Gzip is an HTTP filter which enables Envoy to compress dispatched data from an upstream service upon client request. Compression is useful in situations where large payloads need to be transmitted without compromising the response time.
The window bits is a number that tells the compressor how far ahead in the text the algorithm should be looking for repeated sequence of characters. Due to a known bug in the underlying zlib library, window bits with value eight does not work as expected. Therefore any number below that will be automatically set to 9. This issue might be solved in future releases of the library.
The Gzip filter supports the following runtime settings:
- The % of requests for which the filter is enabled. Default is 100.
How it works¶
When gzip filter is enabled, request and response headers are inspected to determine whether or not the content should be compressed. The content is compressed and then sent to the client with the appropriate headers if either response and request allow.
By default compression will be skipped when:
- A request does NOT contain accept-encoding header.
- A request includes accept-encoding header, but it does not contain “gzip” or “*”.
- A request includes accept-encoding with “gzip” or “*” with the weight “q=0”. Note that the “gzip” will have a higher weight then “*”. For example, if accept-encoding is “gzip;q=0,*;q=1”, the filter will not compress. But if the header is set to “*;q=0,gzip;q=1”, the filter will compress.
- A request whose accept-encoding header includes “identity”.
- A response contains a content-encoding header.
- A response contains a cache-control header whose value includes “no-transform”.
- A response contains a transfer-encoding header whose value includes “gzip”.
- Neither content-length nor transfer-encoding headers are present in the response.
- Response size is smaller than 30 bytes (only applicable when transfer-encoding is not chuncked).
When compression is applied:
- The content-length is removed from response headers.
- Response headers contain “transfer-encoding: chunked” and “content-encoding: gzip”.
- The “vary: accept-encoding” header is inserted on every response.
Every configured Gzip filter has statistics rooted at <stat_prefix>.gzip.* with the following:
|compressed||Counter||Number of requests compressed.|
|not_compressed||Counter||Number of requests not compressed.|
|no_accept_header||Counter||Number of requests with no accept header sent.|
|header_identity||Counter||Number of requests sent with “identity” set as the accept-encoding.|
|header_gzip||Counter||Number of requests sent with “gzip” set as the accept-encoding.|
|header_wildcard||Counter||Number of requests sent with “*” set as the accept-encoding.|
|header_not_valid||Counter||Number of requests sent with a not valid accept-encoding header (aka “q=0” or an unsupported encoding type).|
|total_uncompressed_bytes||Counter||The total uncompressed bytes of all the requests that were marked for compression.|
|total_compressed_bytes||Counter||The total compressed bytes of all the requests that were marked for compression.|
|content_length_too_small||Counter||Number of requests that accepted gzip encoding but did not compress because the payload was too small.|
|not_compressed_etag||Counter||Number of requests that were not compressed due to the etag header. disable_on_etag_header must be turned on for this to happen.|