For service-based systems which are composed of multiple independent stakeholders, correctness cannot be ascertained statically. Continuous monitoring is required to assure that runtime behavior of the systems complies with specified properties. However, most existing work considers only the temporal constraints of messages exchanged between services, ignoring the actual data contents inside the messages. As a result, it is difficult to validate some dynamic properties such as how message data of interest is processed between different participants. To address this issue, this paper proposes an efficient, online monitoring approach to dynamically analyze data-centric properties in service-based systems. By introducing
-BCL - a Parametric Behavior Constraint Language for Web services - various data-centric properties can be specified and monitored. To keep runtime overhead low, we statically analyze the monitored properties to generate parameter state machine, and combine two different indexing mechanisms to optimize the monitoring. The experiments show that the proposed approach is efficient.