In case of events related to user session, ie. to handle properly issue with syncing state of MessageCarbons it is required to pass not only event but also information about user session - instance of XMPPResourceConnection. But as we need this event to be clustered then on destination node we need also instance of XMPPResourceConnection available on destination node. To be able to implement this we need to implement prehandles for events which will be fired on destination node for incoming clustered events and will be able to set instance of some object based on value of other field in this event which is serializable (ie. string, jid or barejid).