Windows Systems Programming: Spring 2004

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Web Services

Internet for computers not people.
Based on existing and emerging standards:

• HTTP (HyperText Transport Protocol)

• SOAP (Simple Object Access Protocol)

• UDDI (Universal Description, Discovery and Integration)

• WS-POLICY (Web Services Policy)

Can be implemented using many current Internet pieces (web servers, protocols)

Remember the "calc" example (our first WebForm) - that was for users who wanted to add two numbers and used the web form for data entry.

But what is a program wanted to Add two numbers

• It could emulate the form submission by sending in a HTTP request and then parse the HTTP response to get the answer (sometimes called screen scraping)

• It could contact a web application oriented to programs NOT people - that is web services - don't need to worry about the appearance of the output - and uses XML to more precisely define the structure of the output - lots of XML technology can be used to ease the programming task

For example

This example adds 2 and 2:

POST /calc.asmx HTTP/1.1
Host: www.wintellect.com
Content-Type: text/xml; charset=utf-8
Content-Length: 338
SOAPAction:  "http://tempuri.org/Add"

<?xml version="1.0" encoding="utf-8"?> 
<soap:Envelope xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xmlns:xsd=http://www.w3.org/2001/XMLSchema
  xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/">
  <soap:Body>
    <Add xmlns="http://tempuri.org/">
      <a>2</a> 
      <b>2</b>
    </Add>
  </soap:Body>
</soap:Envelope>

And here’s how the Web service would respond:

HTTP/1.1 200 OK
Content-Type: text/xml; charset=utf-8
Content-Length: 353

<?xml version="1.0" encoding="utf-8"?>
<soap:Envelope xmlns:xsi=http://www.w3.org/2001/XMLSchema-instance
  xmlns:xsd=http://www.w3.org/2001/XMLSchema
  xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/">
  <soap:Body>
    <AddResponse xmlns="http://tempuri.org/">
      <AddResult>4</AddResult>
    </AddResponse>
  </soap:Body>
</soap:Envelope>

//// also supported is the GET method

GET /calc.asmx/Add?a=2&b=2 HTTP/1.1
Host: www.wintellect.com

The Web service responds as follows:

HTTP/1.1 200 OK
Content-Type: text/xml; charset=utf-8
Content-Length: 80

<?xml version="1.0" encoding="utf-8"?>
<int xmlns="http://tempuri.org/">4</int>

//// Or alternatively

POST /calc.asmx/Add HTTP/1.1
Host: www.wintellect.com
Content-Type: application/x-www-form-urlencoded
Content-Length: 7

a=2&b=2

And here’s the Web service’s response:

HTTP/1.1 200 OK
Content-Type: text/xml; charset=utf-8
Content-Length: 80

<?xml version="1.0" encoding="utf-8"?>
<int xmlns="http://tempuri.org/">4</int>

Web Services Example

<%@ WebService Language="C#" Class="CalcService" %>

using System;
using System.Web.Services;

[WebService (Name="Calculator Web Service",
    Description="Performs simple math over the Web")]
class CalcService
{
    [WebMethod (Description="Computes the sum of two integers")]
    public int Add (int a, int b)
    {
        return a + b;
    }

    [WebMethod
        (Description="Computes the difference between two integers")]
    public int Subtract (int a, int b)
    {
        return a - b;
    }
}

Testing asmx applications

• Normally a Web Service is accessed by a web service client

• For testing can use a browser - why?

• the magic of WSDL (Web Service Description Language) and SOAP/HTTP makes this possible

For Example

Suppose web services is available at "http://128.82.4.94:8000/cwild/Book1/Chap11/Calc/Calc.asmx"
Putting this URL in a IE browser would give this window

• Notice that it has identified the two web method functions

• Clicking on the "Add" method gives

• It also knows the parameter names and allows you to put in values for them

• I put in values "45" and "3"

• Clicking the "invoke" button

[WebMethod (CacheDuration="10")]
public string GetCurrentTime ()
{
    return DateTime.Now.ToShortTimeString ();
}

class CalcService : WebService
{
    [WebMethod (EnableSession="true",
        Description="Adds an item to a shopping cart")]
    public void AddToCart (Item item)
    {
        ShoppingCart cart = (ShoppingCart) Session["MyShoppingCart"];
        cart.Add (item);
    }
}

Consider the example

<%@ WebService Language="C#" Class="LocatorService" %>

using System;
using System.Web.Services;
using System.Data;
using System.Data.SqlClient;

[WebService (Name="Bookstore Locator Service",
    Description="Retrieves bookstore information from the Pubs database")]
class LocatorService
{
    [WebMethod (Description="Finds bookstores in a specified state")]
    public Bookstore[] FindStores (string state)
    {
        return FindByState (state);
    }

    Bookstore[] FindByState (string state)
    {
        SqlDataAdapter adapter = new SqlDataAdapter
            ("select * from stores where state = \'" + state + "\'",
            "server=localhost;database=pubs;uid=sa;pwd=");
        DataSet ds = new DataSet ();
        adapter.Fill (ds);

        DataTable table = ds.Tables[0];
        Bookstore[] stores = new Bookstore[table.Rows.Count];

        for (int i=0; i<table.Rows.Count; i++) {
            stores[i] = new Bookstore (
                table.Rows[i]["stor_name"].ToString ().TrimEnd
                    (new char[] { ' ' }),
                table.Rows[i]["stor_address"].ToString ().TrimEnd
                    (new char[] { ' ' }),

                table.Rows[i]["city"].ToString ().TrimEnd
                    (new char[] { ' ' }),
                table.Rows[i]["state"].ToString ().TrimEnd
                    (new char[] { ' ' })
            );
        }
        return stores;
    }
}

public class Bookstore
{
    public string Name;
    public string Address;
    public string City;
    public string State;

    public Bookstore () {}

    public Bookstore (string name, string address, string city,
        string state)
    {
        Name = name;
        Address = address;
        City = city;
        State = state;
    }
}

<s:complexType name="ArrayOfBookstore">
  <s:sequence>
    <s:element minOccurs="0" maxOccurs="unbounded"
      name="Bookstore" nillable="true" type="s0:Bookstore" /> 
  </s:sequence>
</s:complexType>
<s:complexType name="Bookstore">
  <s:sequence>
    <s:element minOccurs="1" maxOccurs="1" name="Name"
      nillable="true" type="s:string" /> 
    <s:element minOccurs="1" maxOccurs="1" name="Address"
      nillable="true" type="s:string" /> 
    <s:element minOccurs="1" maxOccurs="1" name="City"
      nillable="true" type="s:string" /> 
    <s:element minOccurs="1" maxOccurs="1" name="State"
      nillable="true" type="s:string" /> 
  </s:sequence>
</s:complexType>

• F1: This is the name of the service being accessed

• F2: This is a SOAP extension in the HTTP header - HTTP is extensible

• F3: asking to invoke the "Add" function

• F4: the body of the HTTP request is an XML document

• F5: XML namespace

• F6: the Add function takes two parameters named "a" and "b"

• F7: Return value of the "Add" function

• F8: embed parameters in query string

• F9: returns an array of BookStore objects - interesting

• F10: this is from DB schema

• F11: trims off any trailing blanks

• F12

Copyright chris wild 1999-2004.
For problems or questions regarding this web contact [Dr. Wild].
Last updated: March 01, 2004.