lecture 5 multi-agent systems lecture 5 university “politehnica” of bucarest 2004 - 2005...

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  • Multi-Agent SystemsLecture 5

    University Politehnica of Bucarest2004 - 2005

    Adina Magda Floreaadina@cs.pub.ro

    http://turing.cs.pub.ro/blia_2005version of ...

  • *promotes a societal view of computinguses mental states to design agents; mental categories appear in the programming languagethe semantics of the PL is related to the semantics of mental statesfirst AOPL: Agent-0mental states of Agent-0: beliefs, commitments, capabilities, decisions1 AOP principles

  • *March P to C: Please inform me what flights you have from San Francisco to New York on April 18. C to P: Flight #354 departs at 08:30. flight #293 departs at 10:00, flight #441 departs at noon, P to C: Please book me on #354. C to P: That is sold out. P to C: Please book me on #293. C to P: That is confirmed: your reservation number is 112358. P to C: Please book me also on #441. C to P: That conflicts with #293: I am not allowed to double book a passenger. P to C: Please get permission to do so. C to S: I request permission for the fallowing double booking: S to C: Permission denied. C to P: Sorry, I cannot get approval.Airline reservation

  • *April 18, at the airport P to C: My name is P; I have a reservation for flight =293. C to P: Here is your boarding pass.Airline reservation

  • 3 Overview of the AOP Framework

    A complete AOP system should have 3 components:A formal (logical) language for defining the mental state of the agentsAn (interpreted) programming language in which to define and program agents, with communication primitives based on speech-act theory; the semantics of the PL must match the semantics of the mental statesAn "agentification" process converting neutral devices into programmable agents*

  • The agents beliefs refer to:states of the world,mental states of other agents, andcapabilities of this and other agents.The actions of an agent are determined by its decisions, or choices.Decisions are constrained (but nor determined) by the agents beliefsDecisions are also constrained by prior decisions.


  • Persistence of Mental StateBeliefs persist by default: agents have perfect memory of their beliefs; a belief is dropped only when a contradictory fact is learned.The absence of belief also persists by default.Obligations persist by default, but there are conditions under which theyre revoked, e.g.:explicit release of the agent by the party to which its obligated,realization by the agent that its no longer able to fulfill the obligation.Since decision is defined in terms of obligation, it inherits the default persistence.*

  • Linear discrete timeBeliefsBat Actionsraise_arm(robot)tObligations (commitments)OBLa,bt (CMTa,bt )Decisions (choices)DECat = OBLa,at Abilities (capabilities)CANat

    Properties of mental categoriesInternal consistency of beliefs and commitmentsGood faithPersistence of beliefsPersistence of commitments*Summary of mental categories in AGENT0

  • Actions to which agents can be committed includearbitrary private actionscommunicative actions (e.g., informing and requesting)Private actions may or may not involve IO.Communicative actions always involve IO and are common to all agents.Actions may also be:unconditional actionsconditional actionsA conditional action relies on a mental condition, which refers to the mental state of the agent.

    *A generic interpreter - cont

  • Initialize mental stateand capabilitiesDefine rules for makingnew commitmentsUpdate mentalstate

    Execute commitmentfor current time

    ClockRepresentationof mental stateand capabilitiesOutgoing messageIncoming messageA flow diagram of the generic agent interpreterControlData

  • Commitment rules - key to AOPConditions for entering a commitment:mental conditionmessage conditionMessage condition = a logical combination of message patternsMessage pattern(sender type content)type: INFORM, REQUEST, UNREQUEST, REFRAINcontent: a fact statement or an action statement, depending on typeCommitment rule(COMMIT message-cond mental-cond (agenti actioni)*)Ex: (COMMIT (?a REQUEST ?action)(B (now (myfriend ?a)))(?a ?action))


  • 4 Agent-04.1 Components of Mental StateTwo mental categories: beliefs and decisionsA third (not per se mental) category: capabilities.A forth category: obligation, or commitment - treat decision as commitment to oneself.*

  • The role of an agent program is to control the evolution of an agents mental state.Actions occur as side-effects of the agent being committed to an action whose time has come.

    The Basic LoopEach agent iterates the following steps at regular intervals:1. Read the current messages and update your mental state, including your beliefs and commitments.(The agent program is crucial for the update.)2. Execute the commitments for the current time, possibly resulting in further belief change.(This is independent of the agent program.)

