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gstrauss, 2021-06-21 10:45


mod_deflate (since lighttpd 1.4.42) enables output compression of responses. (Content-Encoding)

Output compression reduces the network load and can improve the overall throughput of the webserver. All major http-clients support compression by announcing it in the Accept-Encoding header. This is used to negotiate the most suitable compression method. We support deflate, gzip, bzip2, brotli (since 1.4.56), and zstd (since 1.4.59).

deflate (RFC1950, RFC1951) and gzip (RFC1952) depend on zlib while bzip2 depends on libbzip2. bzip2 is only supported by lynx and some other console text-browsers. brotli (RFC7932) is supported in most popular browsers.

Since lighttpd 1.4.56, mod_deflate subsumes and replaces mod_compress. mod_deflate can compress static and dynamic responses, while mod_compress could compress only static files.

Module options:

#deflate.mimetypes = ("text/")       # prefix matches all text/* Content-Type responses
deflate.mimetypes = ("text/html", "text/plain", "text/css", "text/javascript", "text/xml")
deflate.allowed-encodings = ( "br", "gzip", "deflate" ) # "bzip2" and "zstd" also supported

## optional

deflate.cache-dir = "/path/to/compress/cache" 
#deflate.max-compress-size = 131072   # measured in kilobytes, so 131072 indicates 128 MB
#deflate.min-compress-size = 256      # measured in bytes
#deflate.compression-level = 9
#deflate.output-buffer-size = 8192 = 2048
#deflate.max-loadavg = "3.50" 

deflate.max-compress-size is the largest response size that will be compressed.
deflate.min-compress-size is the smallest response size that will be compressed.

deflate.compression-level is compression level or quality tuning for the underlying compressor.
(more info: man gzip (1..9); man bzip2 (1..9); man -s 3brotli encode.h (0..11)

deflate.output-buffer-size is a per connection buffer for compressed output, it can
help decrease the response size (fewer chunks to encode). If it is set to
zero, a shared buffer will be used. is the number of kilobytes to compress at one time, it allows
the webserver to do other work (network I/O) in between compression.

deflate.max-loadavg is max system loadavg before bypassing compression (since 1.4.43)

deflate.cache-dir (since 1.4.56) is the location under which to store cache of compressed files. Cleaning the cache is left to the user. A cron job deleting files older than
10 days could do it: find /path/to/compress/cache -type f -mtime +10 | xargs -r rm

deflate.allowed-encodings lists the encodings from which the server will select. Since lighttpd 1.4.60, the ordering of this list is preserved and the server selects the first encoding from this list which matches a value in the client request Accept header.

Known Limitations

mod_deflate currently does not stream compressed content in chunks. This affects very large dynamic responses, or dynamic responses sent in chunks with large time lapses between chunks. If the entire response is not ready when the response header is sent, mod_deflate does not process the response. (e.g. dynamic response from backend and is set > 0) While mod_deflate does not handle this use case, the backend producing the streaming response is able to apply an appropriate Content-Encoding itself (instead of mod_deflate).

mod_deflate deflate.cache-dir, if set, contains cached output of static files, but does not cache dynamic responses unless the response contains an ETag response header. (dynamic responses, if eligible, are still compressed by mod_deflate before the response is sent to the client.)

mod_deflate must be listed after mod_setenv in server.modules if mod_setenv might be used to force setting "Content-Encoding" response header.


For selection/negotiation of pre-compressed files in the document root, e.g. or main.html.gz when main.html is requested, use mod_magnet with custom lua code for Content-Negotiation

Updated by gstrauss over 2 years ago · 36 revisions