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SOAP - Simple Object Access Protocol
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) as its message format, and usually relies on other Application Layer protocols (most notably Remote Procedure Call (RPC) and HTTP) 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.

As a layman's example of how SOAP procedures can be used, a SOAP message could be sent to a web service enabled web site (for example, a house price database) with the parameters needed for a search. The site would then return an XML-formatted document with the resulting data (prices, location, features, etc). Because the data is returned in a standardized machine-parseable format, it could then be integrated directly into a third-party site.

The SOAP architecture consists of several layers of specifications for message format, message exchange patterns (MEP), underlying transport protocol bindings, message processing models, and protocol extensibility. SOAP is the successor of XML-RPC, though it borrows its transport and interaction neutrality and the envelope/header/body from elsewhere (probably from WDDX).

SOAP Transport Methods
SOAP makes use of an internet application layer protocol as a transport protocol. Critics have argued that this is an abuse of such protocols, as it is not their intended purpose and therefore not a role they fulfill well. Proponents of SOAP have drawn analogies to successful uses of protocols at various levels for tunneling other protocols.

Both SMTP and HTTP are valid application layer protocols used as Transport for SOAP, but HTTP has gained wider acceptance as it works well with today's Internet infrastructure; specifically, HTTP works well with network firewalls. SOAP may also be used over HTTPS (which is the same protocol as HTTP at the application level, but uses an encrypted transport protocol underneath) with either simple or mutual authentication; this is the advocated WS-I method to provide web service security as stated in the WS-I Basic Profile 1.1. This is a major advantage over other distributed protocols like GIOP/IIOP or DCOM which are normally filtered by firewalls. SOAP over AMQP is yet another possibility that some implementations support.

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 retard processing speed and 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.

Advantages of SOAP
  1. Using SOAP over HTTP allows for easier communication through proxies and firewalls than previous remote execution technology.
  2. 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 are also usable (e.g., SMTP).
  3. SOAP is platform independent.
  4. SOAP is language independent.
Disadvantages of SOAP
  1. 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.
  2. 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.
  3. Most uses of HTTP as a transport protocol are done in ignorance of how the operation would be modeled in HTTP. This is by design-similar to how different protocols sit on top of each other in the IP stack. But this analogy is imperfect; the application protocols used as transport protocols aren't really transport protocols. As a result, there is no way to know if the method used is appropriate to the operation. This makes good analysis at the application-protocol level problematic with sub-optimal results-for example, a POST operation is used when it would more naturally be modeled as a GET. The REST architecture has become a web service alternative that makes appropriate use of HTTP's defined methods.
  4. When relying on HTTP as a transport protocol, a firewall designed to only allow web browsing is forced to perform more detailed (and thus more costly) analysis of the HTTP packages.
  5. Although SOAP is an open standard, not all languages offer appropriate support. Java, Curl, Delphi, .NET and Flex offer excellent SOAP integration and/or IDE support. Some Perl support exists. Python and PHP support is much weaker.