GZIP is a lossless data compression algorithm used to make files smaller without losing any information. This is particularly effective for text-based files like HTML, CSS, and JavaScript because they often contain repetitive code and syntax.

GZIP works by using a combination of two methods:

• LZ77 Algorithm: This is the core of GZIP. It scans a file for repeated strings of data. When it finds a repeated sequence, instead of writing it out again, it replaces the repeated string with a reference to the first time that string appeared. The reference consists of a distance to the original string and the length of the string.
• Huffman Coding: After the LZ77 algorithm has replaced the repeated strings, Huffman coding is applied. This method assigns a shorter binary code to characters that appear frequently and a longer code to those that appear less frequently. This further reduces the overall file size.

When a web server is configured to use GZIP, it automatically compresses the files before sending them to a user’s browser. The browser, in turn, recognizes the compression and quickly decompresses the files to display the website. This process happens in a fraction of a second, resulting in a much faster page load time and a better user experience.

Here’s an excellent video that provides a visual explanation of how GZIP and other compression algorithms work: How Gzip Compression Works.

Explaind in detail

The process of GZIP compression can be broken down into a few key steps:

1. String Matching: The algorithm, which is based on the DEFLATE compression method, first scans the data for repeating sequences of bytes, or “strings.” It creates a dictionary of these recurring strings and their corresponding positions.
2. Repetitive Data Replacement: Once a repeating string is identified, GZIP doesn’t store the full string again. Instead, it replaces every subsequent instance with a pointer. This pointer consists of two pieces of information:
   • Distance: The number of bytes to look back from the current position to find the original occurrence of the string.
   • Length: The number of bytes in the string that should be repeated.
3. Huffman Coding: After the repetitive data is replaced, the remaining data and the newly created pointers are encoded using Huffman coding. This is a form of variable-length coding where the most frequently occurring characters (or in this case, the pointers and remaining unique strings) are assigned the shortest codes, while less frequent ones get longer codes. This further reduces the overall file size.
4. Encoding and Decoding: When a user’s browser requests a web page, the web server checks if the browser supports GZIP (almost all modern browsers do). If it does, the server compresses the HTML, CSS, and JavaScript files using GZIP before sending them. The compressed file is delivered with an HTTP header (Content-Encoding: gzip). Upon receiving the file, the browser recognizes the header, automatically “unzips” the file, and renders the content. This entire process happens almost instantaneously and is transparent to the user.

For example, consider a simple HTML file with a lot of repeated code. GZIP would find a repetitive string like div class=”container” and replace every subsequent instance with a short pointer, such as (200, 15) (meaning “go back 200 bytes and copy 15 bytes”). This is much more efficient than storing the entire string each time.

The final compressed file is significantly smaller, which means it takes less time to travel over the internet, resulting in faster page load times. This is why enabling GZIP compression is a fundamental step in website performance optimization.

What are the benefits of using GZIP over other compression methods?

The main benefits of GZIP over other compression methods are its high compression efficiency for text-based files, its widespread adoption, and its low resource usage. It’s the standard for web compression because it strikes the ideal balance between performance, compatibility, and effectiveness.

Key Advantages of GZIP

1. High Compression Ratio for Web Content

GZIP is specifically optimized for text-based files like HTML, CSS, and JavaScript, which contain a lot of repetitive code and keywords. Its algorithm excels at finding and replacing these long, repeated strings with short pointers. This results in a very high compression ratio for the types of files that make up most of a website, significantly reducing their size and speeding up transfer times.

2. Universal Compatibility

GZIP has been a standard for web compression for decades. It’s supported by virtually every modern web browser and server. This means you don’t have to worry about compatibility issues; if you enable GZIP compression on your server, you can be confident that nearly all of your visitors’ browsers will be able to decompress and display your content correctly. Other, more modern compression methods like Brotli are excellent but are not yet as universally supported.

3. Low Server and Client Resource Usage

The GZIP algorithm is fast and doesn’t require a lot of processing power to compress or decompress a file. This is crucial for a web server that might be handling hundreds or thousands of requests simultaneously. Similarly, a user’s browser can decompress the file with minimal CPU usage, which is important for mobile devices with limited battery life and processing power. While other algorithms might achieve slightly better compression, they often do so at the cost of higher CPU usage and slower compression/decompression times. GZIP’s efficiency ensures a quick and seamless user experience.