What idea$($s$)$ are used by parameter server? $($A$)$ Each worker group has its own independent parameter namespace, but cannot share the same namespace with other groups $($B$)$ Task dependency only implements sequential consistency model $($C$)$ The task scheduler may schedule a task replacement upon the departure by a worker $($D$)$ A worker can push its updates into the servers, and similarly pulls the updates by others using range push and pull