1. Patterns of agents Massimo Cossentino Massimo Cossentino, Luca Sabatucci ICAR/CNR High performance computing and networks institute Italian National Research Council

2. The Goal of This Work We aim to drastically affect the cost of developing a multi-agent application. But… it is not easy to achieve this goal without simplifying the problem. We decided of using FIPA-compliant platforms and this reduces effectively the dimension of the problem (almost all of these platforms are JAVA-based and have a similar structure). By now we refer to two different platforms: FIPA-OS and JADE

3. Difference in implementing a simple agent behavior in FIPA-OS and JADE FIPA-OS JADE FIPA-OS JADE Different use of the done method in the two platforms Same structure of behaviors but different name of methods Constructor and setup method in both the platforms Shutdown method in FIPA-OS only

4. What is a pattern Christopher Alexander* definition: “Each pattern describes a problem which occurs over and over again in our environment, and then describes the core of the solution to that problem, in such a way that you can use this solution a million times over, without ever doing it the same way twice” *C. Alexander, S. Ishikawa, M. Silverstein, M. Jacobson, I. Fiksdahl- King, and S. Angel. A pattern language. Oxford Univ. Press, New York, 1977

5. OO Patterns classification E. Gamma and R. Helm and R. Johnson and J. Vlissides - "Design Patterns Elements of Reusable Object Oriented Software“ - "Addison-Wesley", 1994 According to Gamma *, patterns can be classified using two criteria: Purpose: what the pattern does. They can belong to the following categories: creational (dealing with the process of object creation), structural (dealing with the composition of classes and objects) or behavioural (describing the interactions of classes/objects) patterns. Scope: classification is directed to separate patterns that apply to classes or object. Class patterns deal with the relationships among classes (usually structural, for example inheritance), Object patterns include relationships among object (usually dynamic and established at runtime).

6. AO patterns classification Lind 1 classifies agent-oriented patterns in accordance to the views defined by the author in the MASSIVE methodology.  Categories reported in this work are: Interaction, Role, Architecture, Society, System, Task, and Environment. Aridor 2 clusters patterns in these categories:  traveling (dealing with agent mobility issues),  task (regarding the breakdown of agents tasks and the delegation of them from one agent to another)  interaction (dealing with agents communications) categories. (1) Jurgen Lind, "Patterns in Agent-Oriented Software Engineering", in Proc. Of AOSE Workshop at AAMAS 2002. July 2002, Bologna(Italy) (2) Y. Aridor and D. B. Lange, "Agent Design Patterns: Elements of Agent Application Design", in Proc. Of the Autonomous Agents '98 Conf., 1998.

7. Our concept of patterns We consider a pattern of agent as composed of its design level description and the corresponding JAVA code. More in detail each pattern is composed of:  A structure Usually a base agent class and a set of task/behavior classes. Described using UML class diagrams  A behavior Expressed by the agent using its structural elements Detailed in UML dynamic diagrams (activity/state chart diagrams)  A portion of code Some lines of code implementing the structure and behavior described in the previous diagram

8. The proposed AO classification We think that:  the basic duality (structure/dynamics) that exists in software is captured by the Gamma's approach  we also consider the importance of enriching it in order to fit the specific context (agents not objects). We classify the patterns using two main criteria (like in Gamma et al.) and include agent specific subcategories in them. Classification Criteria:  Application context: it regards the structural aspects of the pattern, whether it is to be applied to one agent, more agents or to their composing elements (tasks/behaviors).  Functionality: it regards the functional/behavioral aspects of the pattern.

9. Application Context patterns category Sub-categories: Action patterns. They address a functionality of the system; for instance they can be implemented as a method of either an agent class or a task class. Behavior patterns. They address a specific behavior of an agent; we can look at each of them as a collection of actions. Component patterns. They are entire agent patterns; these patterns propose a solution composed of the entire structure of an agent together with its tasks. Service pattern. Concerned about the collaborations between two or more agents; they can be thought as an aggregation of component patterns.

10. Functionality patterns category Sub-categories: Access to local resources. They deal with information retrieval and manipulation of data source. Communication. They represent the solution to the problem of making two agents communicate by an interaction protocol. Elaboration. They are used to deal with the agent's functionality devoted to perform some kind of elaboration on its knowledge. Mobility. These patterns describe the possibility for an agent to move from a platform to another

11. Patterns in our repository

12. Patterns documentation Name  The name of the pattern Classification  The classification of the pattern according to the categories reported in the previous slides Intent  A description of what the pattern does and its rationale and intent Motivation  A scenario that illustrates a design problem and how the agents and their tasks in the pattern solve the problem. Pre-conditions  The initial situation in which the pattern can be applied. Post-conditions  The consequences of the application of the pattern: what changes the pattern introduces into the system Structure  A graphical representation of the structure of the agent and its tasks (usually done with a class diagram)

13. Patterns documentation Participants  A description of the agents involved in the pattern and their roles Collaborations  A (graphical) representation of the collaborations of the agents involved in the pattern (if any) Implementation availability  Availability of the implementation code for the FIPA-OS/JADE platforms. Availability of the UML diagrams of the solution (XMI) for importing them in the existing system design Implementation description  Comments on the most significant code fragments to illustrate the pattern implementation in the specific agent platforms Implementation Code  FIPA-OS/JADE code of the solution Related Patterns  Patterns that should be used in conjunction with this one

