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Chapter 18: Communication. Service-Oriented Computing: Semantics, Processes, Agents – Munindar P. Singh and Michael N. Huhns, Wiley, 2005. Highlights of this Chapter. Agent Communication Languages Communicative Act Theory Semantics Interaction Patterns Contract Net Protocol - PowerPoint PPT Presentation
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Chapter 18:Communication
Service-Oriented Computing: Semantics, Processes, Agents– Munindar P. Singh and Michael N. Huhns, Wiley, 2005
Chapter 18 2Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and
Michael Huhns
Highlights of this Chapter Agent Communication Languages
Communicative Act Theory Semantics Interaction Patterns
Contract Net Protocol Business Protocols
Chapter 18 3Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and
Michael Huhns
Interaction and Communication
Interactions occur whenever agents share an environment
Resource contention, e.g., bumping into each other Communications are interactions understood
to preserve the participants’ autonomy Realized by physical actions (e.g., shared memory or
messaging middleware) that may not preserve autonomy
Meaning based on shared conventions Offer loose coupling: essential for services
A message is a unit of communication A protocol structures communications
What kind of structure is appropriate?
Chapter 18 4Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and
Michael Huhns
Syntax, Semantics, Pragmatics
Syntax: structure of a (communication) language
Semantics: meaning based solely on the terms used
Requires a structured vocabulary and a shared framework of knowledge, e.g., an ontology
Pragmatics: meaning based on the context (“here”)
Knowing whom to communicate with and how to find them
Knowing how to initiate and maintain an exchange Knowing the effect of the communication on the
recipient
Chapter 18 5Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and
Michael Huhns
A Classification of Message Classifications
Structure-based (syntactic) Distinguish messages based on grammatical forms in
natural language Meaning-based (semantic)
Distinguish messages based on a notion of intrinsic meaning
E.g., prohibitive is different from directive, despite syntactic similarity
Use-based (pragmatic) Distinguish messages based on their functions in
specific classes of protocols E.g., assertion is different from acknowledgment
Chapter 18 6Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and
Michael Huhns
Communicative Act Theory
Also called speech act theory (though not specific to speech)
Developed for human language Views communication as action Contrasts with traditional logic, which is about true
or false assertions Considers three aspects of a message:
Locution, or how it is phrased, e.g., “It is hot here” or “Turn on the air conditioner”: strings or XML documents
Illocution, or how it is meant by the sender or understood by the receiver, e.g., a request to turn on the air conditioner or an assertion about the temperature: message type plus proposition
Perlocution, or how it influences the recipient, e.g., turns on the air conditioner, opens the window, ignores the speaker
Illocution is the core aspect
Chapter 18 7Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and
Michael Huhns
Applying the Theory Classifications of illocutions motivate message
types, but are typically designed for natural language
Rely on NL syntax, e.g., conflate directives and prohibitives
Address subtleties in natural language understanding: infer agents’ beliefs and intentions to determine which illocution
For networked services and multiagent systems, determining the
Message type is trivial, because it is explicitly encoded
Agents’ beliefs and intentions is impossible, because the internal details of the agents are not known
Chapter 18 8Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and
Michael Huhns
ACL SemanticsWhat does a inform, promise, request, query
mean? Mentalist: a state of solo or mutual beliefs or
intentions (under assumption of sincerity) Inform: speaker believes true Promise: speaker intends to make true Request: speaker intends listener to make true Query: speaker intends listener to inform if true
Public: conventions based on laws and observations
Inform: speaker’s commitment to the veracity of Promise: speaker’s commitment to bring about
Evaluation: For open systems, public semantics is essential because it underlies compliance
Chapter 18 9Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and
Michael Huhns
FIPAFIPA is the Foundation for Intelligent
Physical Agents Incorporated as an IEEE standards body Specifies standards for heterogeneous,
interoperating agent-based systems Concerned with agency as it relates to
Autonomous behavior Communication with other agents
Chapter 18 10Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and
Michael Huhns
FIPA StandardsSupport interoperability, but at a low level
Ways of interpreting communications in a way that respects their intended meanings
Message types (called communicative acts): inform, request, …
Public ontologies Transport and infrastructure
Superseded by Web services and messaging standards
Programming model and container Compatible with containers in application servers
FIPA Standards Provide … An architecture for interoperability A few fixed primitives (message
types) Formal semantics based on mental
notions A few interaction protocols, but
extensible No formal semantics Over-specified via low-level details
Chapter 18 11Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and
Michael Huhns
Chapter 18 12Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and
Michael Huhns
Contract Net ProtocolAn important generic
protocol Manager announces
tasks via a (selective) multicast
Agents evaluate the announcement; some bid
Manager awards a contract to a bidder
Manager and contractor communicate privately as necessary
Chapter 18 13Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and
Michael Huhns
RFQ: Task Announcement Eligibility specification: criteria that an
agent must meet to be eligible to submit a bid
Task abstraction: a description of the task to be executed
Bid specification: a description of the expected format of the bid
Expiration time
Chapter 18 14Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and
Michael Huhns
Bid and Award Messages A bid specified the provider’s
capabilities An award consists of a task specification
A complete specification of the task
Chapter 18 15Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and
Michael Huhns
Contract Net Evaluated Applies best when problem has a well-
defined hierarchy of tasks With coarse-grained decomposition No interference with each other Yields robustness: failure can be treated as
a manifestation of autonomy Lacks support for specifying service
agreements and contracts Purely operational Hard-coded into the domain Combines agent reasoning with interaction
Chapter 18 16Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and
Michael Huhns
Commitment Protocols
Protocols expressed in terms of Participants’ commitments Actions for performing operations on
commitments (to create and manipulate them)
Constraints on the above, e.g., captured in temporal logic
Examples: escrow, payment, RosettaNet (over 100 mostly request-response Partner Interface Processes or PIPs) Traditionally, specified in operational terms Can be expressed via commitments
Chapter 18 17Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and
Michael Huhns
NetBill Payment Protocol Checking compliance is
easy but the representation is rigid
Some obvious variations are not allowed by the FSM:
The merchant may initiate the protocol by sending an offer
The customer may send an accept prior to offer
The merchant may send the goods prior to accept
How do we design a new FSM?
