W3C’s SOAP provides a way to package messages or requests in XML envelopes.
SOAP is a W3C framework or protocol for the exchange of information between “peers in a decentralized, distributed environment” (http://www.w3.org/TR/soap12-part0/#L1153). SOAP is managed by the W3C’s XML Protocol Working Group (http://www.w3.org/2000/xp/Group/). Originally, SOAP was an acronym for Simple Object Access Protocol, but that interpretation has been dropped.
The original SOAP attracted a lot of interest early on, but as applications of SOAP rolled out (such as the API for Google, http://www.google.com/apis/), many people complained that non-SOAP interfaces were easier to use (http://www.prescod.net/rest/googleapi/). Amazon’s Web Services developer kit offers SOAP and REST (or Representational State Transfer, which is basically XML plus HTTP) interfaces, but the uptake for the REST interface seems to have been greater than for SOAP. Some prefer to avoid SOAP and commercial web services solutions altogether. Nevertheless, SOAP remains an approach at least worth examining.
The main concept behind SOAP is the remote procedure call (RPC), whereby one computer on a network can send a message or command to another computer on the network. The computer that receives the call can hand such an RPC to a local program. Then, if desired, the local program can send a reply—such as a return value—back to the original machine it came from.
SOAP is much more than just an XML vocabulary, though an XML vocabulary is an important part of it. The information in a SOAP envelope can be represented in forms other than XML across the wire. SOAP has a basic data model (http://www.w3.org/TR/soap12-part2/#datamodel) that represents structures as “a directed edge-labeled graph of nodes.” SOAP also has a set of optional encoding rules that may be used to encode messages that conform to the SOAP data model (http://www.w3.org/TR/soap12-part2/#soapenc). This hack will demonstrate what a SOAP envelope looks like in XML and some of the features that surround it.
Example 4-6 shows soap.xml , an XML representation of a SOAP message.
Example 4-6. soap.xml
<?xml version="1.0" encoding="UTF-8"?> <env:Envelope xmlns:env="http://www.w3.org/2003/05/soap-envelope"> <env:Header> <dt:instant xmlns:dt="http://www.wyeast.net/date" env:role="http://www.w3.org/2003/05/soap-envelope/role/next" env:mustUnderstand="true"> <dt:date year="2004" month="06" day="30"/> </dt:instant> </env:Header> <env:Body> <tz:time timezone="PST" xmlns:tz="http://www.wyeast.net/time" env:encodingStyle= "http://www.w3.org/2003/05/soap-encoding"> <tz:hour>11</tz:hour> <tz:minute>59</tz:minute> <tz:second>59</tz:second> <tz:meridiem>p.m.</tz:meridiem> <tz:atomic signal="true"/> </tz:time> </env:Body> </env:Envelope>
The document element is
Envelope, the container
for the SOAP message. The
env: prefix is the
customary prefix for SOAP envelopes using the namespace
Envelope element has two children,
Header is optional, but
not. Both of these elements contain XML elements for some other
SOAP messages are expected to be sent to at least one SOAP receiver, though there may be more. SOAP intermediaries may be included along the path of a SOAP message, and each of those intermediaries is expected to have some role in processing or handling the message.
role attribute, with the value
indicates that the application receiving the message must look over
and perhaps process the message before sending it on to the next
mustUnderstand attribute with a
true means the header block that contains
it must be understood by (i.e., properly handled by) the application
receiving the message.
The information contained in the
Body element is
considered the payload of the message. The
encodingStyle attribute with a value of
the processing application that contents of structure follow
SOAP’s optional encoding rules.
Finally, SOAP may be transported by a variety of protocols, most prominently HTTP. For example, you can grab SOAP messages with HTTP GET (http://www.w3.org/TR/soap12-part0/#L26854) or post a message with HTTP POST (http://www.w3.org/TR/soap12-part0/#L26854). Other transportation methods for SOAP include web architectures that identify SOAP messages with URIs (http://www.w3.org/TR/soap12-part0/#L3677) and email (http://www.w3.org/TR/soap12-part0/#L26854).
For a more thorough introduction to SOAP, read the SOAP v1.2 primer (Part 0): http://www.w3.org/TR/soap12-part0/
SOAP v1.2 messaging framework (Part 1), including information on the SOAP processing model and message constructs: http://www.w3.org/TR/soap12-part1/
SOAP v1.2 adjuncts (Part 2), including information on the SOAP data model, encoding, and HTTP binding: http://www.w3.org/TR/soap12-part2/