Two Simple Rules for HTTP Caching

作者：httpwatch blog
日期：2007.12.10
链接：http://blog.httpwatch.com/2007/12/10/two-simple-rules-for-http-caching/

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In practice, you only need two settings to optimize caching:

   

1.        
2. Don’t cache HTML        
3. Cache everything else forever    

   

“Wooah…hang on/!”, we hear you say. “Cache all my scripts and     images forever?“

   

Yes, that’s right. You don’t need anything else in between. Caching     indefinitely is fine as long as you don’t allow your HTML to be cached.

   

“But what about if I need to issue code patches to my JavaScript? I     can’t allow browsers to hold on to all my images either. I     often need to update those as well.”

   

Simple - just change the URL of the item in your HTML and it will bypass     the existing entry in the cache.

   

In practice, caching ‘forever’ typically means setting an Expires     header value of Sun, 17-Jan-2038 19:14:07 GMT since that’s the     maximum value supported by the 32 bit Unix time/date format. If you’re using     IIS6 you’ll find that the UI won’t allow anything beyond 31-Dec-2035. The     advantage of setting long expiry dates is that the content can be read from     the local browser cache whenever the user revisits the web page or goes to     another page that uses the same images, script or CSS files.

   

You’ll see long expiry dates like this if you look at a Google web page     with HttpWatch. For example, here are the response headers used for the main     Google logo on the home page:

   

   

   

If Google needs to change the logo for a special occasion like Halloween     they just change the name of the file in the page’s HTML to something like     halloween2007.gif.

   

The diagram below shows how a JavaScript file is loaded into the browser     cache on the first visit to a web page:

   

   

   

On any subsequent visits the browser only has to fetch the page’s HTML:

   

   

   

The JavaScript file can be read directly from the browser cache on the     user’s hard disk. This avoids a network round trip and is typically 100 to     1000 times faster than downloading the file over a broadband connection.

   

The key to this caching scheme is to keep tight control over your HTML as     it holds the references to everything else on your web site. One way to do     this is to ensure that your pages have a Cache-Control: no-cache     header. This will prevent any caching of the HTML and will ensure the     browser requests the page’s HTML every time.

   

If you do this, you can update any content on the page just by changing     the URL that refers to it in the HTML. The old version will still be in the     browser’s cache, but the updated version will be downloaded because of the     modified URL.

   

For instance, if you had a file called topMenu.js and you fixed some bugs     in it, you might rename the file topMenu-v2.js to force it to be downloaded:

   

   

   

Now this is all very well, but whenever there’s a discussion of     longer expiration times, the marketing people get very twitchy and concerned     that they won’t be able to re-brand a site if stylesheets and images are     cached for long periods of time.

   

In fact, choosing an expiration time of anything other than zero or     infinite is inherently uncertain. The only way to know exactly when you can     release a new version to all users simultaneously is to choose a specific     time of day for your cache expiry; say midnight. It’s better to set     indefinite caching on all your page-linked items so that you get the maximum     amount of caching, and then force updates as required.

   

Now, by this point, you might have the marketing types on board but     you’ll be losing the developers. The developers by now are seeing all the     extra work involved in changing the filenames of all their CSS, javascript     and images both in their source controlled projects and in their deployment     scripts.

   

So here’s the icing on the cake; you don’t actually need to     change the filename, just the URL. A simple way to do this is to append a     query string parameter onto the end of the existing URL when the resource     has changed.

   

Here’s the previous example that updated a JavaScript file. The     difference this time is that it uses a query string parameter ‘v2′ to bypass     the existing cache entry:

   

   

   

The web server will simply ignore the query string parameter unless you     have chosen to do anything with it programmatically.

   

There’s one final optimization you can make. The Cache-Control:     no-cache response header works well for dynamic pages as it ensures     that pages will always be refreshed from the server; even when pressing the     Back button. However, for HTML that changes less frequently it is better to     use the Last-Modified header instead. This will avoid a     complete download of the page’s HTML, if it has not changed since it was     last cached by the browser.

   

The Last-Modified header is added automatically by IIS for     static HTML files and can be added programmatically in dynamic pages (e.g.     ASPX and PHP). When this header is present, the browser will revalidate the     local, cached copy of an HTML page in each new browser session. If the page     is unchanged the web server returns a 304 Not Modified response     indicating the browser can use the cached version of the page.

   

So to summarize:

   

1.        
2. Don’t cache HTML        
   •            
   • Use Cache-Control: no-cache for dynamic HTML pages            
   • Use the Last-Modified header with the current file             time for static HTML        
                  
3. Cache everything else forever        
   •            
   • For all other file types set an Expires header to             the maximum future date your web server will allow        
                  
4. Modify URLs by appending a query string in your HTML to any page         element you wish to ‘expire’ immediately.