If you are familiar with SOAP web services, you may have heard of the Web Service Definition Language (WSDL). It is an XML description of the interface of the service and defines an endpoint URL for invocation. It is mandatory for a SOAP web service to be described by such a WSDL definition.

Similar to SOAP web services, RESTful services can also make use of a description language, called WADL. WADL stands for Web Application Definition Language. Unlike WSDL for SOAP web services, a WADL description of a RESTful service is optional, that is, consuming the service has nothing to do with its description.

Here is a sample part of a WADL file that describes the GET operation of our balance service:

<application xmlns="http://wadl.dev.java.net/2009/02" 
xmlns:xsd="http://www.w3.org/2001/XMLSchema" 
xmlns:service="http://localhost:8080/data/balance"> 
<grammer> 
<include href="balance.xsd"/> 
<include href="error.xsd"/> 
</grammer> 
<resources base="http://localhost:8080/data/balance/"> 
<resource path="{date}"> 
<method name="GET"> 
<request> 
<param name="date" type="xsd:string" style="template"/> 
</request> 
<response status="200"> 
<representation mediaType="application/xml" element="service:balance"/> 
<representation mediaType="application/json" /> 
</response> 
<response status="404"> 
<representation mediaType="application/xml" element="service:balance"/> 
</response> 
</method> 

</resource> 
</resources> 
</application>

This extract of a WADL file shows how application-exposing resources are described. Basically, each resource must be a part of an application. The resource provides the URI where it is located with the base attribute, and describes each of its supported HTTP methods in a method. Additionally, an optional doc element can be used at resource and application to provide additional documentation about the service and its operations.

Though WADL is optional, it significantly reduces the efforts of discovering RESTful services.