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MODELLING OF MOBILE AGENT SYSTEMS USING dynPLA APPROACH. Agn ė Paulauskaitė-Tarasevičienė Henrikas Pranevičius. Mobile agent systems – variable structure systems. Models of variable structure systems define the systems, which in time change not only their state but the structure as well.
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Models of variable structure systems define the systems, which in time change not only their state but the structure as well.
The structure of system models can change by:
Variable structure systems are complex and mostly critical therefore it is beneficial to describe them using formal methods (Petri Nets, DEVS, -calculus), which enable mathematical description and verification.
Dynamic DEVS notations:
dynDEVS (A. M. Uhrmacher),
ML-DEVS (A. M. Uhrmacher),
M-DEVS (J.H. Kim, T.G. Kim).
Piece–Linear Aggregates (PLA) (H. Pranevičius) method is used for complex systems formalization, verification and simulation.
The PLA is taken as an object defined by:
a state Z,
input signals X,
output signals Y.
transition operators H,
output operators G.
Purpose – to presentthenewconceptformodellingthe mobile agentsystemsusingaggregatemethodwithincludedAbstract Data Types.
1) Structural changes in dynPLA model are performed by adding and removing the elements of variable sets.
2) The external aggregate(s) is excluded in the system that would be responsible for the execution of structural changes.
1. Addition of new link; 2. Removal of link
3. Addition of aggregate 4. Removal of aggregate
5. Aggregate migration form one system to another
In dynPLA aggregate system five types of structural changes are excluded.
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Abstract Data Types (ADT) are used for definition of structural change operations in dynPLA.
ADT structure –aggregate system.
ADT operations – structural changes in aggregate system.
To define ADT in dynPLA, Object-Z specification language has been used.
unambiguous definition of structural change operations,
the preconditions, which have to be met in order to properly perform the structural changes in dynPLA model,
ADT definition in object oriented manner.
to avoid the descriptions of many additional actions used in fulfillment any structural change and thereby present the specification in shorthand way;
to produce an unambiguous statement for these actions needed while performing the structural changes;
to avoid specification errors, since one can manipulate only with operations that are defined in advance.
User, willing to find a certain product on the internet, communicates to a User Interface Agent (UIA).
UIAcreates a MA and delegates the task to it.
MA migrates across the internet to the Data Directory (D).
MA collects all the information from the providers and returns it to UIA, which, in turn, deletes the mobile agent after getting its response.
Two mobile agent migration ways are analyzed in this example:
1. Itinerary – the mobile agent visits all suppliers from the list and returns to the host server after visiting the last supplier;
2. Branching – after getting the list of providers the mobile agent clones itself according to the number of providers. Every cloned agent visits the provider assigned to it and moves to the host with gathered information.
Structural changes of model are described using such ADT operations:
it migrates in branching pattern;
The model was implemented in JADE software platform.
JADE has an integrated mobile agent platform.
Agent migration is realized using FIPA mobile requests protocol.
JADE provides a cloning mechanism that takes snapshot copy of an existing agent.
dynPLA model with included ADT for implementation of MA creation, removal, migration and cloning operations can be used to create various MA systems such as intellectual, distributed network and other.
The usage of ADT for description of mobile agents systems allows to significantly reduce the size of formal specification, since there is no need to additionally specify the operations that alter the system model.
In addition, there is a possibility to extend the specification by introducing the new operations if needed, that greatly increase the flexibility of structure-change modeling.