The Model And Analysis Of Distributed Database Accessing System With Mobile Agent Based On Petri Net

With the development of DDBS, traditional C/S mode may give birth to bottle-neck effect. Mobile agent is brought to the design of distributed database accessing system to reduce the dependence on network bandwidth. And as mobile equipments progresses, more mobile terminals being added to the network, mobile agent system which do not depend on the continuous network can be apply to more fields.This dissertation designs a distributed database accessing system basing on mobile agent and describes the whole structure of the system and the function descriptions of main agents including intelligence user agent, proxy agent, coordinating information query agent, local information query agent. When a client gives a query request, intelligent user agent decomposes the request to task sequence. Then one mobile agent taking the task sequence is dispatched to different sites to finish the tasks. The midi-date of the tasks is also taken with the mobile agent. When all tasks are finished, the result data is returned to the client.Based on this system structure, Petri net model is constructed in which the itineraries of the mobile agent in the system carrying the task sequence are simulated. Analysing the reachablity tree of the model, optimistic itineraries are got. As several itineraries which may have the same priority, the tasks which have the same parent task are set different priority, and then a Petri net model which tasks have different priorities is constructed. Following the way mentioned above dealing with the reachablity tree of the model, more exact optimistic itineraries are got. The site address information can also be added to the Petri net model to construct a colored Petri net model. From the model, the access amount of nodes are obtained when mobile agent running, so the database capacity of sites can be adjusted according with the access amount.