14. The phases of the pattern production/reuse process Meta-pattern XSLT platform specialization XSL rules pattern constraints resolver Constraints + Java skeleton Action pattern Java agent complete code Class Diagram(s) Activity Diagram XSLT Multi-Platform RepresentationPlatform-Specific Representation Design Representation UMLXMLJAVA

15. The design level representation of the pattern Meta-pattern XSLT platform specialization XSL rules pattern constraints resolver Constraints + Java skeleton Action pattern Java agent complete code Class Diagram(s) Activity Diagram Design Representation UML XSLT Design level representation of the pattern using UML class and activity diagrams … QueryReceiveTask

16. The XML meta-pattern representation Meta-pattern XSLT platform specialization XSL rules pattern constraints resolver Constraints + Java skeleton Action pattern Java agent complete code Class Diagram(s) Activity Diagram Design Representation UML XSLT Meta-patterns are platform independent Meta-patterns contain all the common elements of patterns for different MA platforms … Multi-Platform Representation XML public constructor@generic_agent setup@generic_agent shutdown@generic_agent public constructor@query_initiator_task setup@query_initiator_task public handle_refuse@query_initiator_task

17. From meta-patterns to patterns – XSL rules Meta-pattern XSLT platform specialization XSL rules pattern constraints resolver Constraints + Java skeleton Action pattern Java agent complete code Class Diagram(s) Activity Diagram Design Representation UML XSLT Each pattern is specific for one of the selected platforms. In order to obtain a FIPA-OS pattern we apply to the meta- pattern the XSLT transformation shown on the right … FIPAOSAgent public private … Multi-Platform Representation XML

18. From meta-patterns to patterns Meta-pattern XSLT platform specialization XSL rules pattern constraints resolver Constraints + Java skeleton Action pattern Java agent complete code Class Diagram(s) Activity Diagram Design Representation UML XSLT The resulting pattern is localized to a specific platform The pattern is still general and can be customized for the specific application … Multi-Platform Representation XML Meta-pattern Pattern XSL Transformation public FIPAOSAgent public constructor@generic_agent public setup@generic_agent …

19. Patterns introduction in the project generates constraints When a pattern is applied to a project, it modifies the context in which it is placed (introducing new functionality into the system) The relationship between the pattern and existing elements could be expressed with a constraint. A constraint is a rule composed of three elements:  A target that specifies what agent/task will be influenced by the rule.  A platform that specifies which is the application context (FIPA-OS or JADE)  A content that expresses the changes to be applied when the pattern is inserted into the project (it could be an aggregation of attributes, constructors or methods). An example in FIPA-OS:  When we insert a communication task pattern into an existing agent, the listener task (IdleTask) should have a handleX method to catch performative acts of a particular type (i.e. QueryIf, Request, Inform, …)

20. Constraints Meta-pattern XSLT platform specialization XSL rules pattern constraints resolver Constraints + Java skeleton Action pattern Java agent complete code Class Diagram(s) Activity Diagram XSLT … Multi-Platform RepresentationPlatform-Specific Representation Design Representation UMLXMLJAVA From the previous example: here we have the constraint used to introduce the handleQueryIf method in the IdleTask class of a FIPA-OS agent public a_function …

21. From skeleton of classes to complete code With the previous steps we obtained a skeleton of the agent with its tasks/behaviors that is complete down to the method interfaces of each class In order to (partially) fill the skeleton with the remaining code, action patterns are applied. An action pattern is a portion of JAVA code realizing some kind of behavior. It is specific for each platform.  For example the registration to the DF service (Directory Facilitator, the yellow pages of the platform) it is part of the setup@generic_agent action pattern and it is introduced in the setup method of the agent Action patterns are stored in a database of pieces of code and the correct one for each method is selected referring to the value of the Code tag for the specific module (see the XML pattern representation)

22. Constraints Meta-pattern XSLT platform specialization XSL rules pattern constraints resolver Constraints + Java skeleton Action pattern Java agent complete code Class Diagram(s) Activity Diagram XSLT try { registerWithDF( AGENT_TYPE ); DIAGNOSTICS.println("Registered with DF", this,DIAGNOSTICS.LEVEL_MAX ); } catch (DFRegistrationException dfre) { DIAGNOSTICS.println(dfre, this, DIAGNOSTICS.LEVEL_MAX ); String reason = dfre.getExceptionReason(); if ( reason == null || !reason.equals(FIPAMANCONSTANTS. AGENT_ALREADY_REGISTERED) ) { shutdown(); return; } } … The action pattern for the setup method of a generic FIPA-OS agent Multi-Platform RepresentationPlatform-Specific Representation Design Representation UMLXMLJAVA

23. The AgentFactory web application

24. Screenshots Create a new multi agent system Select the agent platform (FIPA-OS or Agent)

25. Screenshots Add a component pattern to the MAS Select the Generic Agent pattern

26. Screenshots Static, dynamic structure of the pattern and tree representation of the system

27. Screenshots Add a behaviour pattern to ‘myTask’ Select the FIPA Query Initiator pattern

28. Screenshots Static structure of the FIPA Query Initiator pattern Select the previously created agent from the list

29. Screenshots The result of the pattern application: the agent structure, the automatically generated code…

30. Screenshots … the static structure and the dynamic behavior

31. Conclusions and future works

32. Experimental results are encouraging but the amount of code automatically produced depends on the dimension of the pattern repository It is difficult to synchronize the DB of patterns for many different platforms An high level representation of patterns is needed in order to facilitate reuse in early stages of design processes