View source for SOAP

{{Short description|Messaging protocol for web services}}
{{About|the computer network protocol|the IBM 650 assembly language|Symbolic Optimal Assembly Program|surfactants used for cleaning|Soap||Soap (disambiguation)}}
{{Infobox technology standard
| title             = SOAP
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| native_name       = <!-- Name in local language. If more than one, separate using {{plain list}} -->
| native_name_lang  = <!-- ISO 639-1 code e.g. "fr" for French. If more than one, use {{lang}} inside native_name items instead -->
| status            = Active
| year_started      = <!-- {{Start date|YYYY|MM|DD|df=y}} -->
| first_published   = Initially as [[XML-RPC]] in {{Start date and age|df=yes|June 1998}}
| version           = 1.2
| version_date      = {{Start date and age|2007|04|27|df=yes}}
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| domain            = [[Text-based protocol]]
| license           = Copyright by [[W3C]], implementations are [[royalty-free]]
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| website           = {{URL|https://w3.org/TR/soap12/}}
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{{Internet protocol suite|application=[[SOAP]]}}
{{Sources|date=December 2023}}
'''SOAP''' (originally an acronym for '''Simple Object Access Protocol'''){{efn|Since version 1.2 of the specification it is no longer an [[acronym]] for Simple Object Access Protocol}} is a messaging [[protocol (computing)|protocol]] specification for exchanging structured information in the implementation of [[web service]]s in [[computer network]]s. It uses [[XML Information Set]] for its [[message format]], and relies on [[application layer]] protocols, most often [[Hypertext Transfer Protocol]] (HTTP), although some legacy systems communicate over [[Simple Mail Transfer Protocol]] (SMTP), for message negotiation and transmission.

==Characteristics==
SOAP provides the Messaging Protocol layer of a [[web services protocol stack]] for web services. It is an XML-based protocol consisting of three parts: 
* an envelope, which defines the message structure<ref>
{{cite book
| last1                 = Hirsch
| first1                = Frederick
| last2                 = Kemp
| first2                = John
| last3                 = Ilkka
| first3                = Jani
| title                 = Mobile Web Services: Architecture and Implementation
| url                   = https://books.google.com/books?id=v5f0ORBgd5IC
| publisher             = John Wiley & Sons
| publication-date      = 2007
| page                  = 27 
| isbn                  = 9780470032596
| access-date            = 2014-09-15
| quote                 = Simple Object Access Protocol (SOAP) defines a messaging envelope structure designed to carry application payload in one portion of the envelope (the message body) and control information in another (the message header). 
| date                = 2007-01-11
}}
</ref> and how to process it
* a set of encoding rules for expressing instances of application-defined datatypes
* a convention for representing procedure calls and responses

SOAP has three major characteristics:

#''extensibility'' (security and [[WS-Addressing]] are among the extensions under development)
# ''neutrality'' (SOAP can operate over any protocol such as [[HTTP]], [[SMTP]], [[Transmission Control Protocol|TCP]], [[SOAP-over-UDP|UDP]])
# ''independence'' (SOAP allows for any [[programming model]])

As an example of what SOAP procedures can do, an application can send a SOAP request to a server that has web services enabled—such as a real-estate price database—with the parameters for a search. The server then returns a SOAP response (an XML-formatted document) with the resulting data, e.g., prices, location, features. Since the generated data comes in a standardized machine-parsable format, the requesting application can then integrate it directly.

The SOAP architecture consists of several layers of specifications for:

* message format
* [[Message Exchange Pattern]]s (MEP)
* underlying transport protocol bindings
* message processing models
* protocol extensibility

SOAP evolved as a successor of [[XML-RPC]], though it borrows its transport and interaction neutrality from Web Service Addressing<ref>{{Cite web|url=https://www.w3.org/Submission/ws-addressingCMIS/|title=Web Services Addressing (WS-Addressing)|website=www.w3.org|access-date=2016-09-15|archive-date=2016-09-25|archive-url=https://web.archive.org/web/20160925013438/http://www.w3.org/Submission/ws-addressing/|url-status=live}}</ref> and the envelope/header/body from elsewhere (probably from [[WDDX]]).{{Citation needed|date=August 2011}}

