An Operational Framework
for the Semantics of
Agent Communication Languages
Giovanni Rimassa
AOT Lab - DII
Università degli Studi di Parma
[email protected]
Mirko Viroli
DEIS
Università degli Studi di Bologna
[email protected]
Outline
• Semantic interoperability by ACL semantics
• Operational vs. intentional stance
• Modelling agent interactions
• The operational description of an ACL semantics
• Future works
ESAW 2002, 16-17 Sept, Madrid
Mirko Viroli, DEIS, Università di Bologna
2
Agents and software engineering
Agent abstraction is of particular interest for software engineering
• Allows us to deal with complexity and unknowability
– as wrapper for legacy systems, intelligent systems,
heterogeneous sources
• In particular, tackling interoperability and composability
– See [Bergenti@ESAW2002]
• Semantic interoperability/composability
– which interpretation of an agent behaviour (stance)?
ESAW 2002, 16-17 Sept, Madrid
Mirko Viroli, DEIS, Università di Bologna
3
The intentional stance
• See also [Zambonelli&Van Dike Parunak@ESAW2002]
• Design stance: a system is understood in terms of what is known
about its design (or internal architecture)
• Intentional stance: a system is understood in terms of an entity
with intentional properties such as beliefs, desires, goals
– abstracting away from an actual architecture
– generally exploited to understand the behaviour of complex system
ESAW 2002, 16-17 Sept, Madrid
Mirko Viroli, DEIS, Università di Bologna
4
Agent communication languages
Standardised interoperability (e.g. FIPA ACL)
• Syntactic interoperability
– specifying the syntax of messages exchanged by agents
• Dealing with intentional stance by speech acts
– resembling a human-like way of speaking (INFORM, REQUEST,..)
– automatically equipped by an informal semantics
• Semantic interoperability
– a formal semantics is to be assigned to performatives
– why messages are sent
– what is the (expected) effect of receiving messages
• ACL semantics
ESAW 2002, 16-17 Sept, Madrid
Mirko Viroli, DEIS, Università di Bologna
5
FIPA ACL semantics
• An agent behaviour is understood as a BDI-like entity
– not as actual implementation, is just a stance!
– B (beliefs), D (desires), U (uncertain beliefs), C (intentions)
• Each performative is associated to
– feasibility preconditions (FP)
• a predicate on B, D, U that must hold for the sender
– rational effects (RE)
• expected effect of a message on the receiver’s BDU
• Furthermore
– rationality principle: behaving so as to pursue the goals
– e.g. sending a message if the REs are actually intended
• Promoting formal approaches to agent development?
ESAW 2002, 16-17 Sept, Madrid
Mirko Viroli, DEIS, Università di Bologna
6
Do we need formal models?
• At which development step are they useful?
• Let’s promote (and use) formal models for the design!!!!
– precise description of a system behaviour at an early stage
– allows for checking the model consistency
– allows for checking conformance w.r.t. desired properties
 guides to more likely correct implementations
• Which applications?
– where designer’s early experience fails to cover new scenarios
– where a system complexity needs to be formally tackled
 they are important for well grounded supporting technologies:
• APIs, middlewares, compilers, methodologies, tools
• ACL semantics:
– should provide hint on how to build interoperable agents!
ESAW 2002, 16-17 Sept, Madrid
Mirko Viroli, DEIS, Università di Bologna
7
Limits of current ACL semantics
(Known) open issues
• How to verify compliance?
– Which tests to validate an implementation?
– At least, how to verify compliance of an agent design?
• How do I build a compliant agent?
– What about non-BDI agents? (e.g. wrapping a legacy system)
– Is heterogeneity really promoted?
Trying to overcome the problem...
ESAW 2002, 16-17 Sept, Madrid
Mirko Viroli, DEIS, Università di Bologna
8
Operational models
• The intentional stance
– Provides an abstract model for an agent
– Suitable for modelling the agent internal behaviour
– Is it practical for dealing with interactions?
• Operational stance
– A system described in terms of its single-step abilities of evolving
its state and performing interactions with the environment
– Is promoted by traditional formal frameworks for describing the
interaction issue
• process algebras, labelled transition systems, automata
• Which applicability to ACL semantics?