Verify it?
FSM Representation
C: rfq
M: offer
C: accept
C: pay
M: receipt
M: goods
Chapter 18 18Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and
Michael Huhns
NetBill using Commitments
Meanings:
1. true
2. request
3. offer
4. Cm,cgoods accept promiseReceipt
5. goods Cc,mpay promiseReceipt
6. goods pay Cm,creceipt
7. goods pay receipt
8. goods promiseReceipt
9. accept
Final states: all detached commitments are discharged
C: rfq
M: offer
C: accept
C: pay
M: receipt
M: goods
M: offer
M: goods
C: pay
C: accept
M: g
oods
Chapter 18 19Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and
Michael Huhns
Patterns for Commitment Operations Ensure that information about commitment
operations flows to the right parties To enable local decisions
x y z
create(x,c)
delegate(x,z,c)
delegate(x,z,c)
discharge(x,c)
x y z
create(x,c)
assign(x,y,z)
discharge(x,c)
assign(x,y,z)
Chapter 18 20Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and
Michael Huhns
Complying with Commitment Protocols
Compliance means commitments are discharged
Directly or indirectly after delegates and assigns How can an agent check if others comply with
specified protocols? Agent x has grounds of complaint against agent y if
and only if Cy,x(True, p), for some p Commitment protocols are specified in terms of
Main roles and sphere of commitment Roles essential for coordination Domain-specific propositions and actions
Chapter 18 21Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and
Michael Huhns
Verifying Compliance Specification
Execution models based on potential causality
Commitments Run-time verification (no access to
implementations) Respects heterogeneity Local verification from observed
messages: each party checks the others’ behavior
Chapter 18 22Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and
Michael Huhns
Run-Time Compliance Checking
An agent maintains Pending commitments of which it is debtor
or creditor A local model: the messages sent or
received It uses this local model to verify that
each commitment is discharged – or replaced by a commitment that is
Invariant: a creditor of a commitment can determine if it is violated – if the commitment doesn’t refer to a third party’s actions
Chapter 18 23Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and
Michael Huhns
Fish-Market Sample Execution
Auctioneer A Bidder B1
m1 “50”: [1,0,0]
m3 “No”: [1,2,0]
m5 “40”: [5,2,2]
m8 “No”: [6,2,4]
Bidder B2
m2 “50”: [2,0,0]
[0,0,0] [0,0,0] [0,0,0]
[2,0,1]
[4,2,2]
[3,2,0]
[5,3,2]
m6 “40”: [6,2,2] [6,2,3]
m7 “Yes”: [5,4,2][7,4,2]
fishmoney
[1,1,0]
[8,4,4]
m4 “No”: [2,0,2]
Based on a vector clock
Chapter 18 24Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and
Michael Huhns
Fish-Market Local Observations
Auctioneer A
[1,0,0]
[2,0,0]
[3,2,0]
[4,2,2]
start
s(m1)
s(m2)
s(m4)
r(m3)
r(m4)
[2,0,2]
[5,2,2]
[6,2,2]
[7,4,2]
end
s(m5)
s(m6)
r(m7)
Bidder B1start
s(m1)
[1,0,0]
r(m1)
s(m3)
r(m5)
s(m7)
end
[1,1,0]
[1,2,0]
[5,2,2][5,3,2]
[5,4,2]
s(m5)
The discharge of a commitment mustbe reachable from its create
Chapter 18 25Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and
Michael Huhns
Fish-Market Compliance Auctioneer can verify if the bidders
comply Sees each message as sender or
receiver An individual bidder cannot verify
if the auctioneer complies Doesn’t see each message If bidders pool their observations,
then they can verify if the auctioneer complies
Asymmetry indicates need for third party
Chapter 18 26Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and
Michael Huhns
Chapter 18 Summary Communication enables loose coupling Communications are actions – thus
outside the purview of traditional logic Protocols capture important patterns of
communications Business protocols can be understood
using commitments Model message meanings, not just syntax Check compliance with respect to
commitments
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