Compression

Overview

OLS supports both Gzip and Brotli compression for HTTP responses. Compression reduces bandwidth usage and improves page load times, particularly for text-based content like HTML, CSS, JavaScript, and JSON.

Enable Compression

Compression is configured in httpd_config.conf under the tuning section:

tuning {
  enableGzipCompress      1
  enableBrotliCompress    1
  enableDynGzipCompress   1
  gzipCompressLevel       6
  brotliCompressLevel     5
  compressibleTypes       default
  gzipAutoUpdateStatic    1
  gzipStaticCompressLevel 6
  gzipMaxFileSize         10M
  gzipMinFileSize         300
}

Parameters

Parameter	Default	Description
enableGzipCompress	1	Enable Gzip for static files
enableBrotliCompress	1	Enable Brotli compression
enableDynGzipCompress	1	Enable Gzip for dynamic (PHP) responses
gzipCompressLevel	6	Gzip level for dynamic content (1-9)
brotliCompressLevel	5	Brotli level for dynamic content (0-11)
compressibleTypes	default	MIME types to compress
gzipAutoUpdateStatic	1	Auto-create .gz files for static resources
gzipStaticCompressLevel	6	Gzip level for pre-compressed static files
gzipMaxFileSize	10M	Maximum file size to compress
gzipMinFileSize	300	Minimum file size to compress (skip tiny files)

Compression Levels

Gzip (1-9)

Level	CPU usage	Compression ratio	Recommended for
1	Very low	~60%	High-traffic sites where CPU is scarce
4-6	Moderate	~75-80%	General purpose (recommended)
9	High	~82%	Static pre-compression only

Brotli (0-11)

Brotli provides better compression ratios than Gzip at similar CPU cost. However, levels above 6 become significantly more CPU-intensive and are only suitable for static pre-compression.

Level	CPU usage	Compression ratio	Recommended for
1-4	Low	Better than Gzip 6	Dynamic content
5-6	Moderate	Significantly better	General purpose (recommended)
7-11	Very high	Maximum compression	Static pre-compression only

Compressible MIME Types

The default value for compressibleTypes covers common text-based types. To customize:

tuning {
  compressibleTypes       text/*, application/javascript, application/json, \
                          application/xml, application/xhtml+xml, \
                          application/rss+xml, application/atom+xml, \
                          application/x-javascript, application/x-httpd-php, \
                          application/x-font-ttf, application/vnd.ms-fontobject, \
                          image/svg+xml, image/x-icon, font/opentype, font/ttf, \
                          font/eot, font/otf
}

Types to Compress

• text/html, text/css, text/javascript, text/xml, text/plain
• application/javascript, application/json, application/xml
• image/svg+xml (SVG is text-based XML)
• application/x-font-ttf, font/woff (some font formats benefit)

Types NOT to Compress

Do not compress already-compressed formats:

• image/jpeg, image/png, image/gif, image/webp
• video/*, audio/*
• application/zip, application/gzip, application/pdf
• font/woff2 (already Brotli-compressed)

Static File Pre-Compression

OLS can pre-compress static files and serve the compressed version when available. This avoids runtime CPU usage.

When gzipAutoUpdateStatic is 1, OLS automatically creates .gz versions of static files on first request. The pre-compressed file is updated when the original file changes.

For Brotli pre-compression, use an external tool:

# Pre-compress static assets with Brotli
find /var/www/example.com -type f \
  \( -name "*.css" -o -name "*.js" -o -name "*.html" -o -name "*.svg" \) \
  -exec brotli -f -Z {} \;

This creates .br files alongside the originals. OLS will serve these when the client supports Brotli.

Per-VHost Compression

Override compression settings for a specific virtual host:

virtualhost example {
  ...
  tuning {
    enableGzipCompress    1
    enableDynGzipCompress 1
    gzipCompressLevel     4
  }
}

.htaccess Compression (LiteHTTPD)

With the LiteHTTPD module, compression can also be configured via .htaccess:

<IfModule mod_deflate.c>
  AddOutputFilterByType DEFLATE text/html text/css text/javascript
  AddOutputFilterByType DEFLATE application/javascript application/json
  AddOutputFilterByType DEFLATE application/xml image/svg+xml
</IfModule>

Note

OLS processes the mod_deflate directives from .htaccess for compatibility, but compression behavior is primarily controlled by the server-level configuration. The .htaccess directives serve as hints.

Verify Compression

Check that responses are compressed:

# Test Gzip
curl -H "Accept-Encoding: gzip" -I https://example.com

# Test Brotli
curl -H "Accept-Encoding: br" -I https://example.com

Look for the Content-Encoding header:

Content-Encoding: gzip

or

Content-Encoding: br

Troubleshooting

Responses not compressed:

• Verify enableGzipCompress is 1
• Check that the response MIME type is in compressibleTypes
• Ensure the response body is larger than gzipMinFileSize
• Confirm the client sends Accept-Encoding: gzip or Accept-Encoding: br

High CPU usage from compression:

• Lower gzipCompressLevel (try 4) or brotliCompressLevel (try 3)
• Disable dynamic compression (enableDynGzipCompress 0) and rely on pre-compressed static files
• Use lower Brotli levels for dynamic content

Double compression artifacts:

• Ensure you are not compressing already-compressed formats
• Check that your application is not compressing responses before OLS adds its compression