SOAP

This article is about the computer network protocol. For other uses, see Soap (disambiguation).

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. 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.

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 real-estate 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 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).

History

SOAP structure

SOAP once 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; however SOAP is different from SOA.

SOAP was originally designed by Dave Winer, Don Box, Bob Atkinson, and Mohsen Al-Ghosein in 1998 in a project for Microsoft (where Atkinson and Al-Ghosein were already working at the time), as an object-access protocol. The SOAP specification is currently maintained by the XML Protocol Working Group of the World Wide Web Consortium.

After SOAP was first launched, 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 fuller understanding of the expected role of SOAP compared to how web services have actually developed.

The SOAP specification

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

SOAP 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.

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 function 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.

Message format

XML is 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 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.

Sample SOAP message

POST /InStock HTTP/1.1
Host: www.example.org
Content-Type: application/soap+xml; charset=utf-8
Content-Length: nnn

<?xml version="1.0"?>
<soap:Envelope
xmlns:soap="http://www.w3.org/2001/12/soap-envelope"
soap:encodingStyle="http://www.w3.org/2001/12/soap-encoding">

<soap:Body xmlns:m="http://www.example.org/stock">
  <m:GetStockPrice>
    <m:StockName>IBM</m:StockName>
  </m:GetStockPrice>
</soap:Body>

</soap:Envelope>

Technical critique

Advantages

• 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., JMS, SMTP).
• 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.
• 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 and 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.
• 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. For example, an HTTP request to a SOAP WebService using the idempotent HTTP method GET, could easily be hiding non-idempotent actions inside the SOAP package, so firewalls must inspect the contents of the package and also have some knowledge of the SOAP structure, this may require parsing or string matching which is very costly in firewall terms. This is why it is generally a bad idea to use a transfer protocol such as HTTP as a transport protocol for which it was not designed.
• Although SOAP is an open standard, not all languages offer appropriate support. Java, Curl, Delphi, PHP, .NET and Flex offer SOAP integration and/or IDE support. Some Perl and Python support exists.

See also

• SOAP with Attachments
• SOAP with Attachments API for Java
• List of web service protocols
• Extensible User Interface Protocol (XUP) – a SOAP based UI protocol
• SOAPjr – a hybrid of SOAP and JR (JSON-RPC)
• Web Services Security

External links

• W3C SOAP page
• SOAP Version 1.2 specification
• SOAP Tutorial
• gSOAP - Open Source, Cross Platform, C/C++ SOAP Toolkit