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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 Speech Act Theory Semantics Interaction Patterns Combining ACLs with Web Services

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chapter 18 communication
Chapter 18:Communication

Service-Oriented Computing: Semantics, Processes, Agents– Munindar P. Singh and Michael N. Huhns, Wiley, 2005

highlights of this chapter
Highlights of this Chapter
  • Agent Communication Languages
    • Speech Act Theory
    • Semantics
    • Interaction Patterns
  • Combining ACLs with Web Services
  • Contract Net Protocol
  • Business Protocols
    • Commitments
    • Compliance

Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

interaction and communication
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 (for signaling) 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

Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

informing
Informing

How can one agent tell another agent something?

  • Send the information explicitly 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")

Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

querying
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")

Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

syntax semantics pragmatics
Syntax, Semantics, Pragmatics

Consider communication as synonymous with 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: is usually context-sensitive
    • 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

Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

a classification of message classifications for soc
A Classification of Message Classifications for SOC
  • 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

Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

speech act theory
Speech Act Theory

Speech act theory, developed for natural language, views communication as action

  • Differs from traditional logic
  • 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

Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

speech act theory applied
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 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

Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

acl semantics
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 the content of that promise come true
  • Public: semantics depends on laws, protocols, and observable behavior
    • An agent promises something if it says so in the appropriate circumstances
  • Evaluation: For open systems, public semantics is appropriate, because a semantics without compliance doesn’t make sense

Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

fipa 1
FIPA: 1

FIPA is the Foundation for Intelligent Physical Agents, with website at www.fipa.org

  • Recently moving into the IEEE
  • 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.

Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

fipa 2
FIPA: 2
  • 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, logic, and public ontologies, offer standard ways of interpreting communication between agents in a way that respects the intended meaning of the communication
    • Much more ambitious than XML Schema, which only aims to standardize the syntax of documents
    • More specific than even OWL, which provides a way to express ontologies but does not standardize ways of communication

Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

agent message transport
Agent Message Transport
  • Agent Message Transport (AMT) defines a message as an envelope plus a body
    • Guidelines for various transport protocols (e.g., IIOP, HTTP, WAP)
    • Message envelope formatting (e.g., XML for HTTP, bit-efficient for WAP)
    • FIPA ACL representions (e.g., string encoding, XML encoding, bit-efficient encoding)

Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

patterns and protocols
Patterns and Protocols

Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

combining agents with traditional web services
Combining Agents with Traditional Web Services

Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

the contract net protocol
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

Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

task announcement message
Task Announcement Message
  • Eligibility specification: criteria that a contractor 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

Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

bid and award messages
Bid and Award Messages
  • A bid consists of a agent 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

Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

applicability of contract net
Applicability of Contract Net

The Contract Net is a

  • High-level communication protocol
  • Way of distributing tasks
  • Means of self-organization for a group of agents

Best used when the

  • Application has a well-defined hierarchy of tasks
  • Problem has a coarse-grained decomposition
  • Subtasks minimally interact with each other, but positively when they do interact

Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

commitment protocols
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)

Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

example netbill protocol
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

FSMs simplify compliance checking but are rigid

FSM Representation

Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

netbill enhanced with commitments
NetBill Enhanced with Commitments

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

Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

compliance with protocols
Compliance with Protocols

Compliance means all commitments are taken care of (discharged directly or indirectly)

  • How can we 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

Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

verifying compliance with commitment protocols
Verifying Compliance with Commitment Protocols
  • Specification
    • Models based on potential causality
    • Commitments based on branching-time TL
  • Run-time Verification
    • Respects heterogeneity
    • Uses model-checking based on local observations
    • Local verification based on observed messages

Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

run time compliance checking
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

Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

vector timestamps
Vector Timestamps
  • No global clock
  • Each agent maintains a local clock, which it increments whenever
    • A message is sent
    • A message is received
    • A local computation takes place
  • Vector clock = vector of local clocks; different vectors are maintained by different agents, up to date only for the agent’s own component of the vector
  • A message that is sent carries the vector clock as known to the sender when the message is sent
  • A recipient of a message updates its vector to the element-wise max of the vector on the message and its prior vector

Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

fish market sample execution
Fish-Market Sample Execution

Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

fish market local observations
Fish-Market Local Observations

Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

fish market compliance
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

Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

message patterns for commitment operations
Message Patterns for Commitment Operations
  • For efficient checking, ensure that the discharge of a commitment is reachable from its create (discharge has a greater vector timestamp than create)
  • That is, ensure that information about commitment operations flows to the right parties
  • The patterns below accomplish this by sending extra messages for delegate (add message x to y) and assign (add message y to z)

Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

chapter 18 summary
Chapter 18 Summary
  • Communication is the key kind of interaction
  • Speech act theory provides a basis for understanding communication that steps out of traditional logic
  • Protocols capture important patterns of communications
  • Business protocols can be understood using commitments
    • Modeling captures their meaningful content, not just message tokens
    • Commitments provide a basis for compliance

Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

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