Multi-Agent Systems Lecture 5 University

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Multi-Agent Systems Lecture 5 University

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... with #293: I am not allowed to double book a passenger. ... (robot,cup)t ... time-constant ::= m | h | d | y ; m (minute) =60, h (hour) 3600, etc. 20. 4.4 ... – PowerPoint PPT presentation

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Title: Multi-Agent Systems Lecture 5 University

1
Multi-Agent SystemsLecture 5University
Politehnica of Bucarest2004 - 2005Adina
Magda Floreaadina_at_cs.pub.rohttp//turing.cs.pub
.ro/blia_2005
2
Agent Oriented ProgrammingLecture outline

1 AOP principles
2 An agent scenario
3 Overview of the AOP framework
4 AGENT-0
5 A sample program

3
1 AOP principles

promotes a societal view of computing
uses mental states to design agents mental
categories appear in the programming language
the semantics of the PL is related to the
semantics of mental states
first AOPL Agent-0
mental states of Agent-0 beliefs, commitments,
capabilities, decisions

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1.1 OOP versus AOP
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2 An agent scenario

Airline Reservation
Agents
P is a passenger.
C is an airline clerk, a program.
S is Cs supervisor.
By confirming a reservation, the airline enters
into a commitment to issue a boarding pass to the
passenger at the appropriate time.

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Airline reservation

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 0830. flight
293 departs at 1000, flight 441 departs at
noon, ¼
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 ¼
S to C Permission denied.
C to P Sorry, I cannot get approval.

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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.

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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 agents
An (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 states
An "agentification" process converting neutral
devices into programmable agents

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4 Agent-0

4.1 Components of Mental State
Two mental categories beliefs and decisions
A third (not per se mental) category
capabilities.
A forth category obligation, or commitment -
treat decision as commitment to oneself.

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The agents beliefs refer to
states of the world,
mental states of other agents, and
capabilities 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 beliefs
Decisions are also constrained by prior
decisions.

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4.2 Representation of Belief, Obligation, and
Capability
Time
We adopt a point-based temporal language
holding(robot,cup)t
Action
We dont distinguish between actions and facts
the occurrence of an action is represented by the
corresponding fact holding,
e.g., instead of saying that the robot took the
action raise-arm at time t, we say that the
sentence
raise-arm(robot)t
is true.
Since actions are facts, theyre instantaneous.

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Belief
Using modal operator B,
Bta j
At time t agent a believes that j,
where j is a (recursively defined) sentence.
B3a B10b like(a,b)7
At time 3 agent a believes that at time 10 agent
b will believe that at time 7 a liked b.
Obligation (commitment)
OBLta,b j
At time t agent a is obligated (committed) to
agent b about j.
Decision (choice)
Decision is defined to be obligation to oneself
DECta j def OBLta,a j

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Capability
CANtaj
At time t agent a is capable of j.
CAN5robotopen(door)8
At time 5 the robot can ensure that the door is
open at time 8.
ABLE is the immediate version of CAN.
time(j) - the outermost time occurring in
sentence j, time(open(door)t)t
ABLEa j def CANtime(j)aj

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Persistence of Mental State
Beliefs 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.

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Summary of mental categories in AGENT0

Linear discrete time
Beliefs Bat ?
Actions raise_arm(robot)t
Obligations (commitments) OBLa,bt ? (CMTa,bt ?)
Decisions (choices) DECat ? OBLa,at ?
Abilities (capabilities) CANat ?
Properties of mental categories
Internal consistency of beliefs and commitments
Good faith
Persistence of beliefs
Persistence of commitments

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4.3 A generic interpreter

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 Loop
Each 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.)

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A generic interpreter - cont

Actions to which agents can be committed include
arbitrary private actions
communicative 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 actions
conditional actions
A conditional action relies on a mental
condition, which refers to the mental state of
the agent.

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A generic interpreter - cont

Assumptions about Message Passing
the platform can pass messages to other agents
addressable by name.
the interpreter determines when messages are
sent.
Assumptions about the Clock
The clock initiates iterations of the two-step
loop at regular intervals.
The length of these intervals is determined by
the variable timegrain.
Assume that a single iteration through the loop
lasts less than the time grain.

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Control
Initialize mental state and capabilities Define
rules for making new commitments
Data
Incoming message
Update mental state
Clock
Representation of mental state and capabilities
Execute commitment for current time
Outgoing message
A flow diagram of the generic agent interpreter
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4.4 Agent language

Fact statements
Fact statements are used to specify both the
content of actions and conditions for their
execution.
(t (employee smith acm))
(NOT (t (employee jones acm)))
ltfactgt (lttimegt (ltpredicategt ltarggt))
lttimegt ltintegergt now lttime-constantgt
( lttimegt lttimegt) (- lttimegt lttimegt)
( lttimegt lttimegt)
Time may be a ltvariablegt when
it appears in a commitment rule
lttime-constantgt m h d y
m (minute) 60, h (hour) 3600, etc.

