Towards Realizing Dynamic QoS-aware Web Service Composition

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In this presentation, we identify two major issues related to Web service composition: the lack of formal specifications for services and service compositions and the inability of current service composition approaches to support dynamicity and QoS-awareness in an effective and scalable way. We analyze the underlying research challenges for each of these issues and propose a tentative research plan to address them.

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Towards Realizing Dynamic QoS-aware Web Service Composition

  1. 1. Towards Realizing Dynamic QoS-aware Web Service Composition PhD Symposium, ECOWS ‘10 George Baryannis Supervisor: Dimitris PlexousakisComputer Science Department Institute of Computer Science University of Crete Foundation for Research & Technology - Hellas
  2. 2. Outline Introduction • Research on Web Services • Formal Specification of Web services and service compositions • Automated Web Service Composition Formal Web Service Specifications • Addressing the Frame Problem • The Ramification and Qualification Problems in Web Service Specifications • Automatic Derivation of Composite Specifications Automated Web Service Composition • Requirements for Automated Web Service Composition • Classification and Comparison of Existing Approaches • QoS-Awareness, Dynamicity and Scalability in AI Planning techniques Research Plan & ConclusionsTowards Realizing Dynamic QoS-aware Web Service George Baryannis 2Composition
  3. 3. Research on Web Services• The last decade has seen an exceptional wealth of research on service-related topics• European Union FP6 and FP7 funded projects and networks• Research covers the full life-cycle of a Web service or a Service-based Application: – Service description − Service execution – Service discovery − Service monitoring – Service composition − Service adaptation – Service deploymentTowards Realizing Dynamic QoS-aware Web Service Composition George Baryannis 3
  4. 4. Formal Specification of Web Services• Complete formal specifications are crucial for both – Service providers, to advertise more effectively the offered services – Service consumers, to make informed choices by knowing the exact way in which a service is expected to perform• Automatically processing specifications facilitates faster service matchmaking and composition – Provided that a right balance between formality and tractability is achieved• A series of issues need to be addressed – The frame, ramification and qualification problems – The problem of automatically deriving specifications for service compositionsTowards Realizing Dynamic QoS-aware Web Service Composition George Baryannis 4
  5. 5. Automated Web Service Composition• Service composition is the process of combining and coordinating a set of services in order to achieve functionality unachievable by any single service• Many approaches that involve automation have been proposed, with varying degrees of success• We conducted an extensive comparative literature review based on a series of requirements – No approaches satisfy the complete spectrum of said requirements – AI Planning techniques seem to be more successful• The following issues were identified – Lack of dynamicity in existing AI Planning approaches – Introducing QoS-awareness in AI Planning techniques – Providing a detailed account of how scalability may be achievedTowards Realizing Dynamic QoS-aware Web Service Composition George Baryannis 5
  6. 6. Outline Introduction • Research on Web Services • Formal Specification of Web services and service compositions • Automated Web Service Composition Formal Web Service Specifications • Addressing the Frame Problem • The Ramification and Qualification Problems in Web Service Specifications • Automatic Derivation of Composite Specifications Automated Web Service Composition • Requirements for Automated Web Service Composition • Classification and Comparison of Existing Approaches • QoS-Awareness, Dynamicity and Scalability in AI Planning techniques Research Plan & ConclusionsTowards Realizing Dynamic QoS-aware Web Service George Baryannis 6Composition
  7. 7. The Frame, Ramification and QualificationProblems• A common characteristic of major Web service description frameworks, such as OWL-S and WSMO, is the use of logic-based knowledge representation for the description of service functionality (IOPEs)• This makes them vulnerable to three well-known problems – The Frame Problem: how to state, in a concise way, that nothing else changes, except when stated otherwise – The Ramification Problem: how to represent and infer information about the knock-on and indirect effects of an action or an event – The Qualification Problem: how to list all preconditions that must be satisfied for an action to have a desired effect and how to update them when new statements become part of our knowledgeTowards Realizing Dynamic QoS-aware Web Service Composition George Baryannis 7
