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

Chapter 182Service-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 183Service-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 184Service-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 185Service-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 186Service-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 187Service-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 188Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns ACL Semantics What 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 189Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns FIPA FIPA 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 1810Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns FIPA Standards Support 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 1811Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

Chapter 1812Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns 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 1813Service-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 1814Service-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 1815Service-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 1816Service-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 1817Service-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

Chapter 1818Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns NetBill using Commitments Meanings: 1. true 2. request 3. offer 4. C m,c goods  accept  promiseReceipt 5. goods  C c,m pay  promiseReceipt 6. goods  pay  C m,c receipt 7. goods  pay  receipt 8. goods  promiseReceipt 9. accept Final states: all detached commitments are discharged

Chapter 1819Service-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

Chapter 1820Service-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 C y,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 1821Service-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 1822Service-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 1823Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns Fish-Market Sample Execution Based on a vector clock

Chapter 1824Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns Fish-Market Local Observations The discharge of a commitment must be reachable from its create

Chapter 1825Service-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 1826Service-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