I just transitioned my blog in two huge ways: (a) I reassigned it to a different domain name, and (b) I changed the blogging engine I was using, which incidentally used a file organization structure that's incompatible with my new engine.

Furthermore, I decided to import all of my old posts into my new blog rather than leave the old and start anew.

I don't regret any of the changes I've made, because I've laid groundwork that I'll be much happier with in the future. But these changes can wreak havoc on my site's perceived status, since any links to my pages and any stale search engine referrals will now result in ugly 404 errors. Sure, we could set up a friendly 404 page that notifies the user of what's happened. However, why don't we eliminate the embarrassment of notifying the user at all and simply redirect them as best as possible?

An Example Scenario
In my case, I was moving my blog from the root of my site (www.nazin.com) to a sub-domain (http://blog.nazin.com). Furthermore, I was migrating from Blogger.com, which publishes an HTML page for each post, to WordPress, which loads everything from index.php and uses the post slug in the query string to determine what post to display. For instance, one highly trafficked page was www.nazin.com/2004/08/airtunes-concerns-answered.htm, and my new site housed this post under http://blog.nazin.com/index.php/airtunes-concerns-answered/. Once all the old HTML files were removed from the old site, those pages would come up as 404 Page Not Found errors.

The Technology Behind the Solution
To redirect the requests, assign a custom 404 page via the system administration for your site. This will naturally vary from one situation to the next depending on your server situation (Do you have direct server access? Do you have shared hosting?). Most Web servers will communicate the error and the address in the CGI query string. We'll use the original address the user attempted to navigate to as a means to redirect them to the post on the newly migrated blog. As always, I'll do this with CF.

Building a ColdFusion 404 Handler
Through the CGI structure, we can take a look at the input we're getting from the Web server for the error and address. Let's create a preliminary 404 handler page. Let's name it 404.cfm.

<cfoutput]#CGI.QUERY_STRING#[/cfoutput>

If we were to implement this and attempt to browse to the scenario page above, we'd get output similar to this:

404;http://www.nazin.com:80/2004/08/airtunes-concerns-answered.htm

To the human eye, it's immediately apparent how to take this output and create a redirect. At this point, it's simply a matter of text manipulation. Perhaps we might write some code like:

<cfscript>
    searchTerm=ListLast(CGI.QUERY_STRING,"/") ;
    searchTerm=ListDeleteAt(searchTerm,ListLen(searchTerm,"."),".") ;
    newURL="http://blog.nazin.com/index.php/" & searchTerm & "/" ;
    </cfscript>
    <cflocation addtoken="no" url="#newURL#">

Here, we're effectively telling ColdFusion:
(1) Grab just the filename of the address (producing "airtunes-concerns-answered.htm").
(2) Delete the filename's extension (producing "airtunes-concerns-answered"). There are UDFs that have been written to perform this function as well, such as ripExt() at cflib.org.
(3) Build the new URL and
(4) Redirect the browser to that location.

Try browsing to the deleted page, and the redirection works like a charm. However, there's a serious issue lingering in our solution: What if your post slugs don't always match from the old blog to the new blog, if you imported your old posts like I did?

Addressing Mismatched Slugs: A Different Approach
For my scenario page, the previous example worked perfectly. However, during the import process, WordPress didn't always create post slugs with exactly the same names as the HTML files generated by Blogger.com. Is it still possible to redirect the user?

Sure. Instead of attempting to redirect the user to the new post's address, try redirecting them to the more forgiving search page.

For the next example, let's use another high-traffic page that breaks the previous solution: www.nazin.com/2005/06/how-to-install-itunes-on-windows-xp.htm. On the new blog, this page is http://blog.nazin.com/index.php/how-to-install-itunes-on-windows-xp-sp2/; as you can see, the slug is slightly different because Blogger.com truncated it. But this isn't the only thing that could cause different slugs. Perhaps certain characters are treated differently in the different engines, or perhaps a customized slug was created. In any case, relying on the new blog's search engine will address the problem.

The code is actually simple:

<cfscript>
    searchTerm=ListLast(CGI.QUERY_STRING,"/") ;
    searchTerm=ListDeleteAt(searchTerm,ListLen(searchTerm,"."),".") ;
    searchTerm=Replace(searchTerm,"-","+","ALL") ;
    newURL="http://blog.nazin.com/?s=" & searchTerm ;
    </cfscript>
    <cflocation addtoken="no" url="#newURL#">

The only difference with this approach is that it converts all the dashes (-) in the filename to pluses (+), which are used in the URL string to indicate spaces. Thus, the URL generated by this code (http://blog.nazin.com/?s=how+to+install+itunes+on+windows+xp) would be the equivalent of someone searching for "how to install itunes on windows xp" in the search box.

The user will thus be redirected to the new blog, with a link directly to the page desired.

Further Enhancements
The final step you could take to really polish the process would be to modify the search results page on your blog to redirect the user to the search results page in the event that only a single record is returned. The code to do this would vary from one engine to the next, but it would achieve the same effect as the previous solution, even when your post slugs differ.

Migrating your blog to a new domain name or to a new blogging engine can be disruptive, especially when you have other sites or search engines linking to your old, out-dated URLs. With very little time and effort, however, a simple 404 handler that utilizes the techniques we just considered can take the pain out of the migration.