  8. 8. Addressing the Frame Problem• The Frame Problem stems from explicitly stating which predicates or functions each procedure (in our case, service) does not change – Instead, we can state which services change each predicate or function – 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 α has executed successfully• This solution was successfully applied to OWL-S service descriptions – An algorithm was implemented, to automatically transform existing service descriptions accordingly• No effort has been made to address the qualification and ramification problems in the same contextTowards Realizing Dynamic QoS-aware Web Service Composition George Baryannis 8
  9. 9. The Ramification Problemin Web Service Specifications (1)• We want to create a specification for a money withdrawal service: – The balance of the account should at least be equal to the amount of money about to be withdrawn. Also, the credit card used for the withdrawal must be valid. – There is a daily limit for the amount of money an account holder can withdraw in a day. – A successful execution means that the new balance is equal to the old one minus the amount of money withdrawn. – Also, the total money withdrawn for the day should be increased by the same amount.Towards Realizing Dynamic QoS-aware Web Service Composition George Baryannis 9
  10. 10. The Ramification Problemin Web Service Specifications (2)• As it has been shown in previous work, such a specification is incomplete due to the frame problem – A representation that handles the frame problem is shown below• Suppose that a ramification of the increase in the total amount withdrawn from the account due to the execution of the service is that if the daily limit is surpassed, the credit card must be temporarily made invalid.Towards Realizing Dynamic QoS-aware Web Service Composition George Baryannis 10
  11. 11. The Ramification Problemin Web Service Specifications (3)• The lack of knowledge of an indirect effect may lead to the assumption that a composition is valid and correct while that particular effect may contradict a precondition of another participating service• The specification below precludes any indirect effects. How do we include the previously mentioned ramification?Towards Realizing Dynamic QoS-aware Web Service Composition George Baryannis 11
  12. 12. The Qualification Problemin Web Service Specifications• Suppose that we are told that there is an initial period after the initial activation of a credit card when it cannot yet be used for purchases (i.e. no money can be withdrawn) in other words a card cannot be valid and at the same time be in a provisional state.• How can this new knowledge, that may be expressed as be assimilated in the existing specification? What new precondition must be inferred? What if this leads to an inconsistent specification (especially in the case of composite services)?Towards Realizing Dynamic QoS-aware Web Service Composition George Baryannis 12
  13. 13. Automatic derivationof composite specifications (1)• No service description frameworks or service composition approaches attempt to derive a complete specification of the inputs, outputs, preconditions and effects (IOPEs) that should be provided to a service consumer – Such specifications promote and facilitate the reusability of composite services – They also allow for the detection of inconsistencies to decide whether a set of services is composable• The composite specification is directly linked to the composition schema and the way the participating services are orchestrated – Which part of the participating services’ specifications should be exposed? • The full set of preconditions and effects of all participating services? • Only the preconditions of the services whose inputs are exposed (and the postconditions of the services whose outputs are exposed)?Towards Realizing Dynamic QoS-aware Web Service Composition George Baryannis 13
  14. 14. Automatic derivation of composite specifications (2) • Let’s examine the following simple example: InpI InpI Service I InpA Service A OutA PreI=true PostI=true PreA PostA Service I Service A Service IPreC=PreI=true PostC=PostI=true Service C ??? ??? • The solution should be somewhere in the middle: find the minimal subset of the participating services’ specifications that should form the composite specification – Insight on this matter can be given by programming language specifications, weakest precondition and the Craig-Lyndon interpolation theorem Towards Realizing Dynamic QoS-aware Web Service Composition George Baryannis 14
  15. 15. Outline Introduction • Research on Web Services • Formal Specification of Web services and service compositions • Automated Web Service Composition Formal Web Service Specifications • Addressing the Frame Problem • The Ramification and Qualification Problems in Web Service Specifications • Automatic Derivation of Composite Specifications Automated Web Service Composition • Requirements for Automated Web Service Composition • Classification and Comparison of Existing Approaches • QoS-Awareness, Dynamicity and Scalability in AI Planning techniques Research Plan & ConclusionsTowards Realizing Dynamic QoS-aware Web Service George Baryannis 15Composition
