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 when agents exist and act in close proximity:
resource contention, e.g., bumping into each other
Communications are the interactions that preserve autonomy of all participants, but maybe realized through physical actions that don’t:
Communications are inevitable when composing services.
Communications can be realized in several ways, e.g.,
through shared memory
because of shared conventions
by messaging passing
Chapter 18
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© Singh & Huhns

4. 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
prohibitive is different from directive, despite syntactic similarity
Use-based (pragmatic)
distinguish messages based on their roles in specific classes of protocols
assertion is different from acknowledgment
Chapter 18
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns
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5. Service-Oriented Computing: Semantics, Processes, Agents
August 2004
Speech Act Theory
Speech act theory, developed for natural language, views communication as action
It considers three aspects of a message:
Locution, or how it is phrased, e.g., "It is hot here" or "Turn on the air conditioner"
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
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
Here is an interesting example of speech act theory
Chapter 18
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns
© Singh & Huhns

6. Speech Act Theory Applied
Service-Oriented Computing: Semantics, Processes, Agents
August 2004
Speech Act Theory Applied
Classifications of illocutions motivate message types, but are typically designed for natural language
rely on NL syntax, e.g., they conflate directives and prohibitives
Most research in speech act theory is about determining the agents’ beliefs and intentions, e.g., how locutions map to illocutions
For Web Services,
determining the message type is trivial, because it is explicitly encoded
determining the 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

7. Service-Oriented Computing: Semantics, Processes, Agents
August 2004
ACL Semantics
What is the semantics of queries, requests, promises?
Mentalist: each agent has a knowledge base that its messages refer to. An agent promises something if it intended to make that promise
Public: semantics depends on laws, protocols, and observable behavior
Evaluation: For open systems, public semantics is appropriate, because a semantics without compliance doesn’t make sense
Chapter 18
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns
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8. Service-Oriented Computing: Semantics, Processes, Agents
August 2004
Informing
How can one agent tell another agent something?
Send the information in a message (message passing)
Write the information in a location where the other agent is likely to look (shared memory)
Show or demonstrate to the other agent (teaching)
Insert or program the information directly into the other agent (master --> slave; controller --> controllee; "brain surgery")
Chapter 18
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9. Service-Oriented Computing: Semantics, Processes, Agents
August 2004
Querying
How can one agent get information from another agent?
Ask the other agent a question (message passing)
Read a location where the other agent is likely to write something (shared memory)
Observe the other agent (learning)
Access the information directly from the other agent ("brain surgery")
Chapter 18
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns
© Singh & Huhns

10. Syntax, Semantics, Pragmatics
Service-Oriented Computing: Semantics, Processes, Agents
August 2004
Syntax, Semantics, Pragmatics
For message passing
Syntax: requires a common language to represent information and queries, or languages that are intertranslatable
Semantics: requires a structured vocabulary and a shared framework of knowledge-a shared ontology
Pragmatics:
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

11. Agent Communication Languages (ACL)
Service-Oriented Computing: Semantics, Processes, Agents
August 2004
Agent Communication Languages (ACL)
KQML: Knowledge Query and Manipulation Language
FIPA ACL
ACL
ACL
Application
Program
Agent
Agent
Chapter 18
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns
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12. Service-Oriented Computing: Semantics, Processes, Agents
August 2004
FIPA
FIPA is the Foundation for Intelligent Physical Agents, with website at
Specifies standards for heterogeneous, interoperating agent-based systems.
Concerned with agency as it relates to
Autonomy (goal-driven)
Communal integration; mostly communication, but also cooperation.
Chapter 18
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns
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13. Service-Oriented Computing: Semantics, Processes, Agents
August 2004
FIPA (contd.)
Mission Statement: The promotion of technologies and interoperability specifications that facilitate the end-to-end interworking of intelligent agent systems in modern commercial and industrial settings.
Goal: through a combination of speech acts, predicate logic and public ontologies, FIPA can offer standard ways of interpreting communication between agents in a way that respects the intended meaning of the communication. This is much more ambitious than, for example, XML, which only aims to standardize the syntactic structure of documents.
Chapter 18
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns
© Singh & Huhns

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

15. Agent Message Transport
Service-Oriented Computing: Semantics, Processes, Agents
August 2004
Agent Message Transport
Agent Message Transport (AMT) defines a message as an envelope plus a body. Together they handle
Guidelines for various transport protocols (e.g., IIOP, HTTP, WAP)
Message envelope representation (e.g., XML for HTTP, bit-efficient for WAP).
FIPA ACL representions (e.g., string encoding, XML encoding, bit-efficient encoding).
Chapter 18
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns
© Singh & Huhns

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

17. Combining Agents with Traditional Web Services
Service-Oriented Computing: Semantics, Processes, Agents
August 2004
Combining Agents with Traditional Web Services
Chapter 18
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns
© Singh & Huhns

18. Planning and Control Needed
Service-Oriented Computing: Semantics, Processes, Agents
August 2004
Planning and Control Needed
How to maintain global coherence without explicit global control? Important aspects include how to
Determine shared goals
Determine common tasks across services
Avoid unnecessary conflicts
Pool knowledge and evidence
Chapter 18
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns
© Singh & Huhns