==History==
SOAP was designed as an object-access protocol and released as [[XML-RPC]] in June 1998 as part of [[UserLand Software|Frontier]] 5.1 by [[Dave Winer]], [[Don Box]], Bob Atkinson, and Mohsen Al-Ghosein for [[Microsoft]], where Atkinson and Al-Ghosein were working.<ref>{{cite web |url=http://dotnet.sys-con.com/node/45908 |title=Exclusive .NET Developer's Journal "Indigo" Interview with Microsoft's Don Box |publisher=Dotnet.sys-con.com |access-date=2012-10-04 |archive-date=2019-01-06 |archive-url=https://web.archive.org/web/20190106063021/http://support.sys-con.com/404/ |url-status=dead }}</ref>  The specification was not made available until it was submitted to [[IETF]] 13 September 1999.<ref>{{cite web |url=http://xml.coverpages.org/soap.html |title=XML Cover Pages on the history of SOAP |publisher=Coverpages.org |access-date=2003-07-22 |archive-date=2001-03-03 |archive-url=https://web.archive.org/web/20010303194003/http://xml.coverpages.org/soap.html |url-status=live }}</ref><ref>{{cite news |url=https://tools.ietf.org/html/draft-box-http-soap-00 |title=SOAP: Simple Object Access Protocol |newspaper=Ietf Datatracker |date=September 1999 |access-date=2015-09-20 |archive-date=2021-02-25 |archive-url=https://web.archive.org/web/20210225055937/https://tools.ietf.org/html/draft-box-http-soap-00 |url-status=live }}</ref> According to Don Box, this was due to politics within Microsoft.<ref>{{cite web |url=http://www.xml.com/pub/a/ws/2001/04/04/soap.html |title=Don Box on the history of SOAP |publisher=XML.com |date=2001-04-04 |access-date=2015-09-20 |archive-date=2015-06-18 |archive-url=https://web.archive.org/web/20150618225840/http://www.xml.com/pub/a/ws/2001/04/04/soap.html |url-status=live }}</ref> Because of Microsoft's hesitation, Dave Winer shipped [[XML-RPC]] in 1998.<ref>{{cite web|url=http://davenet.userland.com/1998/07/14/xmlRpcForNewbies |title=XML-RPC for Newbies |date=1998-07-14 |url-status=dead |archive-url=https://web.archive.org/web/19991012055302/http://davenet.userland.com/1998/07/14/xmlRpcForNewbies |archive-date=October 12, 1999 }}</ref>

The submitted [[Internet Draft]] did not reach [[Request for Comments|RFC]] status and is therefore not considered a "web standard" as such. Version 1.1 of the specification was published as a W3C Note on 8 May 2000.<ref>{{cite web |url=http://www.w3.org/TR/2000/NOTE-SOAP-20000508/ |title=W3C Note on Simple Object Access Protocol (SOAP) 1.1 |publisher=W3C |date=2000-05-08 |access-date=2015-09-20 |archive-date=2021-03-04 |archive-url=https://web.archive.org/web/20210304073820/http://www.w3.org/TR/2000/NOTE-SOAP-20000508/ |url-status=live }}</ref> Since version 1.1 did not reach [[W3C Recommendation]] status, it can not be considered a "web standard" either. Version 1.2 of the specification, however, became a [[World Wide Web Consortium|W3C]] recommendation on June 24, 2003. ''SOAP'' originally stood for "Simple Object Access Protocol" but version 1.2 of the standard dropped this acronym.<ref>{{cite web |date=April 27, 2007 |title=SOAP Version 1.2 Part 1: Messaging Framework (Second Edition) |url=http://www.w3.org/TR/soap12-part1/#intro |url-status=live |archive-url=https://web.archive.org/web/20120619182230/http://www.w3.org/TR/soap12-part1/#intro |archive-date=2012-06-19 |access-date=2012-06-15 |publisher=[[World Wide Web Consortium|W3C]] |quote=Note: In previous versions of this specification the SOAP name was an acronym. This is no longer the case. (Underneath section 1. Introduction)}}</ref>

The SOAP specification<ref>{{cite web |url=http://www.w3.org/TR/soap/ |title=SOAP Specifications |publisher=W3C |access-date=2014-03-29 |archive-date=2021-04-15 |archive-url=https://web.archive.org/web/20210415140903/http://www.w3.org/TR/soap/ |url-status=live }}</ref> was  maintained by the XML Protocol Working Group<ref>{{cite web |url=http://www.w3.org/2000/xp/Group/ |title=W3C XML Protocol Working Group |publisher=W3C |access-date=2014-03-29 |archive-date=2018-12-25 |archive-url=https://web.archive.org/web/20181225164613/http://www.w3.org/2000/xp/Group/%20 |url-status=live }}</ref> of the [[World Wide Web Consortium]] until the group was closed 10 July 2009. 

