Is there a way to clear a session cache key for all users?

You can't get all the current Sessions from all users
(Frankly speaking, you can, but I don't recommend this, and still you can read only InProc sessions this way).

So the only thing you can do is something like this:

• Store in Session only ID of the user cart
• Add item with corresponding ID to the Cache
• Then adding the item to the Cache, set the dependency to the Item
• Then just generate event to change whole carts in Cache.
• Also you should think about re-creating items if for some reasons Cache is being emptied - you should provide the CacheItemRemovedCallback callback for this event.