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

Fundamental Coordination Processes

• Mutual Adjustment

• Direct Supervision

• Standardization

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

Organizational Structures

• Types:– Hierarchical

– Heterarchical

– Flat

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