The demand on mobile and real-time database application and service in distributed environment is becoming increasingly extensive, since the workload cannot be precisely predicted so that they can become overloaded possibly. Low bandwidth in mobile environment will lead to aborting or restarting for transactions because of competing for the limited system resources severely. In this paper, we propose a novel performance metric for example deadline miss ratio of transactions and data freshness based on the relationship of data items in the DAG (directed acyclic graph) to ensure the QoS (quality of service ) in mobile and real-time database systems. Feedback control architecture and MODDFT (modifying on-demand breath-first traversal algorithm) are proposed in our system to decrease miss ratio of transactions and improve data freshness. The MODDFT algorithm can guarantee the QoS performance will not be beyond the reference by database administrator through simulation experiments.