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Recently, I discussed Sending and Receiving TrackBacks on this blog. The TrackBack API is not the only mechanism which blog engines use to communicate between each other about the links that are included in individual posts. Wikipedia lists three methods to keep track of which articles are cross linked: Refback, Trackback, and Pingback. Of these, perhaps the trickiest to implement is the Pingback because it uses the xml-rpc style of communication rather than SOAP or REST that most .Net programmers are familiar with.

Thankfully, Cook Computing has released an open source implementation for xml-rpc which makes programming for it similar to programming .Net services by relying on attributes to specify the methods and contracts that a class consumes. The source and documentation can be found at http://www.xml-rpc.net/. Once we add a reference to the CookComputing.XmlRpcV2.dll, we can begin coding our Pingback implementation.

Unlike WCF, we don't need to perform a lot of configuration steps. We simply add a Generic Handler (.ashx) file and point it to the class that will perform the implementation. In this case, our handler will be called PingbackService.ashx and consists of the following:

<%@ WebHandler Language="VB" Class="LinqBlog.BO.Services.PingbackService" %>

As they say in the Staples advertisement, "That was easy!" Next, we implement the PingbackService class in our business tier. Similar to the Trackback, we'll separate this process into the sending operation and the receiving operation.

Sending Pingbacks

The steps to send a pingback are similar to those for a Trackback:

  • Find the links in our post.
  • Check the post's to see if the hosting server supports the Pingback API
  • Send the pingback to the hosting server's URI

The first step is identical to the TrackBack, so refer to the ParseAndSendTrackbacks method from my previous post for that code. Before we can send the ping, we need to check the server for our link to see if it supports Pingbacks. The Pingback Specification allows for two options for server discovery: based on the presence of a X-Pingback HTTP header looking like:

X-Pingback: http://www.ThinqLinq.com/Api/PingbackService.ashx

Or a <link> tag in the web page as follows:

<link rel="pingback" href="http://www.ThinqLinq.com/Api/PingbackService.ashx" />

Thus we need to check for the presence of either of these auto discovery mechanisms in our check to find the URI for the pingback server. Here's a sample implementation which takes a post's url and returns the url of the server's service (or an empty string if the server doesn't support Pingbacks).

Private Function GetPingbackServer(ByVal destination As String) As String
    Dim destUri As Uri = Nothing
    If Not Uri.TryCreate(destination, UriKind.Absolute, destUri) Then
        'Make sure we have a valid uri and that it isn't from this site (relative uri)
        Return ""
    End If

    Dim server As String = ""
    Dim req = DirectCast(WebRequest.Create(destination), HttpWebRequest)
    req.Referer = "http://www.thinqLinq.com"
    Using resp = DirectCast(req.GetResponse, HttpWebResponse)
        'Check headers for x-Pingback
        server = resp.Headers.Get("x-Pingback")
        If server <> "" Then Return server

        'Check for link element
        If resp.Headers.AllKeys.Contains("Content-Type") AndAlso _
            resp.Headers("Content-Type").StartsWith("text/html") Then

            Dim client As New WebClient()
            client.UseDefaultCredentials = True
            Dim page = client.DownloadString(destination)
            Dim regexString = _
String.Format("<link rel={0}pingback{0} href={0}[a-z0-9:\.\/_\?\-\%]*{0}", _
ControlChars.Quote) Dim match = Text.RegularExpressions.Regex.Match(page, regexString, _
RegexOptions.IgnoreCase).Value If Not String.IsNullOrEmpty(match) Then Dim startIndex As Integer = match.IndexOf("href=") + 6 Dim ret = match.Substring(startIndex, match.Length - startIndex - 1) Return ret End If End If End Using Return "" End Function

In this case, checking the headers is easy. Finding the <link> link tag takes a combination of Regular Expressions and string parsing since the <link> tag can either be HTML or XHTML compliant (and thus we can't use XML parsing on it. Now that we know the address of our post, the address that we're linking to and the address of the linking site's pingback server, we can issue the request to the server using Xmlrpc.Net. Here's the code:

Public Function SendPing(ByVal source As String, ByVal destination As String) As String

    Dim server = GetPingbackServer(destination)
    If Not server = "" Then
        Dim proxy As IPingbackPing = _
DirectCast(XmlRpcProxyGen.Create(GetType(IPingbackPing)), _
IPingbackPing) proxy.Url = server Return proxy.Ping(source, destination) End If Return "" End Function

Typically with the XMLRPC.Net, we specify the server's address in a static attribute on the service type. However, in our case the URL isn't known at compile time. As a result, we use the XmlRpcProxyGen.Create method to create a proxy for the RPC service at runtime. The Create method takes a type as an interface. We define the type with the required attributes as follows:

<XmlRpcUrl("http://ThinqLinq.com/SetAtRuntime.aspx")> _
 Public Interface IPingbackPing
    Inherits IXmlRpcProxy

    <XmlRpcMethod("pingback.ping")> _
    Function Ping(ByVal source As String, ByVal destination As String) As String

End Interface

Notice that the interface does specify a XmlRpcUrl. This is just a place holder which we replace in the SendPing method by setting the proxy.Url to the actual server's address. The act of calling the Ping method is trivial thanks to the generated proxy.

Receiving a Pingback

Switching gears to the server side now, receiving a ping is actually easy to do with the xml-rpc.net implementation. On our class, we inherit from the CookComputing.XmlRpc.XmlRpcService. This takes care of the core handler code and wiring up our method with the RPC call. To associate our method with the RPC method name, we add the CookComputing.XmlRpc.XmlRpcMethod specifying a method name of "pingback.ping" as required by the Pingback specification. This method takes two string parameters: source url and destination url; and returns a message indicating the outcome of the request.

      <XmlRpcMethod("pingback.ping")> _
      Public Function Ping(ByVal source As String, ByVal destination As String) As String
          Using dc As New LinqBlogDataContext
              'Get the post's ID based on the destination url from 
              'the custom URL Rewriting scheme.
              Dim postId As Integer = GetPostId(destination, dc)
              If postId > 0 Then
                  'Make sure we haven't already added this pingback
                  If (From c In dc.CommentBases _
                      Where c.CreatorLink = source And _
                          c.PostId = postId).Count = 0 Then

                      dc.CommentBases.InsertOnSubmit( _
                          New Pingback With {.CreatorLink = source, _
                                             .Description = "Pingback from " & source, _
                                             .EnteredDate = Now, _
                                             .PostId = postId, _
                                             .Creator = "Pingback", _
                                             .CreatorEmail = ""})

                      Return "pingback registered successfully"
                      Return "pingback already registered"
                  End If
                  Return "pingback not registered, no post found"
              End If
          End Using
End Function


In this case, we need to make sure that a) the destination location does include a reference to a post (through the id in the querystring or via URL Rewriting). If we have a valid ID, we then check to see if there is already a linkback associated with this post and the given source address. We don't want to register duplicate linkbacks. This is particularly important since we are implementing both Postbacks and Trackbacks and the calling site could send both requests. We only want to register one. Assuming we have a new Pingback for this post, we will create a new Pingback object (which inherits from CommentBase) and set the necessary values. With LINQ to SQL, applying the change is the standard SubmitChanges call on the context. We finish by letting the client know what happened in our call. This is mostly a courtesy as the specification doesn't require a specific response outside of exception codes.

Feel free to pingback (or trackback) to this post to let me know if this series of posts has been helpful to you.

© 2005 Serge Baranovsky. All rights reserved.
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