||This article's lead section may not adequately summarize key points of its contents. (June 2012)|
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 XML Information Set for its message format, and usually relies on other Application Layer protocols, most notably Hypertext Transfer Protocol (HTTP) or 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, TCP, or JMS) and Independence (SOAP allows for any programming model). As an example of how SOAP procedures can be used, a SOAP message could be sent to a web site that has web services enabled, 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-parsable format, it can then be integrated directly into a third-party web site or application.
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 was designed as an object-access protocol in 1998 by Dave Winer, Don Box, Bob Atkinson, and Mohsen Al-Ghosein for Microsoft, where Atkinson and Al-Ghosein were working at the time. The SOAP specification is currently maintained by the XML Protocol Working Group of the World Wide Web Consortium.
SOAP originally stood for 'Simple Object Access Protocol' but this acronym was dropped with Version 1.2 of the standard. Version 1.2 became a W3C recommendation on June 24, 2003. The acronym is sometimes confused with SOA, which stands for Service-oriented architecture, but the acronyms are unrelated.
After SOAP was first introduced, it became the underlying layer of a more complex set of Web Services, based on Web Services Description Language (WSDL) and Universal Description Discovery and Integration (UDDI). These services, especially UDDI, have proved to be of far less interest, but an appreciation of them gives a more complete understanding of the expected role of SOAP compared to how web services have actually evolved.
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
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.
SOAP Building Blocks
A SOAP message is an ordinary XML document containing the following elements:
- A required Envelope element that identifies the XML document as a SOAP message. - An optional Header element that contains header information. - A required Body element that contains call and response information. - An optional Fault element that provides information about errors that occurred while processing the message.
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.
SOAP also has the advantage over DCOM that it is unaffected by security rights being configured on the machines which require knowledge of both transmitting and receiving nodes. This allows SOAP to be loosely coupled in a way that is not possible with DCOM.
There is also the SOAP-over-UDP OASIS standard.
XML Information Set was chosen as the standard message format because of its widespread use by major corporations and open source development efforts. Typically, XML Information Set is serialized as XML. 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. XML messages by their self documenting nature usually have more 'overhead' (Headers, footers, nested tags, delimiters) than actual data in contrast to earlier protocols where the overhead was usually a relatively small percentage of the overall message .
In financial messaging SOAP was found to result in a 2-4 times larger message than previous protocols (FIX (Financial Information Exchange) and CDR Common Data Representation) 
It is important to note that XML Information Set does not require to be serialized in XML. For instance, a CSV or JSON XML-infoset representation exists. There is also no need to specify a generic transformation framework. The concept of SOAP bindings allows for specific bindings for a specific application. The drawback is that both the senders and receivers have to support this newly defined binding.
POST /InStock HTTP/1.1 Host: www.example.org Content-Type: application/soap+xml; charset=utf-8 Content-Length: 299 SOAPAction: "http://www.w3.org/2003/05/soap-envelope" <?xml version="1.0"?> <soap:Envelope xmlns:soap="http://www.w3.org/2003/05/soap-envelope"> <soap:Header> </soap:Header> <soap:Body> <m:GetStockPrice xmlns:m="http://www.example.org/stock"> <m:StockName>IBM</m:StockName> </m:GetStockPrice> </soap:Body> </soap:Envelope>
- 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, SMTP or Message queue are also usable.
- Since the SOAP model tunnels fine in the HTTP post/response model, it can tunnel easily over existing firewalls and proxies, without modifications to the SOAP protocol, and can use the existing infrastructure.
- When using standard implementations and the default SOAP/HTTP binding, the XML infoset is serialized as XML. Because of the verbose XML format, SOAP can be considerably slower than competing middleware technologies such as CORBA or ICE. 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.
- SOAP with Attachments
- SOAP with Attachments API for Java
- List of web service protocols
- Message Transmission Optimization Mechanism - MTOM
- XML-binary Optimized Packaging - XOP
- Extensible User Interface Protocol (XUP) – a SOAP-based UI protocol
- SOAPjr – a hybrid of SOAP and JSON-RPC
- Web Services Security
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