Agents The Communication Aspects

Agents

The Communication

Aspects

NZDIS Team
November 2000
University of Otago

Outline



Introduction to Agent Communication



Agent Messaging



Agent Communication Language (ACL)



Ontologies



Conversations



Standards bodies



Comparison to other technologies

Introduction



Agents and Objects



What is messaging?



What is Communication Language
(ACL)?



Why not to use API instead of ACL?

Agents and Objects



Better and more tight encapsulation
of the behaviour and control



More goal/task oriented design and
modelling approach



More process and behaviour-centric
programming model



Independence from the underlying
implementation machinery

Agent Messaging



The concept of a message



Message delivery

– push vs. pull models
– peer-to-peer
– tuple spaces
– content based addressing



The message structure

– outer layer (routing, encoding, type)
– inner layer (content, ontology)

Pull vs. Push models

Push

Pull

sender

receiver



Delivery notification



Delivery failure recovery



Participant crash recovery

Peer-to-peer messaging



Drawbacks

– Explicit addressing schema
– The need for facilitators
– Not fully decoupled participants



Solutions

– Tuple spaces
– Content based addressing

Tuple Space



Fully decoupled senders and receivers



No bootstrapping problem



Easy backup and recovery



But single point of failure

Space

receiver

senders

Content-based addressing



Combines some of the elements from pure peer-to-peer and tuple space
models



No need for explicit addressing



More flexible routing than in peer-to-peer model



The need for facilitators

sender

receivers

Routing

Agent

Communication

Language



Language Elements

– Communicative Acts
– Actions and Propositions
– Logical Operators an predicate logic



Communicative acts

– denote the actions related to communication;

this is based on the speech act theory
(Searle, et al) and related utterance theories

Example of

Communicative Acts



Inform

– informs the recipient about a proposition

which the senders believes is true



Query-if

– asks the recipient if a given proposition

is true or not



Request

– requests the recipient to execute a given

action

Message Example



text-based format for:

– human readability
– cross-process compatibility (the

participant implementation language is
irrelevant)



ongoing work on binary formats
and specific API for a given
programming language

Example Inform Message

(inform
   :sender AgentA
   :receiver AgentB
   :content (and
                   (is_a AgentCommunication presentation)
                   (is_in_progress AgentCommunication))
   :ontology PresentationOntology
)

The need to agree on meaning of
objects: presentation,
and predicates: is_a, is_in_progress

Ontology



Conceptualization of the domain of
discourse



Defines the meaning and relations
between all the concepts (objects,
actions, predicates, relations,
associations, etc) in the domain of
discourse

Example Ontology



presentation

– class of objects which denote the

presentation. Subclass of a class: action.
Can contain associations to concepts:
venue, date, time, subject.



is_a

– relationship between an object and its

class



is_in_progress

– state of the object of class action, which

denotes that the action is in progress

Ontology (cont.)



The need for:

– formal ontology specification
– formal ontology language
– ontology interchange
– reasoning and inference capabilities
– easy management and human readability



Solution:

– Unified Modelling Language and Object

Constraint Language (UML+OCL)

Protocols and

Conversations



Protocol

– Complex and dynamic interaction schemas
– Interoperability issues for the standard

interactions schemas

– Reducing the complexity of the full ACL by

constraining the possible interactions



Conversation

– ongoing sequence of communicative acts,

conforming to one or several protocols

Modelling Conversations

Terminated

Process request

Preprocess request

Terminated

Refuse

Agree

Done

Prepare request

Sender (initiator)

Receiver

Modelling Conversations



Robust and easy modelling formalism



Graphical representation of static and
dynamic structures



complex and concurrent structures
support



direct mapping to the implementation



possibility of reusing the structures as
building blocks of complex conversations



easy monitoring of the progress of a
conversation

Standards Organizations



ACL standards:

– Foundation of Intelligent Physical Agents

(FIPA-ACL)

– Knowledge Query and Manipulation Language

(KQML)



Ontology

– FIPA
– Open Knowledge Base Connectivity (OKBC) and

Ontolingua (Stanford University)

– Knowledge Interchange Format (KIF)

FOR MORE INFO...

FIPA - http://www.fipa.org

FIPA - architecture



Peer-to-peer messaging model



Specified delivery model



Specified standard services: ACC, DF,
AMS,



Semantic Language (SL) as a content
language



Work in progress on formal ontology
specifications

Messaging technologies



Peer-to-peer

– Java Messaging Service, CORBA

Messaging Services

– Remote Procedure Calls (RPC), Simple

Object Access Protocol (SOAP)



Tuple spaces

– TSpaces from IBM
– JavaSpaces from Sun Microsystems

ACL vs. API



Full featured language

– high flexibility, robustness
– expressive power
– encapsulation of domain specific

concepts inside ontology

– implementation/efficiency

considerations



Mapping the language to the API

Document Outline