2023年10月11日 23:49
Everything you wanted to know about web content compression
Compression of outbound Web server traffic provides benefits both for Web clients who see shorter response times, as well as for content providers, who experience lower consumption of bandwidth.
Most recently, content compression for web servers has been provided mainly through use of thegzipencoding. Other (non perl) modules are available that provide so-calleddeflatecompression. Both approaches are very similar recently and use the LZ77 algorithm combined with Huffman coding. Luckily for us, to make use of them, there is no real need for most of us to understand all the details of the obscure underlying mathematics of these techniques. Apache handlers available from CPAN can usually do the dirty work. Apache addresses content compression through handlers configured in its configuration file.
Compression is, by its nature, a content filter: It always takes its input as plain ASCII data that it converts to another binary form, and outputs the result to some destination. That is why every content compression handler usually belongs to a particular chain of handlers within the content generation phase of the request-processing flow.Achain of handlersis one more common term that is good to know about when you plan to compress data. There are two of them recently developed for Apache 1.3:Apache::OutputChainandApache::Filter. We have to keep in mind that the compression handler developed for one chain usually fails inside another.
Another important point deals with the order of execution of handlers in a particular chain. It's pretty straightforward in Apache::Filter. For example, when you configure...
PerlModule Apache::Filter
<Files ~ "*\.blah">
SetHandler perl-script
PerlSetVar Filter On
PerlHandler Filter1 Filter2 Filter3
</Files>
However, when you configure Apache::OutputChain like...
PerlModule Apache::OutputChain
PerlModule Apache::GzipChain
PerlModule Apaceh::SSIChain
PerlModule Apaceh::PassHtml
<Files *.html>
SetHandler perl-script
PerlHandler Apaceh::OutpuChain Apache::GzipChain Apache::SSIChain Apaceh::PassHtml
</Files>
...execution begins withApache::PassHtml. Then the content will be processed withApache::SSIChainand finally withApache::GzipChain. Apache::OutputChain will not be involved in content processing at all. It is there only for the purpose of joining other handlers within the chain.
It is important to remember that the content compression handler should always be the last executable handler in any chain.
Another important problem of practical implementation of web content compression deals with the fact that some buggy Web clients declare the ability to receive and decompress gzipped data in their HTTP requests, but fail to keep their promises when an actual compressed response arrives. This problem is addressed through the implementation of theApache::CompressClientFixuphandler. This handler serves thefixupphase of the request-processing flow. It is compatible with all known compression handlers and is available from CPAN.
Web content compression noticeably increases delivery speed to clients and may allow providers to serve higher content volumes without increasing hardware expenditures. It visibly reduces actual content download time, a benefit most apparent to users of dialup and high-traffic connections.
Industry leaders like Yahoo and Google are widely using content compression in their businesses.
The compression ratio is highly content-dependent. For example, if the compression algorithm is able to detect repeated patterns of characters, compression will be greater than if no such patterns exist. You can usually expect to realize an improvement between of 3 to 20 times on regular HTML, JavaScript, and other ASCII content. I have seen peak HTML file compression improvements in excess of more than 200 times, but such occurrences are infrequent. On the other hand I have never seen ratios of less than 2.5 times on text/HTML files. Image files normally employ their own compression techniques that reduce the advantage of further compression.
On May 21, 2002 Peter J. Cranstone wrote to the mod_gzip@lists.over.net mailing list:2002/05/21 に Peter J. Cranstone は mod_gzip@lists.over.net メーリングリストに書きました:
"...With 98% of the world on a dial up modem, all they care about is how long it takes to download a page. It doesn't matter if it consumes a few more CPU cycles if the customer is happy. It's cheaper to buy a newer faster box, than it is to acquire new customers.""... 世界の 98% のダイアルアップモデムで, 彼らがケアしているのはページのダウンロードがどのくらい時間を取るかについてです. 顧客がハッピーならそれが CPU サイクルが多少多く消費するとしても問題にはなりません. 最新で高速のボックスの購入は安価です, 新しい顧客を獲得するよりも."
This approach works in most of the cases I have seen. In some special cases you will need to take extra care with respect to the global architecture of your Web application, but such cases may generally be readily addressed through various techniques. To date I have found no fundamental barriers to practical implementation of Web content compression.
All modern browser makers claim to be able to handle compressed content and are able to decompress it on the fly, transparent to the user. There are some known bugs in some old browsers, but these can be taken into account through appropriate configuration of the Web server.
I strongly recommend use of the Apache::CompressClientFixup handler in your server configuration in order to prevent compression for known buggy clients.On the client side, the browser transparently unencrypts and uncompresses the content for the user. It is important to maintain the correct order of operations on server side to keep the transaction secure. You must compress the content first and then apply an encryption mechanism. This is the only order of operations current browsers support.
a mod_perl handler developed by Ken Williams (U.S.),Apache::Compress, can generate gzip output through theApache::Filter. This module accumulates all incoming data and compresses the entire content body as a unit.
a mod_perl handler developed by Slava Bizyayev,Apache::Dynagzipuses the gzip format to compress output dynamically through theApache::Filteror through the internal Unix pipe.
