Overview

Introduction to Web Services

This is an introduction to the development of Web Services with the Genero Web Services Extension.  It is intended to help those who are using Genero Web Services Extension for the first time to understand the basic Web Services concepts, and to quickly start their development with the Genero tools. 

Summary:

• Concepts

• Web Services Standards

• XML

• XML Schema

• SOAP

• WSDL

• HTTP
  

Concepts

Web services are a standard way of communicating between applications over the Internet or the Intranet. They define how to communicate between two entities:

• A server that exposes services

• A client that consumes the services

For example, a server could expose a "StockQuotation" service that responds to an operation "getQuote".  For the "getQuote" operation, the input message is a stock symbol as a string, and the output message is a stock value as a decimal number. 

The "getQuote" operation could be a BDL function published on the server :

01 FUNCTION getQuote( stockSymbol ) 
02 DEFINE	
03   stockSymbol  VARCHAR(16),
04   stockValue   DECIMAL(16,2)
05
06   # … Function body
07
08   RETURN stockValue
09 END FUNCTION

Note: This is not the actual code to write for the server.  See Writing a Server function.

On the client side, the Web service client application will be able to call this function just as if it were a local function:

01 LET svalue = WebService_StockQuotation_getQuote( "MyCompany" )

Web Services Standards

Web services are platform-independent and programming language-independent. The World Wide Web consortium defines the Web services standards. For more information about these standards, you can check the "Web services" section of their web site at http://www.w3.org.

The standards involved in what is commonly called "Web services" are:

XML

XML (Extensible Markup Language) defines a machine-independent way of exchanging data. For example, an XML representation of the following BDL data structure:

01 DEFINE Person 
02 RECORD
03    FirstName	VARCHAR(32),
04    LastName	VARCHAR(32),
05    Age	INTEGER
06  END RECORD

could be:

01 <Person>
02  <FirstName>John</FirstName>
03  <LastName>Smith</LastName>
04  <Age>35</Age>
05 </Person>

XML Schema

XML Schema describes the structure of an XML document. For example, for the above document, the schema could say that the XML document contains an element "Person", and that each "Person" contains one and only one element "FirstName", "LastName", "Age". Also, the types of these elements can be defined.

XML Schema allows checking the correctness of an XML document.

SOAP

SOAP (Simple Object Access Protocol) is the high-level communication protocol between the server and the client. It defines the XML data flow between the server and the client. The "StockQuote" service mentioned in the Concepts section will exchange messages using the following syntax:

Request

<soap:Envelope xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/">
  <soap:Body>
    <getQuote>
      <stockSymbol>MyCompany</stockSymbol>
    </getQuote>
  </soap:Body>
</soap:Envelope>

Response

<soap:Envelope xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/">
  <soap:Body>
    <getQuoteResponse>
      <stockValue>999.99</stockValue>
    </getQuoteResponse>
  </soap:Body>
</soap:Envelope>

SOAP relies on a lower level protocol for the transport layer.

Web services use SOAP over HTTP.

WSDL

The WSDL ( Web Services Description Language) describes the services offered by a server. It contains:

• The description of the operations offered by the server with their input and output messages.
• The location of the SOAP server.
• Internal connection and protocol details (transport layer, encoding, namespaces, …)

A WSDL description is sufficient to get all the information required to communicate with the SOAP server.

HTTP

HTTP (Hypertext Transfer Protocol) is the set of rules for exchanging files (text, graphic images, sound, video, and other multimedia files) on the World Wide Web.