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Action Statements
Private actions
The syntax for a private action is
(DO t p-action)
where t is a time point and p-action is a private
action name.
Communicative actions
The syntax of informing is
(INFORM t a fact)
where t is a time point, a an agent name, and
fact is a fact statement.
The syntax of requesting is
(REQUEST t a action)
where action is an action statement, recursively
defined.
e.g.,
(REQUEST 1 a (DO 10 update-database))
(REQUEST 1 a (REQUEST 5 b
(INFORM 10 a fact)))

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The syntax of canceling a request is
(UNREQUEST t a action)
We can also prevent commitment to a particular
action
(REFRAIN action)
ltactiongt
(DO lttimegt ltprivateactiongt)
(INFORM lttimegt ltagentgt ltfactgt)
(REQUEST lttimegt ltagentgt ltactiongt)
(UNREQUEST lttimegt ltagentgt ltactiongt)
(REFRAIN ltactiongt)
(IF ltmental-condgt ltactiongt)

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Conditional Action Statements
We distinguish between
commitments for conditional actions, which
include conditions
to be tested just before acting, and
conditions for entering into commitments in the
first place
When the time comes to execute the action, the
mental state at that time is examined to see
whether the mental condition is satisfied. So the
agent and time components of the mental state are
left implicit.
A mental condition is then any combination of
modal statements in the temporal-modal language,
with the primary agent and time arguments
omitted.
(IF (B (t1 (employee smith ibm)))
(INFORM t a (t1 (employee smith
ibm))))

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ltactiongt (IF ltmental-condgt ltactiongt)
lt mental-cond gt
lt mental- conjgt (OR lt mental- conjgt)
lt mental- conjgt
lt mental- patterngt (AND lt mental- patterngt)
lt mental- patterngt
(B ltfactgt)
((CMT ltagentgt) ltactiongt)
(NOT lt mental- patterngt)

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Variables
agent names, fact statements, action statements
?x ?!x
Ex (IF (NOT ((CMT ?x) (REFRAIN sing)))
sing)
(IF (B (t (emp ?!x ibm)))
(INFORM a (t (emp ?!x ibm))))
ltvariablegt ?ltalphanumeric_stringgt
?!ltalphanumeric_stringgt
ltagentgt ltalphanumeric_stringgt ltvariablegt
ltpredicategt ltalphanumeric_stringgt
ltarggt ltalphanumeric_stringgt ltvariablegt

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Commitment rules - key to AOP
Conditions for entering a commitment
mental condition
message condition
Message condition a logical combination of
message patterns
Message pattern (sender type content)
type INFORM, REQUEST, UNREQUEST, REFRAIN
content a fact statement or an action
statement, depending on type
Commitment rule
(COMMIT message-cond mental-cond (agenti
actioni))
Ex (COMMIT (?a REQUEST ?action)
(B (now (myfriend ?a)))
(?a ?action))

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Commitment rules
ltcommitrulegt
(COMMIT ltmes-condgt ltmental-condgt
(ltagentgt ltactiongt))
lt mes- condgt
lt mes- conjgt (OR ltmes- conjgt)
lt mes- conjgt
lt mes- patterngt (AND lt mes- patterngt)
lt mes- patterngt
(ltagentgt INFORM ltfactgt)
(ltagentgt REQUEST ltactiongt)
(ltagentgt UNREQUEST ltactiongt)
(ltagentgt REFRAIN ltactiongt)
(NOT lt mes- patterngt)

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A commitment rule adds a commitment to "action"
(COMMIT mes-cond mental-cond (ai actioni))
If
the message condition holds for the incoming
message
the mental condition holds for the current mental
state
the agent is capable of doing all actioni
the agent is not committed to any REFRAIN actioni
if actioniREFRAIN action1 the agent is not
committed to action1
Then, for all i, commit to ai to perform actioni.

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Agent program
fix the time grain
define agent's abilities for all agents
define agents' initial beliefs
state sequence of commitment rules
ltprogramgt
timegrain lttimegt
CAPABILITIES (ltactiongt ltmental-condgt)
INITIAL BELIEFS ltfactgt
COMMITMENT RULES ltcommitrulegt

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The Basic Loop of AGENT0
1. 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 either
as a result of being informed or
as a result of taking a private action -- e.g.,
- a database agent comes to believe a fact after
performing a
retrieval operation.
- a robotic agent comes to believe something
after performing a visual routine.

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The Basic Loop of AGENT0
(2a) Updating commitments
Items 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))))

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Existing commitments are removed either
as a result of UNREQUEST messages
The agent removes the corresponding item from the
commitment
database if it exists, else does nothing.
or
as 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 was
committed (but AGENT-0 doesnt enforce this).

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Removing existing commitments is independent of
the program, but adding them depends on the
program.
Adding commitments see commitment rule
The Basic Loop of AGENT0
2. Execute the commitments for the current time,
possibly resulting in further belief change.

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Conditions for an agent to be capable of an
action
An agent can request and unrequest anything from
anyone.
An agent can inform anyone of a fact he believes.
An agent can inform itself of any fact at all
(useful to implement reasoning in the agent).
An agent is capable of any private action in the
capability database provided the mental condition
associated with that private action by the
database holds at that time.
An agent can refrain from any action provided
hes not already committed to it.
An agent can perform a conditional action (IF
mental-cond action) if he can perform action
under the condition mental-cond.

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Carrying Out Commitments
Each commitment in the commitment database has an
associated time.
In 2) the interpreter executes all actions whose
time is in the interval (now - timegrain, now.
The meaning of execute depends on the type of
action
INFORM, REQUEST, UNREQUEST Send the
appropriate message.
REFRAIN No effect on execution.
DO Consulting the belief and commitment
databases, check the mental condition associated
with the primitive action in the capability
database if it holds, then perform the primitive
action.
IF Consulting the belief and commitment
databases, test the
mental condition if it holds, then
(recursively) execute the action.

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5 A sample program

A program for the initial scenario
The ideas behind the program are that
the relevant activity on the part of the
airline is issuing a boarding pass to the
passenger, and
confirming a reservation is a commitment to
issue a boarding pass at the appropriate time.

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