After SOAP was first introduced, it became the underlying layer of a more complex set of [[web service]]s, based on [[Web Services Description Language|WSDL]], [[XML Schema (W3C)|XSD]] and [[Universal Description Discovery and Integration|UDDI]]. These different services, especially UDDI, have proved to be of far less interest,{{citation needed|date=March 2025}} but an appreciation of them gives a complete understanding of the expected role of SOAP compared to how web services have actually evolved.{{According to whom|date=March 2025}}

== SOAP terminology ==
SOAP specification can be broadly defined to be consisting of the following three conceptual components: protocol concepts, encapsulation concepts and network concepts.<ref>{{Cite web|url=https://www.w3.org/TR/soap12-part1/#terminology|title=SOAP Version 1.2 Part 1: Messaging Framework (Second Edition)|website=www.w3.org|access-date=2016-09-14|archive-date=2016-09-20|archive-url=https://web.archive.org/web/20160920152253/http://www.w3.org/TR/soap12-part1/#terminology|url-status=live}}</ref>

=== Protocol concepts ===
; SOAP: This is a set of rules formalizing and governing the format and processing rules for information exchanged between a SOAP sender and a SOAP receiver.
; SOAP nodes: These are physical/logical machines with processing units which are used to transmit/forward, receive and process SOAP messages. These are analogous to [[Node (networking)|nodes]] in a network.
; SOAP roles: Over the path of a SOAP message, all nodes assume a specific role. The role of the node defines the action that the node performs on the message it receives. For example, a role "''none"'' means that no node will process the SOAP header in any way and simply transmit the message along its path.
; SOAP protocol binding : A SOAP message needs to work in conjunction with other protocols to be transferred over a network. For example, a SOAP message could use [[Transmission Control Protocol|TCP]] as a lower layer protocol to transfer messages. These bindings are defined in the SOAP protocol binding framework.<ref>{{Cite web|url=https://www.w3.org/2000/xp/Group/1/10/12/Binding_Framework_Proposal|title=Binding Framework Proposal|website=www.w3.org|access-date=2016-09-14|archive-date=2017-07-11|archive-url=https://web.archive.org/web/20170711204301/http://www.w3.org/2000/xp/Group/1/10/12/Binding_Framework_Proposal|url-status=live}}</ref>
; SOAP features: SOAP provides a messaging framework only. However, it can be extended to add features such as reliability, security etc. There are rules to be followed when adding features to the SOAP framework.
; SOAP module : A collection of specifications regarding the semantics of SOAP header to describe any new features being extended upon SOAP. A module needs to realize zero or more features. SOAP requires modules to adhere to prescribed rules.<ref>{{Cite web|url=https://www.w3.org/TR/soap12-part1/#soapmodules|title=SOAP Version 1.2 Part 1: Messaging Framework (Second Edition)|website=www.w3.org|access-date=2016-09-14|archive-date=2016-09-20|archive-url=https://web.archive.org/web/20160920152253/http://www.w3.org/TR/soap12-part1/#soapmodules|url-status=live}}</ref>

=== Data encapsulation concepts ===
; SOAP message: Represents the information being exchanged between 2 SOAP nodes.
; SOAP envelope : It is the enclosing element of an XML message identifying it as a SOAP message.
; SOAP header block: A SOAP header can contain more than one of these blocks, each being a discrete computational block within the header. In general, the SOAP ''role'' information is used to target nodes on the path. A header block is said to be targeted at a SOAP node if the SOAP role for the header block is the name of a role in which the SOAP node operates. (ex: A SOAP header block with role attribute as ''ultimateReceiver'' is targeted only at the destination node which has this role. A header with a role attribute as ''next'' is targeted at each intermediary as well as the destination node.)
; SOAP header : A collection of one or more header blocks targeted at each SOAP receiver.
; SOAP body : Contains the body of the message intended for the SOAP receiver. The interpretation and processing of SOAP body is defined by header blocks.
; SOAP fault: In case a SOAP node fails to process a SOAP message, it adds the fault information to the SOAP fault element. This element is contained within the SOAP body as a child element.

=== Message sender and receiver concepts ===
; SOAP sender: The node that transmits a SOAP message.
; SOAP receiver: The node receiving a SOAP message. (Could be an intermediary or the destination node).
; SOAP message path: The path consisting of all the nodes that the SOAP message traversed to reach the destination node.
; Initial SOAP sender: This is the node which originated the SOAP message to be transmitted. This is the root of the SOAP message path.
; SOAP intermediary: All the nodes in between the SOAP originator and the intended SOAP destination. 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 destination receiver of the SOAP message. This node is responsible for processing the message body and any header blocks targeted at it.