Apache::Dynagzip is most useful when one needs to compress dynamic outbound Web content (generated on the fly from databases, XML, etc.) when content length is not known at the time of the request.Apache::Dynagzip's features include:Support for both HTTP/1.0 and HTTP/1.1.
Control over the chunk size on HTTP/1.1 for on-the-fly content compression.
Support for Perl, Java, or C/C++ CGI applications.
Advanced control over the proxy cache with the configurable Vary HTTP header.Optional control over content lifetime in the client's local cache with the configurable Expires HTTP header.Optional support for server-side caching of the dynamically generated (and compressed) content.
Optional extra-light compression
removal of leading blank spaces and/or blank lines, which works for all browsers, including older ones that cannot uncompress gzip format.
an example of the mod_perl filter developed by Lincoln Stein and Doug MacEachern for their book Writing Apache Modules with Perl and C (U.S.), which likeApache::Compressworks throughApache::Filter.Apache::Gzipis not available from CPAN. The source code may be found on the book's companion Web site athttp://www.modperl.com/
a mod_perl handler developed by Andreas Koenig (Germany), which compresses output through Apache::OutputChain using the gzip format.Apache::GzipChaincurrently provides in-memory compression only. Use of this module underperl-5.8or higher is appropriate for Unicode data. UTF-8 data passed toCompress::Zlib::memGzip()are converted to raw UTF-8 before compression takes place. Other data are simply passed through.
In my observations of data compression of files of up to 200K it takes less then 60 ms CPU on a P4 3 GHz processor. I could not measure the lower boundary reliably for dynamic compression, because there are no really measurable latency. From the perspective of global architecture and scalability planning, I would suggest allowing some 10 ms per request on regular Web pages in order to roughly estimate/predict the performance of the application server.
Estimation of connection times is an even less exact matter for of a variety of possible network-related reasons. The worst-case scenario is pretty impressive: a slow dialup connection through an ISP with no proxy/buffering holds the provider's socket for a time interval proportionate to the size of the requested file. At present, gzip reduces this connection time by a factor of approximately 3-20. If the ISP buffers its traffic, however, the content provider might not feel a dramatic impact -- apart of the fact that they are paying their telecom providers for the transmission of considerable unnecessary data.
If your page/application is initially configured like this:
<Directory /path/to/subdirectory>
SetHandler perl-script
PerlHandler Apache::Registry
PerlSendHeader On
Options +ExecCGI
</Directory>
you might replace it with the following:
PerlModule Apache::Filter
PerlModule Apache::Dynagzip
PerlModule Apache::CompressClientFixup
<Directory /path/to/subdirectory>
SetHandler perl-script
PerlHandler Apache::RegistryFileter Apache::Dynagzip
PerlSendHeader On
Options +ExecCGI
PerlSetVar Filter On
PerlFixupHandler Apache::CompressClientFixup
PerlSetVar LightCompression On
</Directory>
You should usually be all set after that.
In more common cases, you will need to replace the line:
PerlHandler Apache::Registry
in your initial configuration file with the following lines:
PerlHandler Apache::RegistryFilter Apache::Dynagzip
PerlSetVar Filter On
Perl FixupHandler Apache::CompressClientFixup
Optionally, you might add:
PerlSetVar LightCompression On
to reduce the size of the stream for clients unable to speak gzip (like Microsoft Internet Explorer over HTTP/1.0).
Finally, make sure you have somewhere declared
PerlModule Apache::Filter
PerlModule Apache::Dynagzip
PerlModule Apache::CompressClientFixup
This basic configuration uses many defaults. See Apache::Dynagzip POD for further fine tuning if required.Note, however, thatApache::RegistryFilteris not yet anotherApache::Registry. You may need to adjust your script in accordance with requirements ofApache::Filerfirst, especially when the script generates any CGI/1.1-specific HTTP headers. You can test your compatibility with theApache::Filterchain using a temporary configuration like:
PerlModule Apache::Filter
<Directory /path/to/subdirectory>
SetHandler perl-script
PerlHandler Apache::RegistryFilter
PerlSendHeader On
Options +ExecCGI
PerlSetVar Filter On
</Directory>
with noApache::Dynagzipinvolved. SeeApache::Filterdocumentation if you have any problems.
