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What is SOAP?
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SOAP is a protocol for exchanging XML-based messages over
a computer network, normally using HTTP. SOAP forms the foundation
layer of the Web services stack, providing a basic messaging
framework that more abstract layers can build on. SOAP can
be used to facilitate a Service-Oriented architectural pattern.
There are several different types of messaging patterns in
SOAP, but by far the most common is the Remote Procedure Call
(RPC) pattern, where one network node (the client) sends a
request message to another node (the server), and the server
immediately sends a response message to the client. Indeed,
SOAP is the successor of XML RPC.
Overview
Originally designed by Dave Winer, Don Box, Bob Atkinson, and
Mohsen Al-Ghosein in 1998 with backing from Microsoft (where
Atkinson and Al-Ghosein worked 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.
Name
The name "SOAP" was originally an acronym for Simple
Object Access Protocol, but the full name was dropped in Version
1.2 of the SOAP specification, because the focus of SOAP shifted
from object access to object inter-operability.
Transport methods
Both SMTP and HTTP are valid application layer protocols for
SOAP, but HTTP has gained wider acceptance as it works well
with today's Internet infrastructure; specifically, SOAP works
well with network firewalls. This is a major advantage over
other distributed protocols like GIOP/IIOP or DCOM which are
normally filtered by firewalls. A key issue under discussion
is whether or not HTTP is the right transport given its inherent
synchronous nature.
XML was chosen as the standard message format because of its
widespread acceptance by major corporations and open source
development efforts. Additionally, a wide variety of freely
available tools significantly ease the transition to a SOAP-based
implementation.
The somewhat
lengthy syntax of XML can be both a benefit and a drawback.
Its format
is easy for humans to read, but can
be complex and can have slow processing times. For example,
CORBA, GIOP and DCOM use much shorter, binary message formats.
On the other hand, hardware appliances are available to accelerate
processing of XML messages. Binary XML (the use of the word "XML" is
controversial here) is also being explored as a means for streamlining
the throughput requirements of XML.
Structure of a SOAP message
A SOAP message is contained in an envelope. Within this envelope
are two additional sections: the header and the body of
the message. SOAP messages use XML namespaces.
The header contains relevant information about the message.
For example, a header can contain the date the message is sent,
or authentication information. It is not required, but, if
present, must always be included at the top of the envelope.
Example SOAP messages
Here is an example of how a client might format a SOAP message
requesting product information from a fictional warehouse web
service. The client needs to know which product corresponds
with the ID 827635:
<soap:Envelope xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/">
<soap:Body>
<getProductDetails xmlns="http://warehouse.example.com/ws">
<productID>827635</productID>
</getProductDetails>
</soap:Body>
</soap:Envelope>
Here is how the warehouse web service might format its
reply message with the requested product information:
<soap:Envelope xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/">
<soap:Body>
<getProductDetailsResponse xmlns="http://warehouse.example.com/ws">
<getProductDetailsResult>
<productName>Toptimate 3-Piece Set</productName>
<productID>827635</productID>
<description>3-Piece luggage set. Black Polyester.</description>
<price>96.50</price>
<inStock>true</inStock>
</getProductDetailsResult>
</getProductDetailsResponse>
</soap:Body>
</soap:Envelope>
Performance of SOAP
Because of the lengthy XML format, SOAP is considerably slower than competing
middleware technologies such as CORBA. Typically, SOAP is about 10 times slower
than binary network protocols such as RMI or IIOP. This may not be an issue
when only small messages are sent.
About
this Tutorial
This tutorial is from The
Wikipedia which is published under the
GNU Free Documentation License.
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