Building a RESTful Service with WCF and F#

There has been a lot of discussion recently, centered on the idea of making services RESTful.  REST (Representational State Transfer) "is a key design idiom that embraces a stateless client-server architecture in which the web services are viewed as resources and can be identified by their URLs." (1)  This post will provide a simple example of creating a RESTful service with WCF and F#. 

Creating the project:

The first step in building our example is to create a new F# Library project.  I chose to name this project "FSharpWCF".  The next step is to add references to System.Runtime.Serialization, System.ServiceModel, and System.ServiceModel.Web.

Building the contracts:

The code looks only slightly different than an equivalent in a language such as C#, though there are a couple of key things that should be pointed out.  First, System.ServiceModel.Web has been opened.  This is the namespace that allows us to use the WebGet attribute.  The WebGet attribute allows us to set a UriTemplate that will be used for navigation to our service.  Our UriTemplate is set to "temp/{value}", which indicates that navigating to the base URL/temp/{value} will return the result of the service.  {value} associates directly with the parameter named value in our GetData operation.  The last important thing to notice is the assignment of the name "value" to the request argument of the GetData operation (i.e. abstract GetData : value:string -> string).  For more information on this, see this post by Ted Neward.

#light
namespace FSharpWCF

open System
open System.Runtime.Serialization
open System.ServiceModel
open System.ServiceModel.Web

[<ServiceContract>]
type IGetDataService = interface 
    [<WebGet(UriTemplate="temp/{value}")>]
    [<OperationContract>]
    abstract GetData : value:string -> string
end

type GetDataService() = 
    interface IGetDataService with
        member this.GetData value =
            sprintf "You entered: %s" value

Adding an app.config file:

Since an F# library doesn't have App.config as an option when adding a new item, you will need to create the file in the project directory manually, then add it as an existing item.  The main thing to notice is the addition of the endpointBehaviors section and the association of this behaviorConfiguration to the endpoint.

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
  <system.serviceModel>
    <services>
      <service name="FSharpWCF.GetDataService" behaviorConfiguration="FSharpWCF.GetDataServiceBehavior">
        <host>
          <baseAddresses>
            <add baseAddress = "http://localhost:8080/FSharpWCF/GetDataService/" />
          </baseAddresses>
        </host>
        <endpoint binding="webHttpBinding" contract="FSharpWCF.IGetDataService" 
                  behaviorConfiguration="Web">
          <identity>
            <dns value="localhost"/>
          </identity>
        </endpoint>
      </service>
    </services>
    <behaviors>
      <serviceBehaviors>
        <behavior name="FSharpWCF.GetDataServiceBehavior">
          <serviceMetadata httpGetEnabled="True"/>
          <serviceDebug includeExceptionDetailInFaults="False" />
        </behavior>
      </serviceBehaviors>
      <endpointBehaviors>
        <behavior name="Web">
          <webHttp/>
        </behavior>
      </endpointBehaviors>
    </behaviors>
  </system.serviceModel>
</configuration>

Testing the service:

To test your new service, do the following:

1. Right click on the FSharpWCF project and select properties.
2. Navigate to the Debug tab, change the start action to "Start external program:" and locate the WcfSvcHost.exe on your development machine (i.e. C:\Program Files\Microsoft Visual Studio 9.0\Common7\IDE\WcfSvcHost.exe).
3. Add the following "command line arguments:" /service:FSharpWCF.dll /config:FSharpWCF.dll.config
4. Set "Working directory:" to the directory of these DLLs.

You should now be able to launch the service by running the project from Visual Studio.  This will host your service with WcfSvcHost and allow you to access your service from a browser.  (Note: WcfSvcHost should only be used for testing purposes.  You should never use this as a production host.)

To use the service, open a browser and navigate to http://localhost:8080/FSharpWCF/GetDataService/temp/2 and you should receive <string>You entered: 2</string> as a result. 

Conclusion:

That's all there is to it.  The service can now be viewed as a resource that can be identified by it's URL. 


Resources:

1. http://java.sun.com/developer/technicalArticles/WebServices/restful/