If your application is initially configured like:
PerlModule HTML::Mason::ApacheHandler
<Drectory /path/to/subdirectory>
<FilesMatch "\.html$">
SetHandler perl-script
PerlHandler HTML::Mason::ApacheHandler
</FilesMatch>
</Directory>
you may wish to replace it with the following:
PerlModule HTML::Mason::ApacheHandler
PerlModule Apache::Dynagzip
PerlModule Apache::CompressClientFixup
<Drectory /path/to/subdirectory>
<FilesMatch "\.html$">
SetHandler perl-script
PerlHandler HTML::Mason::ApacheHandler Apache::Dynagzip
PerlSetVar Filter On
PerlFixupHandler Apache::CompressClientFixup
PerlSetVar LightCompression On
</FilesMatch>
</Directory>
You should be all set safely after that.
In more common cases, you will need to replace the line:
PerlHandler HTML::Mason::ApacheHandler
in your initial configuration file with the following lines:
PerlHandler HTML::Mason::ApacheHandler Apache::Dynagzip
PerlSetVar Filter On
PerlFixupHandler Apache::CompressClientFixup
Optionally, you might add:
PerlSetVar LightCompression On
to reduce the size of the stream for clients unable to speak gzip (like Microsoft Internet Explorer over HTTP/1.0).
Finally, make sure you have somewhere declared
PerlModule Apache::Dynagzip
PerlModule Apache::CompressClientFixup
This basic configuration uses many defaults. See Apache::Dynagzip POD for further fine tuning.Since the author of Apache::Dynagzip is employed by Slav Company, service is effective and consistent.Apache::Dynagzip is the only handler to date that begins transmission of compressed data as soon as the initial uncompressed pieces of data arrive from their source, at a time when the source process may not even have completed generating the full document it is sending. Transmission can therefore take place concurrently with creation of later document content.This feature is mainly beneficial for HTTP/1.1 requests, because HTTP/1.0 does not support chunks.
I would also mention that the internal buffer in Apache::Dynagzip always prevents Apache from the creating too short chunks over HTTP/1.1, or from transmitting too short pieces of data over HTTP/1.0.The benefits of blank space stripping are mostly significant for non-gzipped data transmissions. One can expect some 5-20% reduction in stream size on regular 'structured' HTML, JavaScript, CSS, XML, etc., in this case at negligible cost in terms of CPU overhead and response delay.
After applying gzip compression, the benefits of previously applied blank space stripping are usually reduced to some 0.5-1.0% of the resulting size, because gzip compresses blank spaces very effectively. It is still worthwhile, however, to perform blank space stripping because:
chances are that your handler will ultimately have to send an uncompressed response back to a known buggy client;
it really costs next-to-nothing, and every little bit helps to reduce the cost of data transmission, especially considering the cumulative effect of frequent repetitions.
an Apache handler written in C by Igor Sysoev (Russia).
an Apache handler written in C, originally by Kevin Kiley, Remote Communications, Inc. (U.S.)
See their respective documentation for further details.
For example, vanilla Apache 1.3 is CGI/1.1 compatible. It allows development of CGI scripts/programs that can generate compressed outgoing streams accomplished with specific HTTP headers.
Alternatively, on mod_perl enabled Apache, some application environments carry their own compression code that can be activated through appropriate configuration:
Apache::ASPdoes this with theCompressGzipsetting;
Apache::AxKituses theAxGzipOutputsetting to do this.
See the documentation for the particular packages for details.
mod_deflate for Apache-2 was written by Ian Holsman (USA).Despite its name,mod_deflatefor Apache-2 providesgzip-encoded content. In accordance with the concept of output filters that was introduced in Apache-2,mod_deflateis capable of gzipping outbound traffic from any content generator, including CGI, Java, mod_perl, etc.
This module supports flushing.
It is output filter-compatible.
It has its own set of configuration options to maintain control over buggy clients.
mod_deflatefor Apache-2 is capable of providing all basic functionality required for effective dynamic content compression. The rest can be easily addressed through implementation of the accompanying specific, tiny filters. For instance,Apache::Clean, which is already ported to Apache-2, can be used to strip unnecessary blank spaces from outbound streams.
You can find many mirrors of RFC archives on the Internet. Try, for instance, my favorite at http://www.ietf.org/rfc.htmlYou can useApache::CompressClientFixupto disable compression for these files dynamically on Apache-1.3.Apache::Dynagzipis capable of providing so-calledlight compressionfor these files.
On Apache-2,mod_deflatecan be configured to disable compression for these files dynamically, and theApache::Cleanfilter can be used to strip unnecessary blank spaces.
See it athttp://www.schroepl.net/projekte/mod_gzip/browser.htm During this work, I received a great deal of real help from Kevin Kiley, Igor Sysoev, Michel Schroepl, and Henrik Nordstrom. I'm thankful to all subscribers of mod_perl users mailing list, mod_gzip mailing list, and squid users mailing list for the questions and discussions regarding the content compression. I'm especially thankful to Stas Bekman for the initiative to publish this FAQ on mod_perl Web site. I highly value patient efforts of Dan Hansen in making this text better English...
The maintainer is the person you should contact with updates, corrections and patches.
Only the major authors are listed above. For contributors see the Changes file.