The Frame Problem in Web Service Specifications

The Frame Problem in Web Service

SpecificationsGeorge Baryannis and Dimitris Plexousakis

[email protected], [email protected]

Computer Science DepartmentUniversity of Crete, Greece

PESOS 2009 WorkshopPrinciples of Engineering Service Oriented Systems

Work partially supported by S-Cube, the European Network of Excellence in Software, Services & Systems

2

The Frame Problem

• Definition• The Frame Problem in the Service-Oriented Architecture

A Motivating Example

• Atomic Services• Service Composition

Addressing the Frame Problem

• Change Axioms• An algorithm for producing Change Axioms

Conclusions & Future Work

Outline

The Frame Problem in Web Service SpecificationsGeorge Baryannis and Dimitris Plexousakis

3

Stating only “what is changed” in a formal specification is inadequate› May lead to inconsistencies› May hinder the ability to prove certain

properties of the specification One should also include clauses which state

explicitly that “nothing else changes”› These clauses are called frame axioms

Stating succinctly these frame axioms has been known in literature as the frame problem

The Frame Problem


4

SOA scenario: an application component in need of a specific service describes this need and tasks a directory service to find matching candidates› Formal descriptions of service functionality

and behavior should be available Without having access to the source code Not limited to interface descriptions

› Describe services in terms of preconditions and postconditions The frame problem may arise when using such

notations

The Frame Problem in the Service-Oriented Architecture


5

Online shop that provides wish list and recommendation functionality› After an order is completed, the purchased

item is removed from the wish list› The item most closely associated to the

purchased one is added to the recommendations list

These functions are offered by two distinct services› Let’s describe them in terms of their

preconditions and postconditions

A Motivating Example (1)


6

Wish List ServicePRE1: completed(order, item)

included(buyersWishList, item)POST1: ¬included(buyersWishList, item)

x, y [x ≠ buyersWishList y ≠ item included(x, y) ≡ included(x, y)] x, y [completed(x, y) ≡ completed(x, y)]

Recommendations List ServicePRE2: completed(order, item)

¬ included(buyersRecoms, associatedItem)POST2: included(buyersRecoms, associatedItem)

x, y [x ≠ buyersRecoms y ≠ associatedItem included(x, y) ≡ included(x, y)] x, y [completed(x, y) ≡ completed(x, y)]

A Motivating Example (2)


7

Both services need to be run after the successful completion of an order› They can be executed in parallel

› Let’s describe the composite service in terms of its preconditions and postconditions

A Motivating Example: Service Composition (1)


Wish List Service

Recoms List

Service

PRE1 PRE2 POST1 POST2

8

Composite ServicePRE1: completed(order, item)

included(buyersWishList, item)PRE2: completed(order, item)

¬ included(buyersRecoms, associatedItem)



9

Composite ServicePRE: completed(order, item) included(buyersWishList, item)




10



POST1: ¬included(buyersWishList, item)

x, y [x ≠ buyersWishList y ≠ item included(x, y) ≡ included(x, y)] x, y [completed(x, y) ≡ completed(x, y)] POST2: included(buyersRecoms, associatedItem)

x, y [x ≠ buyersRecoms y ≠ associatedItem included(x, y) ≡ included(x, y)] x, y [completed(x, y) ≡ completed(x, y)]



11



POST: ¬included(buyersWishList, item) included(buyersRecoms, associatedItem) x, y [x ≠ buyersWishList y ≠ item included(x, y) ≡ included(x, y)] x, y [x ≠ buyersRecoms y ≠ associatedItem included(x, y) ≡ included(x, y)] x, y [completed(x, y) ≡ completed(x, y)]



12

Several solutions have been proposed in the AI literature› Linear deductive planning› Situation calculus and successor state axioms› Explanation closure axioms or change axioms

The change axioms approach is based on first-order predicate logic› Can be adapted in order to work on Semantic

Web Service frameworks such as OWL-S, WSMO and SWSO

Addressing The Frame Problem


13

Frame Axioms: Procedure-oriented perspective› State what predicates each procedure (service)

does not change Change Axioms: State-oriented perspective

› State which procedures (services) change each predicate

› Extend first-order predicate logic with: Special predicate Occur, of arity 1 and special

variable α Occur(α) is true iff the service denoted by variable

a has executed successfully

Expressing Change Axioms (1)


14

Let’s revisit the example



POST: ¬included(buyersWishList, item) included(buyersRecoms, associatedItem) x, y [x ≠ buyersWishList y ≠ item included(x, y) ≡ included(x, y)] x, y [x ≠ buyersRecoms y ≠ associatedItem included(x, y) ≡ included(x, y)] x, y [completed(x, y) ≡ completed(x, y)]



15

Let’s revisit the example



CHANGE: α x, item [included (x, item) ¬ included( x, item) Occur(α)] α = updWishList α = updRecList α order, item [completed(order, item)

¬completed(order, item) Occur(α)] false



16

An Algorithm for producing Change Axioms (1)


Create a list of

participating services

Already checked

?

Predicate used in a

precondition too?

Associated change axiom

already exists?

Negation in either the pre

or the postcondition

(but not in both)?

Create a list of

postconditions

Get next service

Get next postcondition

Yes

No

Negation in either the pre

or the postcondition

(but not in both)?

List End

?

List End

?

No

No

Yes

Yes

No

Yes

No

Yes

No Modify existing change axiom

(add disjunction)

Insert change axiom with

Occur in consequent

Yes


false in consequent

No

Create a list of

preconditions

Already checked

?

Associated change axiom

already exists?

Get next postcondition

List End

?


false in consequent

No

No

No

Yes

YesYes

END

Yes

17

The algorithm has been implemented in Java, for the case of OWL-S process model descriptions with SWRL-FOL rules› OWL-S process model files are read using the CMU

OWL-S API (http://www.semwebcentral.org/projects/owl-s-api/)

› Change axioms are added according to the algorithm

› The system also checks for postconditions that are redundant after the addition of change axioms, and removes them

› The modified model is exported as an OWL-S process model file

An Algorithm for producing Change Axioms (2)


http://www.semwebcentral.org/projects/owl-s-api/

18

The frame problem can apparently cause issues when devising complete Web service specifications

The problem can be addressed using the approach of change axioms Change axioms can be used to provide complete descriptions for

atomic and composite services containing all major composition schemas

An algorithm for the automatic production of change axioms was presented

Future work includes › detecting service compositions which contain conflicts, either in

preconditions or postconditions (including change axioms)› adapting the implementation in order to support WSMO and SWSO

specifications› using the implementation as part of an automatic composition framework,

allowing it to take into account the frame problem when attempting to compose Web services

› Taking indirect effects into account (ramification problem)

Conclusions & Future Work


Documents

The Frame Problem in Web Service Specifications