I start working on multisite on elgg 1.5rc

You can see it running on the demo site

The points are :

- multisite is not compatible with simple cache, that mean that you have to disable simple cache. I made a trac about that, but the elgg team answer is that the multisite is not yet fully yet supported so adapting simple cache is an ehancement and not a defect to solve.  I fully understand that point of view as multicite is very new. I guess that the elgg 2.0 will be fully multisite so we wont have to hack the code to make it work. So for now, no simple cache until someone brings up a hack for that :)

- The river is still not multisite comptatible as you get river data from other sites mixed into the river of the site you are logged in. The solution is to add a site_guid field in the system_log table and to modify the river so that the site_guid is checked when retrieving log data. I already made that work, ill put it in the documentation of the group soon.

- i do my best to make the new multisite plugin the more out of the core as possible to avoid hacking. Unitl now (except the river pb) i succeeded :) i override or hook the actions that need to be, and keep it inside the plugin instead of hacking  the original files

- Unfortunately, the site switching cannot be hook in a plugin. the sites_init function has a plugin_hook implemented but the hook is triggered before the plugin are loaded :) . The solution i have found is to add a multisite.php file in the engine/lib folder. This way, the siote_init hook can be registered before it is triggered by the sites_init function.

- Unfortunately, le messages standard plugin doesnt fully use the page_handler. The result is that i cannot override the pages that i would need to (the send page ...). So i am going to build a specific multisite message plugin, whhich will be a copy of the original one for most of the functions, but will be compatible with multisite. So the administrators will just have to disable the standar message plugin and enable the multisite compatible one. This is the last work i have to do before releasing a new version comptable with elgg 1.5