Improve performance by compressing files in Azure CDN

File compression is a simple and effective method to improve file transfer speed and increase page-load performance by reducing a file's size before it is sent from the server. File compression can reduce bandwidth costs and provide a more responsive experience for your users.

There are two ways to enable file compression:

  • Enable compression on your origin server. In this case, Azure CDN passes along the compressed files and delivers them to clients that request them.
  • Enable compression directly on the CDN POP servers (compression on the fly). In this case, the CDN compresses the files and serves them to the end users, even if they were not compressed by the origin server.

Important

Azure CDN configuration changes can take some time to propagate through the network:

  • For Azure CDN Standard from Microsoft profiles, propagation usually completes in 10 minutes.
  • For Azure CDN Standard from Akamai profiles, propagation usually completes within one minute.
  • For Azure CDN Standard from Verizon and Azure CDN Premium from Verizon profiles, propagation usually completes in 10 minutes.

If you're setting up compression for the first time for your CDN endpoint, consider waiting 1-2 hours before you troubleshoot to ensure the compression settings have propagated to the POPs.

Enabling compression

The standard and premium CDN tiers provide the same compression functionality, but the user interface differs. For more information about the differences between standard and premium CDN tiers, see Azure CDN Overview.

Standard CDN profiles

Note

This section applies to Azure CDN Standard from Microsoft, Azure CDN Standard from Verizon, and Azure CDN Standard from Akamai profiles.

  1. From the CDN profile page, select the CDN endpoint you want to manage.

    CDN profile endpoints

    The CDN endpoint page opens.

  2. Select Compression.

    CDN compression selection

    The compression page opens.

  3. Select On to turn on compression.

    CDN file compression options

  4. Use the default MIME types, or modify the list by adding or removing MIME types.

    Tip

    Although it is possible, it is not recommended to apply compression to compressed formats. For example, ZIP, MP3, MP4, or JPG.

  5. After making your changes, select Save.

Premium CDN profiles

Note

This section applies only to Azure CDN Premium from Verizon profiles.

  1. From the CDN profile page, select Manage.

    CDN Manage select

    The CDN management portal opens.

  2. Hover over the HTTP Large tab, then hover over the Cache Settings flyout. Select Compression.

    CDN compression selection

    The compression options are displayed.

    CDN file compression options

  3. Enable compression by selecting Compression Enabled. Enter the MIME types you want to compress as a comma-delimited list (no spaces) in the File Types box.

    Tip

    Although it is possible, it is not recommended to apply compression to compressed formats. For example, ZIP, MP3, MP4, or JPG.

  4. After making your changes, select Update.

Compression rules

Azure CDN Standard from Microsoft profiles

For Azure CDN Standard from Microsoft profiles, all files are eligible for compression. However, a file must be of a MIME type that has been configured for compression.

These profiles support the following compression encodings:

  • gzip (GNU zip)
  • brotli

If the request supports more than one compression type, those compression types take precedence over brotli compression.

When a request for an asset specifies gzip compression and the request results in a cache miss, Azure CDN performs gzip compression of the asset directly on the POP server. Afterward, the compressed file is served from the cache.

Azure CDN from Verizon profiles

For Azure CDN Standard from Verizon and Azure CDN Premium from Verizon profiles, only eligible files are compressed. To be eligible for compression, a file must:

  • Be larger than 128 bytes
  • Be smaller than 1 MB

These profiles support the following compression encodings:

  • gzip (GNU zip)
  • DEFLATE
  • bzip2
  • brotli

If the request supports more than one compression type, those compression types take precedence over brotli compression.

When a request for an asset specifies brotli compression (HTTP header is Accept-Encoding: br) and the request results in a cache miss, Azure CDN performs brotli compression of the asset directly on the POP server. Afterward, the compressed file is served from the cache.

Azure CDN Standard from Akamai profiles

For Azure CDN Standard from Akamai profiles, all files are eligible for compression. However, a file must be of a MIME type that has been configured for compression.

These profiles support gzip compression encoding only. When a profile endpoint requests a gzip-encoded file, it is always requested from the origin, regardless of the client request.

Compression behavior tables

The following tables describe Azure CDN compression behavior for every scenario:

Compression is disabled or file is ineligible for compression

Client-requested format (via Accept-Encoding header) Cached-file format The CDN response to the client Notes                                                           
Compressed Compressed Compressed
Compressed Uncompressed Uncompressed
Compressed Not cached Compressed or Uncompressed The origin response determines whether CDN performs a compression.
Uncompressed Compressed Uncompressed
Uncompressed Uncompressed Uncompressed
Uncompressed Not cached Uncompressed

Compression is enabled and file is eligible for compression

Client-requested format (via Accept-Encoding header) Cached-file format CDN response to the client Notes
Compressed Compressed Compressed CDN transcodes between supported formats.
Compressed Uncompressed Compressed CDN performs a compression.
Compressed Not cached Compressed CDN performs a compression if the origin returns an uncompressed file.
Azure CDN from Verizon passes the uncompressed file on the first request and then compresses and caches the file for subsequent requests.
Files with the Cache-Control: no-cache header are never compressed.
Uncompressed Compressed Uncompressed CDN performs a decompression.
Uncompressed Uncompressed Uncompressed
Uncompressed Not cached Uncompressed

Media Services CDN Compression

For endpoints enabled for Media Services CDN streaming, compression is enabled by default for the following MIME types:

  • application/vnd.ms-sstr+xml
  • application/dash+xml
  • application/vnd.apple.mpegurl
  • application/f4m+xml

See also