Application server, related method thereof, and system for executing collaborative sessions

The invention provides an application server, a method for the application server, a system for executing collaborative sessions, and a method for a system comprising the application server and a terminal device. The application server includes the following components of: a receiving unit which receives preference item rules from control party terminal devices which control collaborative sessions between terminal devices which are located at the application server and comprise the control party terminal devices, wherein the preference item rules are used in special conditions when the control party terminal devices get lost from the collaborative sessions, and/or the modes of the control party terminal devices are converted to passive control modes; a selection unit which selects one candidate from candidates of a replacement device based on the preference item rules under the special conditions; and a request unit which requests one candidate to take over control on the collaborative sessions, and requests another candidate from the candidates of the replacement device to take over the control on the collaborative sessions based on the preference item rules when the first candidate refuses to take over the control on the collaborative sessions, wherein the preference item rules contain information for indicating the candidates of the replacement device.