ESAW 2002, 16-17 Sept, Madrid
Mirko Viroli, DEIS, Università di Bologna
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Transition systems
• Formally, a sort of infinite-state automaton <P,,Act>
– P, set of states of the system of interest
– Act, set of actions performed on, by, due to the environment
–   P x Act x P, associates <old_state,action,new_state>
• Transition systems promote the idea of observation semantics
– defining a process observation from its executed/allowed actions
– e.g. trace semantics: sequence of actions executed
• System equivalence
– allowing for the same set of observations
• System refinement (preorder)
– allowing for fewer observations, i.e., more deterministic, executable
ESAW 2002, 16-17 Sept, Madrid
Mirko Viroli, DEIS, Università di Bologna
10
Grey-box approach
Several degrees of completeness in an operational specification
• White-box description
– entirely modelling the agent behaviour and internal architecture
– cannot deal with complexity and unknowability
• Black-box description
– only describing the kinds of interaction, and the protocols
– provide only little “semantics” details
• Grey-box description
– providing a partially specified architecture
– seems the more reasonable for agents
ESAW 2002, 16-17 Sept, Madrid
Mirko Viroli, DEIS, Università di Bologna
11
Our grey-box approach for agents
Agent
Internal
Machinery
Agent
Core
(black)
(white)
• Focuses on the agent part dealing with interactions
• Abstracting away from agent internal details
• W.r.t. intentional stance
– higher abstraction in the agent internal behaviour
– less abstraction, greater specification of the agent interface
ESAW 2002, 16-17 Sept, Madrid
Mirko Viroli, DEIS, Università di Bologna
12
Detailed architecture
?
internal
machinery
events
input acts
updates
output acts
agent core
(O)
with
ESAW 2002, 16-17 Sept, Madrid
Mirko Viroli, DEIS, Università di Bologna
13
Grey-box and ACLs
• This model represents
– how to manage incoming messages
 i.e. their effect on the receiving agent
– which system’s evolution results in sending a message
 i.e. the causes of sending a message
• Key idea
– encoding an ACL semantics as operational description of the
agent core
ESAW 2002, 16-17 Sept, Madrid
Mirko Viroli, DEIS, Università di Bologna
14
An example of application
FIPA ACL-like feasibility preconditions
• Observable state as a triple
– pending input acts, allowed output acts, state M
– computation of feasibility preconditons as fp:MPow(Oc)
• Dynamics
– changes in M cause fp to be recomputed
– agent core constrains sending of output acts
ESAW 2002, 16-17 Sept, Madrid
Mirko Viroli, DEIS, Università di Bologna
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Compliant interactive behaviour
internal
machinery
events
input acts
updates
output acts
agent core
(O)
Agent core (ACL semantics)
+
Virtual internal machinery
=
Compliant interactive behaviour
ESAW 2002, 16-17 Sept, Madrid
Mirko Viroli, DEIS, Università di Bologna
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A specific agent implementation
internal
machinery
(I)
events
input acts
updates
output acts
agent core
(O)
Agent core (ACL semantics)
+
Actual internal machinery
=
Agent implementation
ESAW 2002, 16-17 Sept, Madrid
Mirko Viroli, DEIS, Università di Bologna
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Compliance by refinement
Compliant interactive behaviour
Agent implementation
By basic
compositionality
Actual implementation as a
refinement of the compliant
interactive behaviour
Agent implementation R can be safely used where compliant
behaviours are expected, i.e., R is compliant to the ACL
semantics!
ESAW 2002, 16-17 Sept, Madrid
Mirko Viroli, DEIS, Università di Bologna
18
Main applications
• Development of compliant agents
– by a wrapper ensuring compliance
– built by invoking an API, by a middleware, ...
• Compliance of an agent design
– building the agent core for the intended ACL semantics
– building the operational specification of an agent design
– proving refinement by some existing verification tool
ESAW 2002, 16-17 Sept, Madrid
Mirko Viroli, DEIS, Università di Bologna
19
Other future works....
• Helping FIPA?
– identifying specification lacks and inadequacy
– proposing improvements towards implementability
• Deepening comparison w.r.t. intentional stance
– relations and differences
– integrated approach?
ESAW 2002, 16-17 Sept, Madrid
Mirko Viroli, DEIS, Università di Bologna
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