Magento – Saving data to session using Magento observer function

The registry stores data to memory which is specific to that request (rather than user or anything else), and persists for the duration of that request only. The principle is very simple really, the Mage class is instantiated as a singleton object for every request and the instantiated Mage object remains in memory, and is accessible in all classes (and templates) until the request completes and the response is sent.

As the object is a singleton, whenever you access it you get the same object. All that is happening is that you are storing values to this object, so when one class stores a value, and another accesses it they are both working on the same object and the second class is able to retrieve the value the first class set.

When the controller_front_init_before event is dispatched, the session is not initialized yet. Your call to Mage::getSingleton('core/session') will initialize it, but the session model relies on values from the front controller, like the host name of the requets. Since the front controller is not initialized yet, it might be that you end up with a different session id (tbh, I'd have to look into the implementation details to see what exactly goes wrong)

A possible solution is to use Mage::registry() to save the value globally for the current request, and if you need to persist it in the session, do that at a later point, for example when you retrieve the value or in a different observer.