19. Service-Oriented Computing: Semantics, Processes, Agents
August 2004
Coordination
A property of interaction among a set of agents performing some activity in a shared state. The degree of coordination is the extent to which they
Avoid extraneous activity
reduce resource contention
avoid livelock
Avoid deadlock
Maintain safety conditions
Cooperation is coordination among nonantagonistic agents. Typically,
Each agent must maintain a model of the other agents
Each agent must develop a model of future interactions
Chapter 18
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns
© Singh & Huhns

20. Goals for Multiagent Control
Service-Oriented Computing: Semantics, Processes, Agents
August 2004
Goals for Multiagent Control
Develop techniques for
Locating and allocating capabilities and resources that are dispersed in the environment
Predicting, avoiding, or resolving contentions over capabilities and resources
Mediating among more agents, with more heterogeneity and more complex interactions
Maintaining stability, coherence, and effectiveness
Chapter 18
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns
© Singh & Huhns

21. Service-Oriented Computing: Semantics, Processes, Agents
August 2004
Control Challenges
The difficulty of control depends on some major characteristics of the overall system, including:
agents comprising the system
problems being solved by the agents individually and/or collectively
critical solution characteristics
Chapter 18
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns
© Singh & Huhns

22. Control Challenges: Agents
Service-Oriented Computing: Semantics, Processes, Agents
August 2004
Control Challenges: Agents
Control gets harder as agents become more:
numerous
complex individually (e.g., more versatile)
heterogeneous (in capabilities, representations, languages)
quantity
complexity
heterogeneity
Chapter 18
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns
© Singh & Huhns

23. Control Challenges: Problems
Service-Oriented Computing: Semantics, Processes, Agents
August 2004
Control Challenges: Problems
Control gets harder as the problems agents solve become more
interrelated
rapidly changing, or pursued in an uncertain and changing world
unforgiving of control failures (e.g., involving irreversible actions)
degree of interaction
volatility
severity of failure
Chapter 18
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns
© Singh & Huhns

24. Control Challenges: Solutions
Service-Oriented Computing: Semantics, Processes, Agents
August 2004
Control Challenges: Solutions
Control gets harder as solutions to agent problems are required to be more
efficient
robust to changing circumstances
cheaper or faster to develop individually and in concert
quality / efficiency
robustness
low overhead
Chapter 18
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns
© Singh & Huhns

25. Technologies for Agent Control
Service-Oriented Computing: Semantics, Processes, Agents
August 2004
Technologies for Agent Control
Broker-based
Matchmaker-based
Market-based; auctions
BDI and commitment based
Decision theoretic
Workflow (procedural) based
Standard operating procedures
Learning / adaptive
Coordinated planning
Conventions / protocols
Stochastic or physics-based
Organizations: teams and coalitions
Constraint satisfaction/ optimization
Chapter 18
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns
© Singh & Huhns

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

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

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

29. Service-Oriented Computing: Semantics, Processes, Agents
August 2004
Task Decomposition
Divide-and-conquer to reduce complexity: smaller subtasks require less capable agents and fewer resources
Task decomposition must consider the resources and capabilities of the agents, and potential conflicts among tasks and agents
The designer decides what operators to decompose over
The system decides among alternative decompositions, if available
Chapter 18
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns
© Singh & Huhns

30. Task Decomposition Methods
Service-Oriented Computing: Semantics, Processes, Agents
August 2004
Task Decomposition Methods
Inherent (free!): the representation of the problem contains its decomposition, as in an AND-OR graph
Composition designer (human does it): decomposition is programmed during implementation. (There are few principles for automatically decomposing tasks)
Hierarchical planning (agents implementing the services do it): decomposition again depends heavily on task and operator representation
Chapter 18
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns
© Singh & Huhns

31. Task Decomposition Examples
Service-Oriented Computing: Semantics, Processes, Agents
August 2004
Task Decomposition Examples
Spatial decomposition: service is info source or decision point:
Functional decomposition: service offers specialized expertise:
Agent 1
Agent 3
Agent 2
Pediatrician
Neurologist
Internist
Cardiologist
Psychologist
Chapter 18
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns
© Singh & Huhns

32. Task Distribution Criteria
Service-Oriented Computing: Semantics, Processes, Agents
August 2004
Task Distribution Criteria
Avoid overloading critical resources
Engage services on tasks where expertise matches
Make an agent with a wide view assign tasks to other agents
Assign overlapping responsibilities to agents to achieve coherence
Assign highly interdependent tasks to agents in spatial or semantic proximity. This minimizes communication and synchronization costs
Reassign tasks if necessary for completing urgent tasks
Chapter 18
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns
© Singh & Huhns

33. Task Distribution Mechanisms
Service-Oriented Computing: Semantics, Processes, Agents
August 2004
Task Distribution Mechanisms
Market mechanisms: tasks are matched to agents by generalized agreement or mutual selection (analogous to pricing commodities)
Contract net: announce, bid, and award cycles
Multiagent planning: planning agents have the responsibility for task assignment
Organizational structure: agents have fixed responsibilities for particular tasks
Recursive allocation: responsible agent may further decompose task and allocate the resultant subtasks
Chapter 18
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns
© Singh & Huhns

