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Transcript of L 9 : Collaborations Why? Terminology Coherence Coordination Reference s :...
L 9 : CollaborationsL 9 : Collaborations
• Why?
• Terminology
• Coherence
• Coordination
Reference s :
http://www.csd.abdn.ac.uk/~mkolling/publications/DesignMultiAgentCoordControlStigmergy.pdf
Why is coordination needed?
• Dependencies between agents actions
• Handle global constraints– E.g. Time, Money, or computational resources
• No individual have sufficient competence or resources to solve the entire problem alone
Coordination is needed when..
• Building a house– Dependencies between agents actions (e.g. electricians and
carpenters)
• Host a big sports arrangement, e.g. Olympics– No individuals have sufficient competence or resources to
solve the entire problem alone
• Allocating lecture rooms to courses at a university– Handle global constraints (limited number of rooms)
Terminology of Coordination - I
• Coordination– An agents reasoning about its own and the anticipated
behavior of other agents in order to ensure that the community acts in a coherent manner
• Coherence– Agents actions can be performed and that they do not
conflict with one another
Terminology of Coordination - II
• Convention– A means of monitoring commitments in changing
circumstances
• Commitment– An agents pledges to undertake a specified course of action
How to reach coherent behavior?
• Complete knowledge– All agents know everything about each other at all times
• Centralized Control– One or a few agents gather and distribute information about
other agents
• Distributed Control– Agents know about themselves and some or only
expectations about other agents
Degrees of control in Coordination - I
1. Explicit Central Control– Procedure calls; master/slave (centralized, little adaption)
2. Explicit Synchronization and Communication– Semaphores, Monitors (operating system level), semi-
centralized (resource-competing agents/threads)
3. Shared-Data Abstraction– Common Language Constructions, concurrent prolog
Degrees of control in Coordination - II
4. Functionally Accurate Cooperative Approach– Asynchronous Control, Adaptive to changes in semantics
and temporal uncertainty (unprecise information)
5. Reasoned Control– Agents use their own and other agents’ knowledge to
build/revise coordination frameworks (decentralized)
6. Evolving Interactions– Agents evolve during interaction (~ evolutionary
computation, e.g. Genetic Algorithms), no shared semantics, fully adaptive
Behavioristic Coordination Approach
1. Internal (micro-level, inside agent)
2. External (macro-level, behavioristic, multi-agent system)
... But when selecting behavioristic perspective– Agents’ actions may be incoherent even if trying to coordinate
Example: 2 robots in appearantly different directions
– Agents’ actions may be coherent even if uncoordinated Example: people running to shelter when raining
Need to consider internal aspects of agents (belief, desires,..)
Coordination as a distributed Goal-Search Problem
Agent 1 Agent 2
O verall G oal 1 O verall G oal 2
Subgoal 2Subgoal 1 Subgoal 3
Sub..Subgoal n
AND OR
Dependencies – Weak and Strong
Agent 1 Agent 2
O verall G oal 1 O verall G oal 2
Subgoal 2Subgoal 1 Subgoal 3
Sub..Subgoal n
W EAKSTRONGSTRONG
W EAK
Dependencies – Uni/Bi-directional
Agent 1 Agent 2
O verall G oal 1 O verall G oal 2
Subgoal 2Subgoal 1 Subgoal 3
Sub..Subgoal n
UNIDIR.UNIDIR.
BIDIR.
Joint Goals
• Joint Goals basis of Joint Actions
• Joint actions– Sophisticated form of cooperation in which a team of agents
decide to pursue a common goal in a cooperative manner
• Properties of joint actions:1. Mutual responsibility
– e.g. Legal
2. Joint Commitment– Pulling in the same direction
3. Mutual Supportive– Backing up each other
Joint Commitments
• Commitment with distributed state– Several agents participate
• Main Problems with joint commitment– Hard to be aware of each others states at all times
Divergating beliefs
Conventions
• Commitments may change over time– Due to changed environment
• Need conventions to describe when to change commitments
1. When to keep commitment (retain)
2. When to revise commitment (rectify)
3. When to remove commitment (abandon)
Example Convention
• Reasons for Re-Assessing Commitment:– Commitment Satisfied– Commitment Unattainable– Motivation for commitment no longer present
• Actions:– R1:
If Commitment Satisfied OR
Commitment Unattainable OR
Motivation for Commitment no longer present
Drop Commitment
Joint Action Example
• Invoke when:– Status of Commitment to Joint Goal changes– Status of Commitment to Attaining Joint Action in
present team changes– Status of Joint Commitment of Team Member Changes
• Actions:R1: If status of Commitment to Joint Action changes OR
Status of Commitment to Present Team Context ChangesInform all team members of change
R2: If status of Joint Commitment of a team member changes
Determine Whether to Continue Joint Commitment
Ingredients in Good Coordination
1. Have structures that enable agents to interact in predictable ways
2. Have flexibility allowing partial and imprecise viewpoint of the community (for each agent)
3. Agents must have sufficient knowledge and reasoning capabilities for exploit structure and flexibility
Coordination ”in a nutshell”
• Coordination =
Commitments +
Conventions +
Social Conventions +
Local Reasoning
Commitment Types
• Goal-Subgoal Relationships– Hierarchical, AND/OR goals
• Goal-Dependency Relationships– Weak/Strong dependencies
– Uni/Bi-directional
Social Convention Types
• Goal-Subgoal Relationships– Believing that each individual agent will do their best to
perform their subgoal
• Goal-Dependency Relationships– Monitoring changes in individual commitments in order to
decide actions
• Joint Action Convention, Joint Commitments and Joint Goals
Enable individuals to make assumptions about the actions of other individuals
Common Coordination Techniques
• Organizational Structures – long term– Pattern of information and control relationships between
individuals
• Meta-Level Information Exchange – medium term– Agents giving each other information about their current
priorities and focus
• Multi-Agent planning – short term– Specify a plan with all future actions and interactions with
respect to achieving a particular objective
Performance of Organization Types
• Product Hierarchy– High Production Cost
– Low Coordination Cost
– High Vulnerability Cost
• Functional Hierarchy– Low Production Cost
– Medium Coordination Cost
– Very High Vulnerability Cost
• Centralized Market– Low Production Cost
– High Coordination Cost
– Quite high vulnerability Cost
• Decentralized Market– Low Production Cost
– High Coordination Cost
– Low vulnerability Cost
Meta-level Exchange
• Agents build local plans which they share with other agents in order to identify improvements in coordination
• Medium-term
• Partial Global Planning
Multi-Agent Planning
• Short-term
• Very high-level of detail – Everyone knows everything
• Two approaches– Centralized - coordinator does the planning
– Distributed – agents propose plans to coordinator which does adjustments
Conclusions
• Coordination – four main structures
1. Commitments
• Agree on doing something in a MAS
2. Convention
• When to do something about commitments
3. Social Conventions
• Team-oriented Conventions
4. Local Reasoning
• Intelligence needed to fulfill conventions and commitments
• Related research fields– Self-organization
– CSCW