Compressing Web Content with mod_gzip and mod_deflate

Reducing costs is a key consideration for every IT budget. One of the items looked at most closely is the cost of a company's bandwidth. Using content compression on a Web site is one way to reduce both bandwidth needs and cost. With that in mind, this article examines some of the compression modules available for Apache, specifically, mod_gzip for Apache 1.3.x and 2.0.x and mod_deflate for Apache 2.0.x.

Most compression algorithms, when applied to a plain-text file, can reduce its size by 70% or more, depending on the content in the file. When using compression algorithms, the difference between standard and maximum compression levels is small, especially when you consider the extra CPU time necessary to process these extra compression passes. This is quite important when dynamically compressing Web content. Most software content compression techniques use a compression level of 6 (out of 9 levels) to conserve CPU cycles. The file size difference between level 6 and level 9 is usually so small as to be not worth the extra time involved.

For files identified as text/.* MIME types, compression can be applied to the file prior to placing it on the wire. This simultaneously reduces the number of bytes transferred and improves performance. Testing also has shown that Microsoft Office, StarOffice/OpenOffice and PostScipt files can be GZIP-encoded for transport by the compression modules.

Some important MIME types that cannot be GZIP encoded are external JavaScript files, PDF files and image files. The problem with Javascript files mainly is due to bugs in browser software, as these files are really text files and overall performance would benefit by being compressed for transport. PDF and image files already are compressed, and attempting to compress them again simply makes them larger and leads to potential rendering issues with browsers.

Prior to sending a compressed file to a client, it is vital that the server ensures the client receiving the data correctly understands and renders the compressed format. Browsers that understand compressed content send a variation of the following client request headers:

Current major browsers include some variation of this message with every request they send. If the server sees the header and chooses to provide compressed content, it should respond with the server response header:

Content-encoding: gzip

This header tells the receiving browser to decompress the content and parse it as it normally would. Alternatively, content may be passed to the appropriate helper application, based on the value of the Content-type header.

The file size benefits of compressing content can be seen easily by looking at a couple of examples, one an HTML file (Table 1) and the other a PostScript file (Table 2). Performance improvements are examined later in this article.

mod_deflate for Apache versions 2.0.44 and earlier comes with the compression ratio set for best speed, not best compression. This configuration can be modified using the tips found at www.webcompression.org/mod_deflate-hack.php. Starting with Apache 2.0.45, a configuration directive is included.