Mobile computing introduces new challenges for data management. Our main concern is to develop a data service whose goal is the processing of location-dependent queries, i.e., queries for which the answer depends on location of objects. We deal with contexts where not only the user issuing the query can change her/his position, but the objects involved in the query can move as well. Examples of location-dependent queries are: "find the teller machines inside a radius of three miles accepting VISA cards and their distances to my current position", "show me the police cars within two miles", and "find the nearest cinema displaying Gladiator within five miles with available tickets". In addition, queries can depend on direction of movement (for example, "find the next gas station that I will find following my current route").
We deal with continuous queries, i.e., queries whose answer must be updated continuously, as opposite to instantaneous queries in which only a single answer is obtained for a query. Instantaneous queries are not very useful in a mobile environment since the answer presented to the user can become obsolete in a short time. Continuous queries introduce new problems in the query processing because the movement of any involved object could change the query answer. Moreover, a continuous query cannot be considered as a sequence of instantaneous queries that are re-sent continuously to the data server. This would lead to a low performance, wireless bandwidth overhead, and the computer issuing the query may disconnect. Besides, delays involved in communication must be considered since they may make an answer outdated. So it is necessary an approach to assure updated data but optimizing communications.