Why use deflate instead of gzip for text files served by Apache

Question

What advantages do either method offer for html  css and javascript files served by a LAMP server  Are there better alternatives   The server provides information to a map application using Json  so a high volume of small files   See also Is there any performance hit involved in choosing gzip over deflate for http compression

User · Accepted Answer

Why use deflate instead of gzip for text files served by Apache?

The simple answer is don't.

RFC 2616 defines deflate as:

deflate The "zlib" format defined in RFC 1950 in combination with the "deflate" compression mechanism described in RFC 1951

The zlib format is defined in RFC 1950 as :

     0   1
     +---+---+
     |CMF|FLG|   (more-->)
     +---+---+

       0   1   2   3
     +---+---+---+---+
     |     DICTID    |   (more-->)
     +---+---+---+---+

     +=====================+---+---+---+---+
     |...compressed data...|    ADLER32    |
     +=====================+---+---+---+---+

So, a few headers and an ADLER32 checksum

RFC 2616 defines gzip as:

gzip An encoding format produced by the file compression program "gzip" (GNU zip) as described in RFC 1952 [25]. This format is a Lempel-Ziv coding (LZ77) with a 32 bit CRC.

RFC 1952 defines the compressed data as:

The format presently uses the DEFLATE method of compression but can be easily extended to use other compression methods.

CRC-32 is slower than ADLER32

Compared to a cyclic redundancy check of the same length, it trades reliability for speed (preferring the latter).

So ... we have 2 compression mechanisms that use the same algorithm for compression, but a different algorithm for headers and checksum.

Now, the underlying TCP packets are already pretty reliable, so the issue here is not Adler 32 vs CRC-32 that GZIP uses.

Turns out many browsers over the years implemented an incorrect deflate algorithm. Instead of expecting the zlib header in RFC 1950 they simply expected the compressed payload. Similarly various web servers made the same mistake.

So, over the years browsers started implementing a fuzzy logic deflate implementation, they try for zlib header and adler checksum, if that fails they try for payload.

The result of having complex logic like that is that it is often broken. Verve Studio have a user contributed test section that show how bad the situation is.

For example: deflate works in Safari 4.0 but is broken in Safari 5.1, it also always has issues on IE.

So, best thing to do is avoid deflate altogether, the minor speed boost (due to adler 32) is not worth the risk of broken payloads.

User · Answer

mod deflate requires fewer resources on your server  although you may pay a small penalty in terms of the amount of compression   If you are serving many small files  I d recommend benchmarking and load testing your compressed and uncompressed solutions - you may find some cases where enabling compression will not result in savings

User · Answer

On Ubuntu with Apache2 and the deflate module already installed  which it is by default   you can enable deflate gzip compression in two easy steps   a2enmod deflate  etc init d apache2 force-reload   And you re away  I found pages I served over my adsl connection loaded much faster   Edit  As per  GertvandenBerg s comment  this enables gzip compression  not deflate

User · Answer

You are likely not able to actually pick deflate as an option  Contrary to what you may expect mod deflate is not using deflate but gzip  So while most of the points made are valid it likely is not relevant for most

User · Answer

if I remember correctly    gzip will compress a little more than deflate deflate is more efficient

User · Answer

There shouldn t be any difference in gzip  amp  deflate for decompression   Gzip is just deflate with a few dozen byte header wrapped around it including a checksum   The checksum is the reason for the slower compression   However when you re precompressing zillions of files you want those checksums as a sanity check in your filesystem   In addition you can utilize commandline tools to get stats on the file   For our site we are precompressing a ton of static data  the entire open directory  13 000 games  autocomplete for millions of keywords  etc   and we are ranked 95  faster than all websites by Alexa   Faxo Search   However  we do utilize a home grown proprietary web server   Apache mod deflate just didn t cut it   When those files are compressed into the filesystem not only do you take a hit for your file with the minimum filesystem block size but all the unnecessary overhead in managing the file in the filesystem that the webserver could care less about   Your concerns should be total disk footprint and access decompression time and secondarily speed in being able to get this data precompressed   The footprint is important because even though disk space is cheap you want as much as possible to fit in the cache