==Specification==
[[File:SOAP.svg|thumb|SOAP structure]]

The SOAP specification defines the messaging framework, which consists of:
* The ''SOAP processing model'', defining the rules for processing a SOAP message<ref name="auto">{{Cite web|url=https://www.w3.org/TR/soap12-part1/Overview.html|title=SOAP Version 1.2 Part 1: Messaging Framework (Second Edition)|website=www.w3.org|access-date=2020-06-24|archive-date=2017-04-02|archive-url=https://web.archive.org/web/20170402224809/https://www.w3.org/TR/soap12-part1/Overview.html|url-status=live}}</ref>
* The ''SOAP extensibility model'' defining the concepts of SOAP features and SOAP modules<ref name="auto"/>
* 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<ref name="auto"/>
* The ''SOAP message construct'' defining the structure of a SOAP message<ref name="auto"/>

==SOAP building blocks==
A SOAP message is an ordinary XML document containing the following elements:

{| class="wikitable"
|-
!Element
!Description
!Required
|-
|Envelope 
|Identifies the XML document as a SOAP message.
|Yes
|-
| Header 
|Contains header information.
|No
|-
| Body 
|Contains call and response information.
|Yes
|-
| Fault 
| Provides information about errors that occurred while processing the message.
|No
|}

==Transport methods==
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 [[firewall (networking)|firewalls]]. SOAP may also be used over [[HTTPS]] (which is the same protocol as HTTP at the application level, but uses an [[Transport Layer Security|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 [[General Inter-ORB Protocol|GIOP/IIOP]] or [[Distributed Component Object Model|DCOM]], which are normally filtered by firewalls. SOAP over [[AMQP]] is yet another possibility that some implementations support.  SOAP also has an advantage over [[Distributed Component Object Model|DCOM]] that it is unaffected by security rights configured on the machines that require knowledge of both transmitting and receiving nodes. This lets SOAP be loosely coupled in a way that is not possible with [[Distributed Component Object Model|DCOM]].  There is also the [[SOAP-over-UDP]] [[OASIS (organization)|OASIS]] standard.

== Message format ==
[[XML Information Set]] was chosen as the standard message format because of its widespread use by major corporations and [[open-source model|open source]] development efforts. Typically, XML Information Set is [[Serialization|serialized]] as [[XML]]. A wide variety of freely available [[software development tool|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 facilitates error detection and avoids interoperability problems such as byte-order ([[endianness]]), it can slow processing speed and can be cumbersome. For example, [[CORBA]], [[General Inter-ORB Protocol|GIOP]], [[Internet Communications Engine|ICE]], and [[Distributed Component Object Model|DCOM]]<!--EDIFACT too :P--> use much shorter, binary message formats. On the other hand, hardware appliances are available to accelerate processing of [[XML]] messages.<ref>{{cite web |url=http://www-306.ibm.com/software/integration/datapower/xa35/ |title=IBM Datapower |publisher=306.ibm.com |date=2011-11-30 |access-date=2012-10-04 |archive-date=2008-06-22 |archive-url=https://web.archive.org/web/20080622054818/http://www-306.ibm.com/software/integration/datapower/xa35/ |url-status=dead }}</ref><ref>{{cite web |url=http://www.research.ibm.com/XML/IBM_Zurich_XML_Accelerator_Engine_paper_2004May04.pdf |title=IBM Zurich XML Accelerator Engine |access-date=2012-10-04 |archive-date=2012-09-30 |archive-url=https://web.archive.org/web/20120930202629/http://www.research.ibm.com/XML/IBM_Zurich_XML_Accelerator_Engine_paper_2004May04.pdf |url-status=dead }}</ref> [[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' (e.g., headers, 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&ndash;4 times larger message than previous protocols [[Financial Information eXchange|FIX]] (Financial Information Exchange) and [[Common Data Representation|CDR]] (Common Data Representation).<ref>{{cite web |url=http://www2003.org/cdrom/papers/alternate/P872/p872-kohlhoff.html |title=Evaluating SOAP for High Performance Business Applications: Real-Time Trading Systems |publisher=Tenermerx Pty Ltd University of Technology, Sydney |date=2011-11-30 |access-date=2013-03-14 |archive-date=2013-08-10 |archive-url=https://web.archive.org/web/20130810002005/http://www2003.org/cdrom/papers/alternate/P872/p872-kohlhoff.html |url-status=live }}</ref>

XML Information Set does not have to be serialized in XML. For instance, CSV and [[JSON]] XML-infoset representations exist. 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.

==Example message (encapsulated in HTTP) ==

The message below requests a stock price for AT&T (stock ticker symbol "T").