34. The Contract Net Protocol
Service-Oriented Computing: Semantics, Processes, Agents
August 2004
The Contract Net Protocol
An important generic protocol
Manager announces tasks via a (possibly selective) multicast
Agents evaluate the announcement. Some submit bids
Manager awards a contract to the most appropriate agent
Manager and contractor communicate privately as necessary
Chapter 18
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns
© Singh & Huhns

35. Task Announcement Message
Service-Oriented Computing: Semantics, Processes, Agents
August 2004
Task Announcement Message
Eligibility specification: criteria that a node must meet to be eligible to submit a bid
Task abstraction: a brief description of the task to be executed
Bid specification: a description of the expected format of the bid
Expiration time: a statement of the time interval during which the task announcement is valid
Chapter 18
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns
© Singh & Huhns

36. Service-Oriented Computing: Semantics, Processes, Agents
August 2004
Bid and Award Messages
A bid consists of a node abstraction—a brief specification of the agent’s capabilities that are relevant to the task
An award consists of a task specification—the complete specification of the task
Chapter 18
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns
© Singh & Huhns

37. Applicability of Contract Net
Service-Oriented Computing: Semantics, Processes, Agents
August 2004
Applicability of Contract Net
The Contract Net is
a high-level communication protocol
a way of distributing tasks
a means of self-organization for a group of agents
Best used when
the application has a well-defined hierarchy of tasks
the problem has a coarse-grained decomposition
the subtasks minimally interact with each other, but cooperate when they do
Chapter 18
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns
© Singh & Huhns

38. JADE Behavior for CNP Initiator Role
Service-Oriented Computing: Semantics, Processes, Agents
August 2004
JADE Behavior for CNP Initiator Role
Chapter 18
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns
© Singh & Huhns

39. Service-Oriented Computing: Semantics, Processes, Agents
August 2004
Commitment Protocols
Protocols enable open systems to be constructed
Interaction 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 (107 request-response PIPs)
Chapter 18
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns
© Singh & Huhns

40. Example: NetBill Protocol
Service-Oriented Computing: Semantics, Processes, Agents
August 2004
Example: NetBill Protocol
Some variations:
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
These variations are not allowed by the FSM
FSM Representation
Chapter 18
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns
© Singh & Huhns

41. NetBill Enhanced by CMs
Service-Oriented Computing: Semantics, Processes, Agents
August 2004
NetBill Enhanced by CMs
Meanings:
1. true
2. request
3. offer
4. Cmgoods  accept  promiseReceipt
5. goods  Ccpay  promiseReceipt
6. goods  pay  Cmreceipt
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
© Singh & Huhns

42. Message Patterns for Commitment Operations
Service-Oriented Computing: Semantics, Processes, Agents
August 2004
Message 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
© Singh & Huhns

43. Compliance with Protocols
Service-Oriented Computing: Semantics, Processes, Agents
August 2004
Compliance with Protocols
In an open environment, agents are contributed by different vendors and serve different interests
How can an application check if the agents comply with specified protocols?
Coordination aspects: traditional techniques
Commitment aspects: representations of the agents’ commitments in temporal logic
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
© Singh & Huhns

44. Verifying Compliance with Commitment Protocols
Service-Oriented Computing: Semantics, Processes, Agents
August 2004
Verifying Compliance with Commitment Protocols
Specification
models based on potential causality
commitments based on branching-time TL
Run-time Verification
respects design autonomy
uses TL model-checking
local verification based on observed messages
Chapter 18
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns
© Singh & Huhns

45. Run-Time Compliance Checking
Service-Oriented Computing: Semantics, Processes, Agents
August 2004
Run-Time Compliance Checking
An agent can keep track of
its pending commitments
commitments made by others that are not satisfied
It uses this local model to see if a commitment has been violated
An agent who benefits from a commitment can always determine if it was violated
Chapter 18
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns
© Singh & Huhns

46. Fish-Market Sample Execution
Service-Oriented Computing: Semantics, Processes, Agents
August 2004
Fish-Market Sample Execution
Chapter 18
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns
© Singh & Huhns

47. Fish-Market Local Observations
Service-Oriented Computing: Semantics, Processes, Agents
August 2004
Fish-Market Local Observations
Chapter 18
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns
© Singh & Huhns

48. Fish-Market Compliance
Service-Oriented Computing: Semantics, Processes, Agents
August 2004
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
© Singh & Huhns

49. Policies and Structure
Service-Oriented Computing: Semantics, Processes, Agents
August 2004
Policies and Structure
Spheres of commitment (SoComs)
abstract specifications of societies
made concrete prior to execution
Policies apply on performing social actions
Policies relate to the nesting of SoComs
Role conflicts can occur when agents play multiple roles, e.g., because of nonunique nesting
- Munindar Singh and Michael Huhns
© Singh & Huhns

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