SOAP intermediary


A SOAP intermediary is both a SOAP receiver and a SOAP sender and is targetable from within a SOAP message. It processes the SOAP header blocks targeted at it and acts to forward a SOAP message towards an ultimate SOAP receiver.


§    Ultimate SOAP receiver


The SOAP receiver that is a final destination of a SOAP message. It is responsible for processing the contents of the SOAP body and any SOAP header blocks targeted at it. In some circumstances, a SOAP message might not reach an ultimate SOAP receiver, for example because of a problem at a SOAP intermediary. An ultimate SOAP receiver cannot also be a SOAP intermediary for the same SOAP message.


Message format:


XML was chosen as the standard message format because of its widespread use by major corporations and open source development efforts. Additionally, a wide variety of freely available tools significantly eases the transition to a SOAP-based implementation. The somewhat lengthy syntax of XML can be both a benefit and a drawback. While it promotes readability for humans, facilitates error detection, and avoids interoperability problems such as byte-order (Endianness), it can slow processing speed and can be cumbersome. For example, CORBA, GIOP, ICE, and DCOM use much shorter, binary message formats. On the other hand, hardware appliances are available to accelerate processing of XML messages. Binary XML is also being explored as a means for streamlining the throughput requirements of XML.



POST /InStock HTTP/1.1


Content-Type: application/soap+xml; charset=utf-8

Content-Length: 299

SOAPAction: ""


<?xml version="1.0"?>


<soap:Envelope xmlns:soap=""> <soap:Header>







<m:GetStockPrice xmlns:m=""> <m:StockName>IBM</m:StockName>







SOAP is versatile enough to allow for the use of different transport protocols. The standard stacks use HTTP as a transport protocol, but other protocols such as JMS and SMTP are also usable.


Since the SOAP model tunnels fine in the HTTP get/response model, it can tunnel easily over existing firewalls and proxies, without modifications to the SOAP protocol, and can use the existing infrastructure.


Disadvantages :


Because of the verbose XML format, SOAP can be considerably slower than competing middleware technologies such as CORBA. This may not be an issue when only small messages are sent. To improve performance for the special case of XML with embedded binary objects, the Message Transmission Optimization Mechanism was introduced. When relying on HTTP as a transport protocol and not using WS-Addressing or an ESB, the roles of the interacting parties are fixed. Only one party (the client) can use the services of the other. Developers must use polling instead of notification in these common cases.

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