1. Service-Oriented Computing: Semantics, Processes, Agents
August 2004
Chapter 18: Communication
Service-Oriented Computing: Semantics, Processes, Agents – Munindar P. Singh and Michael N. Huhns, Wiley, 2005
© Singh & Huhns

2. Highlights of this Chapter
Service-Oriented Computing: Semantics, Processes, Agents
August 2004
Highlights of this Chapter
Agent Communication Languages
Speech Act Theory
Semantics
Interaction Patterns
Combining ACLs with Web Services
Contract Net Protocol
Business Protocols
Chapter 18
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns
© Singh & Huhns

3. Interaction and Communication
Service-Oriented Computing: Semantics, Processes, Agents
August 2004
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 and helps specify open systems
Chapter 18
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns
© Singh & Huhns

4. Syntax, Semantics, Pragmatics
Service-Oriented Computing: Semantics, Processes, Agents
August 2004
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
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns
© Singh & Huhns

5. A Classification of Message Classifications
Service-Oriented Computing: Semantics, Processes, Agents
August 2004
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
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns
© Singh & Huhns

6. Communicative Act Theory
Service-Oriented Computing: Semantics, Processes, Agents
August 2004
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
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns
© Singh & Huhns

7. Service-Oriented Computing: Semantics, Processes, Agents
August 2004
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
In natural language understanding: determining how locutions map to illocutions (inferring agents’ beliefs and intentions)
For services and agents, 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
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns
© Singh & Huhns

8. Service-Oriented Computing: Semantics, Processes, Agents
August 2004
ACL Semantics
What does a inform, promise, request, query mean?
Mentalist: a state of solo or mutual belief (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 veracity of
Promise: speaker’s commitment to bring about
Evaluation: For open systems, public semantics is essential because it underlies compliance
Chapter 18
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns
© Singh & Huhns

9. FIPA FIPA is the Foundation for Intelligent Physical Agents
Incorporated as 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
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

10. FIPA Standards
Ways of interpreting communications in a way that respects their intended meanings
Communicative acts
Public ontologies
Transport and infrastructure
Superseded by Web services and messaging standards
Programming model and container
Compatible with containers in application servers
Compared to existing approaches
XML Schema standardizes grammar, not meaning
OWL standardizes ontology description, not communication
Chapter 18
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

11. Example Low-Level Patterns
Chapter 18
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

12. Combining Agents with Traditional Web Services
ACL is the FIPA ACL standard
Chapter 18
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

13. Service-Oriented Computing: Semantics, Processes, Agents
August 2004
Contract Net Protocol
An 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
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns
© Singh & Huhns

14. RFQ: Task Announcement
Service-Oriented Computing: Semantics, Processes, Agents
August 2004
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
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns
© Singh & Huhns

15. Service-Oriented Computing: Semantics, Processes, Agents
August 2004
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
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns
© Singh & Huhns

16. Contract Net Evaluated
Service-Oriented Computing: Semantics, Processes, Agents
August 2004
Contract Net Evaluated
Applies best when problem has a well-defined hierarchy of tasks
With coarse-grained decomposition
No interference with each other
Lacks support for specifying service agreements and contracts
Yields robustness: failure can be treated as a manifestation of autonomy
Chapter 18
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns
© Singh & Huhns

17. 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)
Chapter 18
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

18. NetBill Payment Protocol
Checking compliance is easy but the representation is rigid
Some obvious variations are not allowed by the FSM:
The merchant may start the protocol by sending a quote
The customer may send an accept prior to offer
The merchant may send the goods prior to accept
FSM Representation
Chapter 18
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

19. 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 state: No open commitments remain
Chapter 18
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

20. Patterns for Commitment Operations
Ensure that information about commitment operations flows to the right parties
To enable local decisions
Chapter 18
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

21. 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?
Commitment protocols are specified in terms of
Main roles and sphere of commitment
Roles essential for coordination
Domain-specific propositions and actions
Chapter 18
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

22. Verifying Compliance Specification Run-time Verification
Models based on potential causality
Commitments based on branching-time TL
Run-time Verification
Respects design autonomy
Uses TL model-checking
Local verification from observed messages: each party checks the others behavior
Chapter 18
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

23. Run-Time Compliance Checking
An agent maintains
Pending commitments of which it is debtor or creditor
A local model of 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
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

24. Fish-Market Sample Execution
Based on a vector clock
Chapter 18
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

25. Fish-Market Local Observations
The discharge of a commitment must
be reachable from its create
Chapter 18
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

26. Fish-Market Compliance
Auctioneer can verify if the bidders comply
An individual bidder cannot verify if the auctioneer complies
If bidders pool their observations, then they can verify if the auctioneer complies
Asymmetry indicates need for third party
Chapter 18
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

27. Service-Oriented Computing: Semantics, Processes, Agents
August 2004
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
Chapter 18
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns
© Singh & Huhns