    *4.3 A generic interpreter

  • Commitment rules ::=(COMMIT ( )*)< mes- cond> ::=< mes- conj> | (OR *)< mes- conj> ::=< mes- pattern> | (AND < mes- pattern>*)< mes- pattern> ::=( INFORM ) |( REQUEST ) |( UNREQUEST ) |( REFRAIN ) |(NOT < mes- pattern>)


  • A commitment rule adds a commitment to "action"(COMMIT mes-cond mental-cond (ai actioni)*)If:the message condition holds for the incoming messagethe mental condition holds for the current mental statethe agent is capable of doing all actionithe agent is not committed to any REFRAIN actioniif actioni=REFRAIN action1 the agent is not committed to action1

    Then, for all i, commit to ai to perform actioni.


  • The Basic Loop of AGENT01. Read the current messages and update your mental state, including your beliefs and commitments.(1a) Update the beliefs.(2a) Update the commitments.

    (1a) The belief database is updated eitheras a result of being informed oras a result of taking a private action -- e.g.,- a database agent comes to believe a fact after performing aretrieval operation.- a robotic agent comes to believe something after performing a visual routine.


  • The Basic Loop of AGENT0(2a) Updating commitmentsItems in the database of commitments are pairs(agent action),the agent to which the commitment was made and the content of the commitment.Items in the database of capabilities are pairs(privateaction mental-cond).The mental condition part prevents commitment to incompatible actions.E.g.,((?!time (rotate wheelbase ?degrees))(NOT ((CMT ?x) ?!time (service wheelbase))))


  • Existing commitments are removed eitheras a result of UNREQUEST messagesThe agent removes the corresponding item from the commitmentdatabase if it exists, else does nothing.oras a result of belief change.Belief change may affect capabilities since the capability of each private action depends on mental preconditions. So, whenever a belief update occurs, the AGENT-0 interpreter examines the current commitments to private actions. It removes those whose preconditions in the capability database have been violated.It should add a commitment to inform the agent to which it wascommitted (but AGENT-0 doesnt enforce this).*

  • Removing existing commitments is independent of the program, but adding them depends on the program.Adding commitments see commitment rule

    The Basic Loop of AGENT02. Execute the commitments for the current time, possibly resulting in further belief change.


  • 5 A sample programA program for the initial scenarioThe ideas behind the program are that:the relevant activity on the part of the airline is issuing a boarding pass to the passenger, andconfirming a reservation is a commitment to issue a boarding pass at the appropriate time.*

  • 5.1 Macros(issue_bp pass flightnum time) (IF (AND (B ((- time h) (present pass))) (B (time(flight ?from ?to flightnum)))) (DO time h (physical_issue_bppass flightnum time)))(query_which t asker askee q) (REQUEST t askee(IF (B q) (INFORM (+ t 1) asker q)))(query_whether t asker askee q) (REQUEST t askee(IF (B q) (INFORM (+ t 1) asker q)))(REQUEST t askee(IF (B (NOT q))(INFORM (+ t 1) asker (NOT q))))*Private action

  • 5.2 Initial BeliefsConcerning the flight schedule:(time (flight from to number))And the number of seats available:(time (remaining_seats time1 flight_number seats)5.3 CapabilitiesThese are issuing boarding passes and updating the count of theavailable seats on flights.((issue_bp ?a ?flight ?time) true)((DO ?time(update_remaining_seats ?time1 ?flight_number?additional_seats))(AND(B (?time (remaining_seats ?time1?flight_number ?current_seats)))(?current_seats >= |?additional_seats|)))*

  • 5.4 Commitment Rules

    (COMMIT(?pass REQUEST(IF (B ?p) (INFORM ?t ?pass ?p)))true?pass(IF (B ?p) (INFORM ?t ?pass ?p)))*

  • (COMMIT(?cust REQUEST(issue_bp ?pass ?flight ?time))(AND(B (?time (remaining_seats ?flight ?n)(?n > 0)(NOT ((CMT ?anyone)(issue_bp ?pass ?anyflight ?time))))(myself(DO (+ now 1)(update_remaining_seats ?time ?flight -1)))(?cust (issue_bp ?pass ?flight ?time)))*

  • 5.5 Sample exchange between a passenger and an airline agent smith (query_which lmarch/l:00 smith airline(18april/?!time (flight sf ny ?!num)))airline (INFORM lmarch/2:00 smith(18april/8:30 (flight sf ny #354)))airline (INFORM lmarch/2:00 smith(18april/10:00 (flight sf ny #293)))airline (INFORM lmarch/2:00 smith(18april/ smith (REQUEST lmarch/3:00 airline(issue_bp smith #354 18april/8:30))smith (query_whether lmarch/4:00 smith airline((CMT smith)(issue-bp smith #354 18april/8:30)))*

  • airline (INFORM lmarch/5:00 smith(NOT ((CMT smith)(issue-bp smith #354 18april/8:30))))smith (REQUEST lmarch/6:00 airline(issue-bp smith #293 18april/10,.00))smith (query-whether

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