Issue link: https://read.uberflip.com/i/1146286
30 | Chapter 4: Rule 4: Gzip Components If the web server sees this header in the request, it may compress the response using one of the methods listed by the client. The web server notifies the web client of this via the Content-Encoding header in the response. Content-Encoding: gzip Gzip is currently the most popular and effective compression method. It is a free for- mat (i.e., unencumbered by patents or other restrictions) developed by the GNU project and standardized by RFC 1952. The only other compression format you're likely to see is deflate, but it's slightly less effective and much less popular. In fact, I have seen only one site that uses deflate: msn.com. Browsers that support deflate also support gzip, but several browsers that support gzip do not support deflate, so gzip is the preferred method of compression. What to Compress Servers choose what to gzip based on file type, but are typically too limited in what they are configured to compress. Many web sites gzip their HTML documents. It's also worthwhile to gzip your scripts and stylesheets, but many web sites miss this opportunity (in fact, it's worthwhile to compress any text response including XML and JSON, but the focus here is on scripts and stylesheets since they're the most prevalent). Image and PDF files should not be gzipped because they are already com- pressed. Trying to gzip them not only wastes CPU resources, it can also potentially increase file sizes. There is a cost to gzipping: it takes additional CPU cycles on the server to carry out the compression and on the client to decompress the gzipped file. To determine whether the benefits outweigh the costs you would have to consider the size of the response, the bandwidth of the connection, and the Internet distance between the client and the server. This information isn't generally available, and even if it were, there would be too many variables to take into consideration. Generally, it's worth gzipping any file greater than 1 or 2K. The mod_gzip_minimum_file_size directive controls the minimum file size you're willing to compress. The default value is 500 bytes. I looked at the use of gzip on 10 of the most popular U.S. web sites. Nine sites gzipped their HTML documents, seven sites gzipped most of their scripts and stylesheets, and only five gzipped all of their scripts and stylesheets. The sites that don't compress all of their HTML, stylesheets, and scripts are missing the opportu- nity to reduce the page weight by up to 70%, as we'll see in the next section, "The Savings." A survey of major web sites and what they choose to compress is shown in Table 4-1.