<syntaxhighlight lang="http">
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" xmlns:m="http://www.example.org">
  <soap:Header>
  </soap:Header>
  <soap:Body>
    <m:GetStockPrice>
      <m:StockName>T</m:StockName>
    </m:GetStockPrice>
  </soap:Body>
</soap:Envelope>
</syntaxhighlight>

==Technical critique ==
{{More citations needed section|date=August 2020}}

===  Advantages    ===
* SOAP's neutrality characteristic explicitly makes it suitable for use with any transport protocol. Implementations often use HTTP as a transport protocol, but other popular transport protocols can be used. For example, SOAP can also be used over SMTP,<ref>{{cite web |url=https://svn.apache.org/repos/asf/webservices/archive/soap/site/faq/faq_chawke_smtp.html |work=Apache-SOAP User's FAQ |title=Making Apache SOAP Invocations using SMTP |author=Jonathan Chawke |date=March 9, 2001}}</ref> [[Java Message Service|JMS]]<ref>{{cite web |url=https://www.ibm.com/support/knowledgecenter/en/SSEQTP_9.0.5/com.ibm.websphere.base.doc/ae/rwbs_soapjmsstd.html |title=SOAP over JMS protocol |publisher=IBM |access-date=March 22, 2020 |archive-date=March 22, 2020 |archive-url=https://web.archive.org/web/20200322212140/https://www.ibm.com/support/knowledgecenter/en/SSEQTP_9.0.5/com.ibm.websphere.base.doc/ae/rwbs_soapjmsstd.html |url-status=live }}</ref><ref>{{cite web |url=https://www.w3.org/2002/ws/soapjms/wiki/SOAP-JMS_FAQ |title=SOAP-JMS FAQ |publisher=SOAP-JMS Binding Working Group |access-date=March 22, 2020 |archive-date=July 17, 2017 |archive-url=https://web.archive.org/web/20170717130309/https://www.w3.org/2002/ws/soapjms/wiki/SOAP-JMS_FAQ |url-status=live }}</ref> and [[message queue]]s.
* SOAP, when combined with HTTP post/response exchanges, tunnels easily through existing firewalls and proxies, and consequently doesn't require modifying the widespread computing and communication infrastructures that exist for processing HTTP post/response exchanges.
* SOAP has available to it all the facilities of XML, including easy internationalization and extensibility with XML Namespaces.

===Disadvantages===
* When using standard implementation and the default SOAP/HTTP binding, the XML infoset is serialized as XML. 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|Web Services Addressing]] or an [[Enterprise Service Bus]], the roles of the interacting parties are fixed. Only one party (the client) can use the services of the other.
* SOAP is less "simple" than the name would suggest. The verbosity of the protocol, low parsing speed of XML, and lack of a standardized interaction model led to the dominance of services using the [[HTTP]] protocol more directly. See, for example, [[Representational state transfer|REST]].
* Being protocol-agnostic, SOAP is unable to take advantage of protocol-specific features and optimizations such as [[Representational state transfer#Uniform interface|REST's Uniform Interface]] or [[Web cache|caching]] – instead having to reimplement them (as with [[WS-Addressing]]).

==See also==
{{Div col|colwidth=30em}}
* [[SOAP with Attachments]]
* [[SOAP with Attachments API for Java]]
* [[SOAP-over-UDP]]
* [[List of web service protocols]]
* [[Message Transmission Optimization Mechanism]] (MTOM)
* [[XML-binary Optimized Packaging]] (XOP)
* [[Extensible User Interface Protocol|Extensible User Interface Protocol (XUP)]]{{snd}} a SOAP-based UI protocol
* [[WebSocket]]
* [[WS-Security|Web Services Security]]
* [[WS-Security based products and services]]
{{Div col end}}

== Notes ==
{{Notelist}}

== References ==
{{Reflist}}

==Further reading==
* Benoît Marchal, "[https://web.archive.org/web/20070819165933/http://www.ibm.com/developerworks/xml/library/x-soapbx1.html Soapbox: Why I'm using SOAP]", IBM
* Uche Ogbuji, "[http://www.ibm.com/developerworks/edu/x-dw-cosoap-i.html Tutorial: XML messaging with SOAP]", Principal Consultant, Fourthought, Inc.

== External links ==
* [http://www.w3.org/TR/soap/ W3C SOAP page]
* [http://www.w3.org/TR/soap12/ SOAP Version 1.2 specification]
* [http://www.jitendrazaa.com/blog/java/create-soap-message-using-java/ Create SOAP Message in Java]

{{Clear}}
{{W3C Standards}}
{{Authority control}}

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[[Category:Computer-related introductions in 1998]]
[[Category:Application layer protocols]]
[[Category:Remote procedure call]]
[[Category:World Wide Web Consortium standards]]
[[Category:XML-based standards]]