The contract between the framework and one observer is composed of two files located on observer side :

• observer.wadl
• observer.rdf

To learn more :  http://www.w3.org/QA/Tools/Unicorn/contract/

observer.wadl

This file describes the communication protocol between an observer and the framework who wants to use it. It defines all communication methods with their parameters and values allowed.

• Step 1 :

Write an XML header, root tag called application and define XML namespace attributs used.

<?xml version="1.0"  encoding="utf-8"?>

<application xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	    xsi:schemaLocation="http://research.sun.com/wadl wadl.xsd"
	    xmlns="http://research.sun.com/wadl"
	    xmlns:xs="http://www.w3.org/2001/XMLSchema"
		xmlns:uco="http://www.w3.org/unicorn/observationresponse">
...

</application>

• Step 2 :

In this part, we are going to define severals ressources of our observer.
In our case, we have just one resource, calculator.php, based on http://localhost/ (Full path of our observer will be http://localhost/calculator.php).

<?xml version...?>
<application...>

   <resources base="http://localhost/">
      <resource uri="calculator.php">
      ...
      </resource>
   </resources>

</application>

• Step 3 :

Now, we have to define methods of our observer.
There is two attributs for tag method :

• name : GET or POST.
• id : method's key name. We'll use this id in observer.rdf.

In our case, there is just one method. It takes 3 parameters (of which two are optional).

• uri : path of a file which contains some simple arithmetic operations.
• x2 : it's a complex type to multiply results by 2. We'll describe this more in details later.
• xn : to multiply results by n.

<method name="GET" id="CalculatorGET">
   <request>
      <query_variable name="uri" />
      <query_variable name="x2" type="tX2"/>
      <query_variable name="xn"/>
   </request>
</method>

• Step 4 :

In step 3, we have defined a parameter "x2" with "tX2" type.
Only two values are possible for this parameter : 0 or 1.
If 1, results will be multiply by 2.
So, we are going now to definie a schema for our new type "tX2.

<xs:schema>
   <xs:element name="tX2">
      <xs:complexType>
         <xs:choice>
	    <xs:element name="0" />
	    <xs:element name="1" />
	 </xs:choice>
      </xs:complexType>
   </xs:element>
</xs:schema>

You can see full observer.wadl here.