A Framework for Contractual Resource Sharing in

Coalitions

Babak Sadighi Firozabai (SICS), Marek Sergot (IC),

Anna Squicciarini (UoM) and Elisa Bertino (UoM)

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Overview Coalition: heterogeneous and independently managed

enterprises collaborating to achieve a common goal

Sharing resources may be part of the collaboration

No single line of authority dictating how resources can be shared among the coalition members

Resource sharing is regulated by a contract between coalition members

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Overview Resources are managed locally and released upon

requests

A coalition policy (contract) specifies obligations of coalition members to share their resources among them

An enterprise is a member of a coalition only if it agrees to follow the coalition contract

A Local policy (or a plan) specifies when and under what conditions a coalition member allows use of its resources by other coalition members

Members publish their local policies within the coalition

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Coalition Policy (Contract) Example:

1. Each member of the coalition is obliged to provide 10GB of disc-space, for two hours a day, to all other members of the coalition.

2. Enterprise E1 is obliged towards E2 to provide access to application A

up to 3 hours, each working day.

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Local Policy of E1

Example:1. 10GB of disc-space will

be provided to other members but only between 8-10 pm.

2. Employees of E2 are entitled to access application A for four hours starting at any time during working hours.

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OverviewTwo independent enterprises E1 and E2

R R R R RR

RRR R R

RC C

Coalition Policy

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Overview Access Request Procedure

R R R R RR

RRR R R

RC C

Coalition Policy

Request

Request

Check

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Overview Each party must publish a local policy complying with

the coalition contract

Parties are not, in practice, forced to follow their local policies (plans)

A party may choose to not grant a request although it has promised it in its local policy

An enterprise may be member of several coalitions and as the result it may have promised access to its resources more than what it can answer

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Overview Obligation as a deontic concept

Obligations can be violated

Entitlement as strong permission If X is entitled to access A, then Y (the controller of A) is

obliged to grant X’s request for accessing A Denying an entitled request is regarded as violation of

corresponding obligation

Publishing an entitlement in a local policy is regarded as an obligation (a promise) by the publisher to the entitled agent

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Overview Local policies may change

A coalition member may change its promises as time passes based on how its resources are actually used

Changing local policy is a way of scheduling and optimising resource usage

Contracts do not change In current version of the framework we do not allow

changes of contractual terms during coalition life-time

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The Framework Coalition elements

Set of resources shared among coalition members

Set of users of the coalition resources

Finite set of enterprises that are coalition members

Set of policies of a coalition, namely the coalition policy (the contract) and the set of local policies of the members (LPt

Ei) Two basic policy components: Obligation and entitlement

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The Framework Resources

A generic specification of resources based on the following metrics

Time (e.g. (22-09-2003,13:54:23) Scalar (some suitable numeric value) Max limit – Min limit (limits on the given metrics)

Generic resource comparison R1 is-less-than R2. R1 and R2 must be of the same type.

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The Framework Obl(P,S,R,I) where

P is the enterprise that is the bearer of the obligation

S is the set of enterprises to whom the obligation is owed

R is the specified amount of a resource

I = [ts, te] is the validity time-period of the obligation

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The Framework EntA(E, R, I)

A is the issuer of the entitlement E is the set of entitled enterprises R is the promised resource by A I is the validity time-period of the entitlement

An entitlement is regarded as an obligation of A to grant access to R upon requests from employees of enterprises E

Promises are obligations which means that they may get violated

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The Framework Local Policy (LP) is a finite set of entitlements

A local policy is in compliance with (meets) an obligation at time t

LPP meets Obl(P, S, R, I) iff There is a set of entitlements that covers I and R, and t I

At each time point of a coalition life-time, each coalition member must have (published) a local policy that meets its obligations stated in the coalition contract.

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The Framework Supported request

Req(u, P, R, t) is supported if there is an Obl(P, S, R’, I) if t I, and u is an employee of an enterprise belonging to S, and R is-less-than R’

Locally supported request Req(u, P, R, t) is supported by a local policy if there is an

entitlement supporting it

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The Framework An obligation Obl(P,S,R,I) is violated at time t I if

1. there is no local policy that meets it at time t, or2. a request supported by it is not granted.

Otherwise the obligation is fulfilled if t is after I

An obligation is fulfilled if no supported request is submitted

”use-it-or-loose-it” principle

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The Framework Contract

Contract blocks Obligation sequences

Obligations

Obligation sequence Obl(P, S, (R1, I1); (R2, I2);…; (Rn, In)) R1,R2,…,Rn are all of the same type I1,I2,…In is an ordered sequence of contiguous time

An obligation sequence is violated at time t if any of its constituent obligations is violated at t.

A local policy is in compliance with an obligation sequence at time t if every constituent obligation is either fulfilled at t or the local policy meets it at time t.

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The Framework A contract block is an expression of the form:

obl(P, S1; Seq1) || … || obl(P, Sn; Seqn)

P is the bearer of the obligations in the block

A contract block is fulfilled at time t iff any of its obligation sequence is fulfilled at time t.

A contract block is violated at time t if all its obligation sequences are violated at t.

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The Framework The idea of contract block is to give a free choice to the

bearer about which of the obligation sequences it wants to fulfil.

In practice, contract blocks can be used to specify obligations which violation of one results in activation of a more stricter obligation to be fulfilled.

Contract blocks are the way we define violation punishments and sanctioning mechanisms in our framework.

Note that an enterprise choose among the obligation sequences of a contract block by specifying an entitlement in its local policy that meets that specific obligation sequence.

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The Framework Finally:

A contract is a finite set of contract blocks.

A contract is fulfilled at time t if all its contract blocks are fulfilled at time t.

A contract is violated at time t if any of its contract blocks is violated at time t.

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Future work An architecture for contractual performance monitoring

A more elaborated resource description language

Use of the framework for resource optimisation and automatic resource-usage scheduling

Mechanisms to decide which obligations to fulfil in order to maximize resource usage and minimize sanctioning costs

Relations to other ongoing work on contracts and service level agreements such as Web Service Level Agreement (WSLA)

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Questions?