Enabling automatic gzipping

  Last updated 2021-09-27

Fastly supports gzip data compression to speed up information transmission. The Fastly gzip feature dynamically fetches content from origin, compresses it, and then caches it. There are two ways to enable gzip:

  • Based on file extension. Enable the default gzip policy to compress content in files with the following extensions: css js html eot ico otf ttf json svg.
  • Based on content type. Set up an advanced gzip policy to customize the content and conditions for compression.

Enabling gzip

To dynamically gzip cacheable content based on file extension or content-type, follow the steps below to enable the default gzip policy:

  1. Log in to the Fastly web interface.
  2. From the All services page, select the appropriate service. You can use the search box to search by ID, name, or domain.
  3. Click the Edit configuration button and then select the option to clone the active version. The Domains page appears.
  4. Click the Content link. The Content page appears.
  5. Click the Default gzip policy switch to enable gzip.
  6. Click the Activate button to deploy your configuration changes.

Setting up an advanced gzip policy

To customize the content that’s compressed and the conditions under which this compression occurs, follow the steps below:

  1. Log in to the Fastly web interface.
  2. From the All services page, select the appropriate service. You can use the search box to search by ID, name, or domain.
  3. Click the Edit configuration button and then select the option to clone the active version. The Domains page appears.
  4. Click the Content link. The Content page appears.
  5. Click the Set up advanced gzip button. The Create a gzip page appears.

  6. Click the Override these defaults link. Additional gzip fields appear.

    Create a new gzip

  7. Fill out the Create a gzip fields as follows:
    • In the Name field, enter an arbitrary name for your new gzip rule.
    • In the Extensions field, enter the file extension for each file type to be dynamically gzipped, separated by spaces. Only enter the three- or four-letter string representing the file extension.
    • In the Content Types field, enter the content-type for each type of content to dynamically gzip, separated by spaces. Do not use regular expressions.
  8. Click the Create button. The new gzip policy appears.
  9. Click the Activate button to deploy your configuration changes.

Automatic normalization

Because gzip is one of the most common reasons to vary output based on a request header, Fastly will normalize the value of Accept-Encoding on incoming requests. The modified header will be set to a single encoding type, or none, and will reflect the best compression scheme supported by the browser. This includes removing Accept-Encoding values in requests from browsers that advertise support for gzip but whose implementation is broken, such as IE6.

Specifically, we run the following steps on inbound requests:

  1. If the User-Agent matches a pattern for browsers that have problems with compressed responses, remove the Accept-Encoding header
  2. Else if the Accept-Encoding header includes the string "gzip", set the entire value to the string "gzip"
  3. Else if the Accept-Encoding header includes the string "deflate", set the entire value to the string "deflate"
  4. Else remove the Accept-Encoding header

Where this normalization process has changed the header value, the original value is made available in the custom header Fastly-Orig-Accept-Encoding.

If a user agent advertises support for Brotli, currently we will normalize this to gzip because we do not support Brotli encoding at the edge. However, if you are doing Brotli encoding at your origin server, you may want to modify our normalization algorithm.

Limitations and considerations

This feature has the following limitations and considerations you should take into account:

  • The automatic gzip feature can only compress cacheable content.
  • Content already compressed at origin will be cached as-is. We will not attempt to re-compress it.
  • As a best practice, we recommend compressing content at the origin whenever possible to improve performance and reduce costs (e.g., cloud egress charges).
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