Web Services

RPC

• XML-RPC provide facility to computer to call procedure from other remote computer and make function across network.
• XML-RPC uses the HTTP protocol to transfer information between a client computer and a server computer. It uses XML vocabulary to describe nature of request and response.
• XML-RPC client specified only a procedure name and parameters  and the server returns fault or response to the client but both are in XML format.
• With XML-RPC and web services, however, the Web becomes a collection of procedural connections where computers exchange information along tightly bound paths.

Why XML-RPC ?

XML-RPC Data Model

XML-RPC simple types mapped onto .NET data types.

XML-RPC simple types are mapped to and from the following .NET types:

The given basics types are always enclosed in a value elements.

• Strings may be enclosed in a value element but omit the string element.
• These basic types may be combined into two more complex types, arrays and structs. Arrays represent sequential information, while structs represent name-value pairs, much like hashtables, associative arrays, or properties.

<value><array><data>
      <value><string>Hello </string></value>
     <value><string>Good </string></value>
   </data></array></value>
.......................................................
<value><array><data>
    <value><int>7</int></value>
    <value><int>124</int></value>
   </data></array>
.......................................................
<value><array>
    <data>
    <value><boolean>1</boolean></value>
    <value><double>42.14159265</double></value>
   </data></array></value>

• It is a combination of HTTP headers and XML contents

• XML content used the data type structure which help to passed parameter according when you call procedure and it also contains additional information about calling procedur.
• The HTTP headers provide a wrapper for request to passing over the Web. Request contained at least single XML document, whose root element is a methodCall element.
• Each methodCallelement contains two element first one is the  methodName and second element is a params element.
• The methodNameelement identifies the name of the procedure to be called.
• The params element contains a list of parameters and their values.

<?xml version="1.0"?>
<methodCall>
   <methodName>AreaSquare</methodName>
      <params><param>
            <value><double>2</double></value>
         </param></params>
</methodCall>

• An XML-RPC response contain only one parameter.
• That parameter may be any type. It may be array type, struct type etc. Therefore it returns multiple values
• It also return boolean values.

<?xml version="1.0"?>
<methodResponse>
   <params><param>
         <value><double>4</double></value>
      </param></params>
</methodResponse>

• A fault will also have an error code which make easy to understand that what type error
• If any problem to processing requests then methodResponse element contain fault element instead of a params element
• Fault element contain a single value like a params element that indicates that something wrong

<?xml version="1.0"?>
<methodResponse>
   <fault>
      <value><string>No Method</string></value>
   </fault>
</methodResponse>
.........................................................
<?xml version="1.0"?>
<methodResponse>
   <fault><value><struct>
            <member>
             <name>name</name>
        <value><int>26</int></value></member>
            <member>
           <name>message</name>
     <value><string>No Method</string></value>
    </member></struct></value></fault>
</methodResponse>

XML-RPC Client

[XmlRpcUrl("http://...")]
public interface IcityName : IXmlRpcProxy
{
    [XmlRpcMethod("examples.cityName")]
    string GetCityName(int cityNumber); 
}

ICityName proxycity = XmlRpcProxyGen.Create<ICityName>();

string cityName = proxycity.GetCityName(45);

The model for XML-RPC Services are SOAP-based Web Services implemented as part of ASP. Net and they running in Microsoft IIS web server environment . An XML-RPC Service is implemented in any language that compiled under CLR (Common Language Runtime). It is create by creating a class which derives from the XmlRpcService base class. Decorating the methods to be exposed via XML-RPC with the XmlRpcMethod attribute.

For example:

public class CityNameService : XmlRpcService 
{
  [XmlRpcMethod("examples.getCityName",
    Description="Return name of city given its number")] 
  public string getCityName(int cityNum)
  {
    if (cityNum == 45)
      return "Delhi";
    else
      return "Unknown";
  }
}

For example:

public interface ICityName
{
    [XmlRpcMethod("examples.getCityName")]
    string GetCityName(int cityNumber); 
}
public interface ICityNameProxy : ICityName, IXmlRpcProxy
{
}
public class CityNameService : XmlRpcService, ICityName
{
  public string getCityName(int cityNum)
  {
    if (cityNum == 45)
        return "Delhi";
    else
      return "Unknown";
  }
}

1. A class may implemented many XML-RPC methods not like a single as shown in above example.
2. The resulting assembly DLL placed in the bin directory of an IIS file directory
3. To dispatch HTTP requests to the custom handler implemented by class XmlRpcService.
4. A web.config file is used.

For example

<configuration>
  <system.web>
    <httpHandlers>
      <add verb="*" path="cityname.rem" 
        type="r4r.CityNameService, CityNameService" />
    </httpHandlers>
  </system.web> 
</configuration>

The Service can be invoked via the XML-RPC protocol at this URL:

http://localhost/xmlrpc/cityname.rem