A wireless sensor node comprises a wireless transceiver to communicate over a wireless communication link with at least one other node in a wireless sensor network, a sensor interface to receive sensor data from a sensor, and a programmable processor coupled to the sensor interface and the wireless transceiver. The programmable processor executes program instructions that implement a virtual machine for executing an event program that is generated from a query. The event program comprises a plurality of instructions, each instruction being associated with an event. The virtual machine allocates a token data structure for each of event and manages the execution of the plurality of instructions using the token data structures.