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## 6.3 Using the cache with Client Side Image Maps

You can also use the cache system for CSIM as well. The cache system interface is slightly different in this case since the cache needs to store both the cached image and the cached image-map. It also needs to change due to the way CSIM HTML paradigm work. The two major differences from the "standard" cache is

1. The cached version will not be stored in the previous defined cache directory. See more below.
2. You must call an extra method, CheckCSIMCache(), to check the cache, see more below.

The performance benefits even for simple CSIM images is around 50% if the cache can be used and can of course be several 1000% if construction of the image requires DB calls and other complex operations which can be avoided.

Before reading further you should have an understanding on how the CSIM works by reading the section "sing Client side image maps".

Please remember that when using CSIM you must end your script with a call to Graph::StrokeCSIM() method instead of the Graph::Stroke() used for non-csim.

To use the cache with CSIM you have to call the Graph::CheckCSIMCache(). As with the caching for non-CSIM you have to supply a name to be used for the cached version as well as an optional timeout value. The default timeout value if nothing else is specified is 60 minutes.

The name argument requires some more explanations. You must specify a relative name here. For example "myimage" or perhaps "firstpage/image3". Depending on your installation of JpGraph this will now end up in the directory specified in the CSIMCACHE_DIR define. This must also be a directory accessible by the normal web server. By default a directory called "csimcache" will be created in the same directory as the image script itself.

This has the drawback that the directory from where the script is executed must be writable by the process running PHP. Best practice for this is to keep the number of writable directory for PHP down to a minimum and re-use the same directory as is used for the standard cache. This however, require that your system administrator setup that cache directory so that it also accessible by the HTTP server from the htdocs root.

The CheckCSIMCache() method checks the cache for an existing cached version and if found it returns it and halts execution of the script. So, this call should be the first call after the creation of the Graph() and before any heavy work is done to create the image so that you can minimize the execution of the script in the case a match is found.

So, the general structure of a script that uses CSIM and the cache is

`$graph = new Graph(400,300 );
// Check cache, 10 min timeout
$graph->CheckCSIMCache( "image1",10);
// !! If cached version exists, execution halts here !!
//
// ... Construct the image with heavy DB calls etc, etc
//
$graph->StrokeCSIM();`

Please note that you do not need to pass any argument to the final call to StrokeCSIM() as you do when not using the cache.

Note: The CSIM caching works by storing two files in the cache directory. One file being the image and the other file being the corresponding image map as a pure HTML file.

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