pf » Supporting If-Modified-Since HTTP header in CFML

The If-Modified-Since header in the HTTP protocol allows user agents (typically RSS aggregators, or Spiders) to check and see if the content they are about to download has changed since their last visit. The user agent sends something like this in the headers:

If-Modified-Since: Fri, 18 Feb 2005 19:24:26 GMT

You can access this header from CFML, compare the date your content was last changed, and then if the content was not modified since the date passed, you return a 304 status code. If the content has been modified, you just return the content as you normally would. Here's some example CFML you can put at the top of your page, you need to update the pageModificationDate:

<cftry>
<cfif StructKeyExists(GetHttpRequestData().headers, "If-Modified-Since")>
	<cfset ifmodDate = ParseDateTime(GetHttpRequestData().headers["If-Modified-Since"])>
	<cfset pageModificationDate = GetHttpTimeString(ParseDateTime(Now()))>
	<cfif DateDiff("n", pageModificationDate, ifmodDate) GT 0>
		<cfheader statuscode="304" statustext="Not Modified"> 
		<cfabort>
	</cfif>
</cfif>
<cfcatch type="any"></cfcatch>
</cftry>

I use the GetHttpRequestData() function to get the value of the If-Modified-Since header (notice that I have to use StructKeyExists instead of IsDefined due to the dash - in the header name).

Also note that the dates need to be formatted properly, use the GetHttpTimeString() function for this.

I also put in a try/catch block to catch any date parsing errors, that way if there is an error parsing a date sent by a bad client, they still get the content, you may decide to log the error, or give a 500 level status code to such clients.

I implemented this with my RSS feed for this blog, so I should see a decrease in bandwidth on my RSS feed. Well written aggregators will only get content when my feed is updated, and just the headers otherwise.