User · Answer

The main reason is that deflate is faster to encode than gzip and on a busy server that might make a difference  With static pages it s a different question  since they can easily be pre-compressed once

User · Answer

The main reason is that deflate is faster to encode than gzip and on a busy server that might make a difference  With static pages it s a different question  since they can easily be pre-compressed once

User · Answer

I think there s no big difference between deflate and gzip  because gzip basically is just a header wrapped around deflate  see RFCs 1951 and 1952

User · Answer

mod deflate requires fewer resources on your server  although you may pay a small penalty in terms of the amount of compression   If you are serving many small files  I d recommend benchmarking and load testing your compressed and uncompressed solutions - you may find some cases where enabling compression will not result in savings

User · Answer

GZip is simply deflate plus a checksum and header footer  Deflate is faster  though  as I learned the hard way

User · Answer

if I remember correctly    gzip will compress a little more than deflate deflate is more efficient

User · Answer

I think there s no big difference between deflate and gzip  because gzip basically is just a header wrapped around deflate  see RFCs 1951 and 1952

User · Answer

On Ubuntu with Apache2 and the deflate module already installed  which it is by default   you can enable deflate gzip compression in two easy steps   a2enmod deflate  etc init d apache2 force-reload   And you re away  I found pages I served over my adsl connection loaded much faster   Edit  As per  GertvandenBerg s comment  this enables gzip compression  not deflate

User · Answer

I think there s no big difference between deflate and gzip  because gzip basically is just a header wrapped around deflate  see RFCs 1951 and 1952

User · Answer

There shouldn t be any difference in gzip  amp  deflate for decompression   Gzip is just deflate with a few dozen byte header wrapped around it including a checksum   The checksum is the reason for the slower compression   However when you re precompressing zillions of files you want those checksums as a sanity check in your filesystem   In addition you can utilize commandline tools to get stats on the file   For our site we are precompressing a ton of static data  the entire open directory  13 000 games  autocomplete for millions of keywords  etc   and we are ranked 95  faster than all websites by Alexa   Faxo Search   However  we do utilize a home grown proprietary web server   Apache mod deflate just didn t cut it   When those files are compressed into the filesystem not only do you take a hit for your file with the minimum filesystem block size but all the unnecessary overhead in managing the file in the filesystem that the webserver could care less about   Your concerns should be total disk footprint and access decompression time and secondarily speed in being able to get this data precompressed   The footprint is important because even though disk space is cheap you want as much as possible to fit in the cache

User · Answer

The main reason is that deflate is faster to encode than gzip and on a busy server that might make a difference  With static pages it s a different question  since they can easily be pre-compressed once

User · Answer

GZip is simply deflate plus a checksum and header footer  Deflate is faster  though  as I learned the hard way

User · Answer

mod deflate requires fewer resources on your server  although you may pay a small penalty in terms of the amount of compression   If you are serving many small files  I d recommend benchmarking and load testing your compressed and uncompressed solutions - you may find some cases where enabling compression will not result in savings

User · Answer

You are likely not able to actually pick deflate as an option  Contrary to what you may expect mod deflate is not using deflate but gzip  So while most of the points made are valid it likely is not relevant for most

User · Answer

GZip is simply deflate plus a checksum and header footer  Deflate is faster  though  as I learned the hard way

User · Answer

GZip is simply deflate plus a checksum and header footer  Deflate is faster  though  as I learned the hard way

User · Answer

The main reason is that deflate is faster to encode than gzip and on a busy server that might make a difference  With static pages it s a different question  since they can easily be pre-compressed once

User · Answer

if I remember correctly    gzip will compress a little more than deflate deflate is more efficient

User · Answer

I think there s no big difference between deflate and gzip  because gzip basically is just a header wrapped around deflate  see RFCs 1951 and 1952

User · Answer

if I remember correctly    gzip will compress a little more than deflate deflate is more efficient

User · Answer

mod deflate requires fewer resources on your server  although you may pay a small penalty in terms of the amount of compression   If you are serving many small files  I d recommend benchmarking and load testing your compressed and uncompressed solutions - you may find some cases where enabling compression will not result in savings

[apache] Why use deflate instead of gzip for text files served by Apache?

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