上QQ阅读APP看书，第一时间看更新

Types of web services

There are many types of web services which have evolved over time. Prominent ones are :

SOAP
UDDI
WSDL
REST

Out of these, SOAP became popular in the early 2000s, when XML was on the top wave. The XML data format is used by various distributed systems to communicate with each other. SOAP is too complex to implement. Criticizers of SOAP point out how bulky the SOAP HTTP request is.

A SOAP request usually consists of these three basic components:

Envelope
Header
Body

Just to perform an HTTP request and response cycle, we have to attach a lot of additional data in SOAP. A sample SOAP request looks like this:

POST /StockQuote HTTP/1.1
Host: www.stockquoteserver.com
Content-Type: text/xml; charset="utf-8"
Content-Length: nnnn
SOAPAction: "Some-URI"

<SOAP-ENV:Envelope
  xmlns:SOAP-ENV="http://schemas.xmlsoap.org/soap/envelope/"
  SOAP-ENV:encodingStyle="http://schemas.xmlsoap.org/soap/encoding/">
   <SOAP-ENV:Body>
       <m:GetLastTradePrice xmlns:m="Some-URI">
           <symbol>DIS</symbol>
       </m:GetLastTradePrice>
   </SOAP-ENV:Body>
</SOAP-ENV:Envelope>

This is a standard example of SOAP from the W3C standard (https://www.w3.org/TR/2000/NOTE-SOAP-20000508/). If we observe carefully, it is in XML format, with special tags specifying the envelope and body. Since XML operates on a lot of namespaces to function, additional information comes into play.