CSP Gateway Configuration Guide - InterSystems Documentation

CSP Gateway Operation and Configuration
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2.6 Compressing the Response to Requests for CSP
Forms (GZIP/ZLIB)
Note:
The CSP Gateway does not support GZIP compression on VMS.
Compressing the response generated by the CSP engine before dispatching it to the client is advantageous because it can
dramatically reduce the network bandwidth required to transport the response to the client. From the client’s perspective
the performance of the application is improved. This is particularly true for clients accessing the application through mobile
devices over slower telecommunications networks. There is, of course, a cost in terms of the web server host’s CPU time
that’s required to actually compress the data but this is a small price to pay for the advantages.
The advantage of serving compressed response data is particularly marked for CSP pages for which large volumes of
response data are generated.
There are two methods for implementing GZIP in a web server environment.
• Using the Gateway’s own interface to the GZIP library described here.
• Using a GZIP output filter as an add-on to the hosting web server.
Most web servers offer add-on facilities for compressing data. Windows/IIS offers a gzip filter (implemented as an ISAPI
filter). The Apache Group offer a compression filter implemented as an add-on module (mod_deflate.c – which, rather
confusingly, implements gzip compression not deflate). There is also a third-party module for Apache called mod_gzip.c.
There are a number of third-party GZIP products available as add-ons for most web servers.
The advantages of implementing a compression solution directly in the CSP Gateway are as follows:
• Ease of setup and configuration.
• Greater flexibility in controlling which CSP files are to be compressed.
• The Gateway receives the response content from Caché in fairly large chunks; therefore the performance of the compression
and the degree of compression achieved is better if the data is submitted to the compressor functions in large
buffers.
• Finally, it has been discovered that if Chunked Transfer Encoding is enabled at the CSP Gateway level and if the
Apache mod_deflate output filter is enabled for the same resources, then Windows browsers are occasionally unable
to display the response content.
The Gateway makes use of the freely available GZIP (or zlib) library for implementing data compression. The compression
algorithm used is described in RFCs (Request for Comments) 1950 to 1952.
2.6.1 Installing the GZIP/ZLIB Library
You can download the GZIP/ZLIB library from the following site:
http://www.zlib.net/
This resource was developed by Jean-loup Gailly and Mark Adler (Copyright (C) 1995-2006).
The library is freely available for all platforms on which the Gateway is supported. It is implemented as a DLL for Windows
(zlib.dll), a shared object (or shared library) for UNIX® systems (libz.so or libz.sl) and a ZLIB library is available as a
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