Improve performance by compressing files in Azure CDN

Article
02/28/2023
4 minutes to read

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's 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 don't get 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.

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

The CDN endpoint page opens.
Select Compression.

The compression page opens.
Select On to turn on compression.
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.
After making your changes, select Save.

Premium CDN profiles

Note

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

From the CDN profile page, select Manage.

The CDN management portal opens.
Hover over the HTTP Large tab, then hover over the Cache Settings flyout. Select Compression.

The compression options are displayed.
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.
After making your changes, select Update.

Compression rules

Azure CDN Standard from Microsoft profiles

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

Be of a MIME type that has been configured for compression
Have only "identity" Content-Encoding headers in the origin response
Be larger than 1 KB
Be smaller than 8 MB

These profiles support the following compression encodings:

gzip (GNU zip)
brotli

If the request supports more than one compression type, brotli compression takes precedence.

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.

If the origin uses Chunked Transfer Encoding (CTE) to send compressed data to the CDN POP, then response sizes greater than 8 MB aren't supported.

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 3 MB

These profiles support the following compression encodings:

gzip (GNU zip)
DEFLATE
bzip2

Azure CDN from Verizon doesn't support brotli compression. When the HTTP request has the header Accept-Encoding: br, the CDN responds with an uncompressed response.

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's 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. Azure CDN from Microsoft doesn't support transcoding between formats and instead fetches data from origin, compresses and caches separately for the format.
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. Azure CDN from Microsoft doesn't support decompression and instead fetches data from origin and caches separately for uncompressed clients.
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