I am new to Expression Engines, actualy as of today, and I must compliment you on how simple and strait-forward the structure for plugins is.

But to the point, I am developing a custom plugin to interface with the <a href=“http://www.flickr.com/services/api”>Flickr API</a>, which responds with xml data. The plugin will be parsing the xml and printing out some of the xml data. What I am wondering is if there is just one instance of the plugin class instanciated the first time it is called per page, or if each “{exp:class:function}” instanciates a new class object. I ask because I need data persistance for as long as it takes to generate the page, in case multiple function calls occur, the goal being to reduce request to the Flickr API.

Every instance of a plugin tag in a template has a new instantiation in the class called.  It is pretty common for plugin files to have default values in class variables, so we wanted a clean slate each time.

You could use either a constant or a superglobal to store data, if you want, since that will remain persistent.  Alternatively, you could create a caching system like those used by various other plugins grabbing data from off site.