Sunday, 07 November, 2004

One problem with rolling your own bloging engine is implementing all the fiddly details that other bloggers take for granted. Little things like the RSS syndication or trackback functionality. I developed the RSS feed quite a while ago and it's how most of my vistors keep up-to-date with my posts so I consider the RSS feed a definite success.

The previous blog post about security was going to be a comment on Schneier's new blog but I couldn't get it to format quite the way I wanted it so I thought: "Damn I'm going to have to do a track-back!" Of course, I'm not using Moveable type or some other blogging software so I was going to have to implement this feature myself. Finding the technical specification is a little more difficult than you might have thought due to the fact that when people write articles on how to do track-back pings they write their article for some particular piece of blogging software and not about the way the underlying system works. Never-the-less if you want to go ahead and implement then I found the full spec here.

It's actually pretty easy to send a trackback ping to another web-site. It's essentially a HTTP POST request to a specfic trackback page with the following fields:

title The title of the blog entry
excerpt A short teaser from your blog entry
url The url to your blog post
blog_name The name of the blog in which your entry is contained

The next question is how on earth do you do this in ASP. I must admit, before I took my job in Manchester I would probably have built a custom COM object to achieve this goal, however, there's actually a much easier way to achieve this result. I suspect that what i'm about to show you isn't terribly well known. A lot of the new customers we take on at work struggle making GET/POST requests to other servers when we first open up our specs to them. I get the feeling that most people probably never need to do this and consequently never learn how to do it.

Using the Microsoft XML COM component it only takes a few lines of code. Consider the following:

set oMsXmlHttp = server.createobject("Msxml2.ServerXMLHTTP")
oMsXmlHttp.setTimeOuts [Resolve], [Connect], [Send], [Recieve]
oMsXmlHttp.Open "POST", [Url], False
oMsXmlHttp.Send [Post Request]
sResult = oMsXmlHttp.responseText

The parameters here are pretty straight-forward: The timeouts are measured in milliseconds, the URL is the track-back URL, sResult is the response the track-back page returns to us and the [Post Request] is the actual trackback instruction we send to the track-back URL. This is passed as a typical URL encoded string as per the format given in RFC 1866.

sResult is an XML string returned from the post request track-back page. The XML response is pretty straight forward. The root element is called response. It has two children: the error and message elements. If no error is returned and the track-back was successful then the error element contains "0" and no message element is returned. If there was a problem then the error element contains a "1" and the reason for the failed track-back attempt is returned in the message element.

So there you go.. Emoticon: Wink About fifteen minutes work to add the ability to perform trackbacks to your blogging system. Of course, this won't allow your blog to recieve track-back pings that's a little more complicated but I'll probably cover that when I implement that part of the specification.


22:24:50 GMT | #Programming | Permalink
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