Friday, September 23, 2011

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How To Optimize Your Site With GZIP Compression

Compression is a simple, effective way to save bandwidth and speed up your site. I hesitated when recommending gzip compression when speeding up your javascript because of problems in older browsers.

But it's the 21st century. Most of my traffic comes from modern browsers, and quite frankly, most of my users are fairly tech-savvy. I don't want to slow everyone else down because somebody is chugging along on IE 4.0 on Windows 95. Google and Yahoo use gzip compression. A modern browser is needed to enjoy modern web content and modern web speed -- so gzip encoding it is. Here's how to set it up.



Setting up the server

The "good news" is that we can't control the browser. It either sends the Accept-encoding: gzip, deflate header or it doesn't.

Our job is to configure the server so it returns zipped content if the browser can handle it, saving bandwidth for everyone (and giving us a happy user).

For IIS, enable compression in the settings.

In Apache, enabling output compression is fairly straightforward. Add the following to your .htaccess file:
# compress text, html, javascript, css, xml:
AddOutputFilterByType DEFLATE text/plain
AddOutputFilterByType DEFLATE text/html
AddOutputFilterByType DEFLATE text/xml
AddOutputFilterByType DEFLATE text/css
AddOutputFilterByType DEFLATE application/xml
AddOutputFilterByType DEFLATE application/xhtml+xml
AddOutputFilterByType DEFLATE application/rss+xml
AddOutputFilterByType DEFLATE application/javascript
AddOutputFilterByType DEFLATE application/x-javascript

# Or, compress certain file types by extension:

SetOutputFilter DEFLATE


Apache actually has two compression options:

mod_deflate is easier to set up and is standard.
mod_gzip seems more powerful: you can pre-compress content.

Deflate is quick and works, so I use it; use mod_gzip if that floats your boat. In either case, Apache checks if the browser sent the "Accept-encoding" header and returns the compressed or regular version of the file. However, some older browsers may have trouble (more below) and there are special directives you can add to correct this.

If you can't change your .htaccess file, you can use PHP to return compressed content. Give your HTML file a .php extension and add this code to the top:

In PHP:



We check the "Accept-encoding" header and return a gzipped version of the file (otherwise the regular version). This is almost like building your own webserver (what fun!). But really, try to use Apache to compress your output if you can help it. You don't want to monkey with your files.

Verify Your Compression

Once you've configured your server, check to make sure you're actually serving up compressed content.

View the headers: Use Live HTTP Headers to examine the response. Look for a line that says "Content-encoding: gzip".



Caveats

As exciting as it may appear, HTTP Compression isn't all fun and games. Here's what to watch out for:

Older browsers: Yes, some browsers still may have trouble with compressed content (they say they can accept it, but really they can't). If your site absolutely must work with Netscape 1.0 on Windows 95, you may not want to use HTTP Compression. Apache mod_deflate has some rules to avoid compression for older browsers.
Already-compressed content: Most images, music and videos are already compressed. Don't waste time compressing them again. In fact, you probably only need to compress the "big 3" (HTML, CSS and Javascript).
CPU-load: Compressing content on-the-fly uses CPU time and saves bandwidth. Usually this is a great tradeoff given the speed of compression. There are ways to pre-compress static content and send over the compressed versions. This requires more configuration; even if it's not possible, compressing output may still be a net win. Using CPU cycles for a faster user experience is well worth it, given the short attention spans on the web.

Enabling compression is one of the fastest ways to improve your site's performance. Go forth, set it up, and let your users enjoy the benefits.