  16. 16. Requirements forAutomated Web Service Composition (1)1. Automation – Minimize user intervention and accelerate the process – Approaches should be at least partially automated2. Dynamicity – Produce an abstract composition schema with service placeholders instead of actual services, which are only bound at run-time – Dynamic schemas can be consistent and executable long after their initial design3. Semantic Capabilities – Allow for more efficient service matchmaking – Semantic descriptions should be exploited, if available4. QoS-Awareness – Composite services that not only provide the required functionality, but also guarantee the best possible quality5. Non-determinism – Support for choice constructs (such as if-then-else) or loops (such as repeat-while) George Baryannis 16
  17. 17. Requirements forAutomated Web Service Composition (2)6. Partial observability – Deal with incomplete (or even wrong) information about the initial world state7. Scalability – Ensure acceptable performance even when the number of candidate services or the complexity of the composition schema rise8. Correctness – Ensure that certain desired properties hold, such as the fact that a certain set of outputs is guaranteed to be produced given a certain set of inputs and conditions.9. Domain independence – The same approach should be applicable to different domains, allowing for the solution of a broad range of problems10. Adaptivity – Adapt to any unexpected or desired changes in order to satisfy new requirements or fit to new situations dictated by the environmentTowards Realizing Dynamic QoS-aware Web Service Composition George Baryannis 17
  18. 18. Classification of Composition Approaches The classification is based on the way service compositions are represented1. Workflow-based approaches – Use workflow languages such as BPEL or YAWL to exploit the fact that a composite service is conceptually similar to a workflow2. Model-based approaches – Employ modeling languages (Petri-nets, UML, FSMs) to represent service compositions3. Mathematics-based approaches – Use logics, calculi or algebras to represent service compositions4. AI Planning – Handle service composition as an AI planning problemTowards Realizing Dynamic QoS-aware Web Service Composition George Baryannis 18
  19. 19. Comparison• Automation, Semantic Capabilities, Correctness and Domain Independence have been well researched, in general• Non-determinism and Partial Observability have only been successfully handled in more recent work in the ASTRO framework• Adaptation is out of the scope of this particular survey• Dynamicity, QoS Awareness and Scalability have not been adequately explored, especially when it comes to service composition as AI planningTowards Realizing Dynamic QoS-aware Web Service Composition George Baryannis 19
  20. 20. QoS-Awareness in AI Planning techniques• QoS-Awareness has been largely ignored in approaches that employ AI planning techniques• Introducing QoS-Awareness involves at least 3 fundamental decisions: 1. Choose the AI planning technique that will be adapted to be QoS-aware 2. Choose the QoS model that will be employed 3. Decide on which phases of the composition process to include QoS characteristics • QoS aspects can be used to speed up the plan generation by pruning the search space (excluding services that don’t meet QoS thresholds) • QoS aspects can be included in the planning goals, although, at first glance, this would complicate the goals and make them harder to satisfyTowards Realizing Dynamic QoS-aware Web Service Composition George Baryannis 20
  21. 21. Dynamicity in AI Planning techniques• The vast majority of AI Planning approaches produce static composition schemas• Only Peer and Klusch include some form of dynamicity. Peer’s approach supports generating new plans but only at design time• OWLS-XPLAN (Klusch et al.) and its replanning module are a step in the right direction – However, it offers no support for nondeterminism and partial observability, QoS-awareness, scalability and any proof of correctness• It is challenging to explore how dynamicity at run-time via replanning can be applied to planning techniques that support most of these features, such as planning as model checkingTowards Realizing Dynamic QoS-aware Web Service Composition George Baryannis 21
  22. 22. Scalability in Automated Web ServiceComposition approaches• While some of the approaches explicitly mention scalability and provide details, the majority does not provide a comprehensive examination of the problem – Any scalability claims should be supported by experimental evaluation• Achieving scalability involves identifying causes of inefficiency and attempting to remove them – This should be done only after evaluating the significance of limiting the approach that way – Trying to achieve scalability may sometimes lead to crippling the overall applicability of the approach.Towards Realizing Dynamic QoS-aware Web Service Composition George Baryannis 22
  23. 23. Outline Introduction • Research on Web Services • Formal Specification of Web services and service compositions • Automated Web Service Composition Formal Web Service Specifications • Addressing the Frame Problem • The Ramification and Qualification Problems in Web Service Specifications • Automatic Derivation of Composite Specifications Automated Web Service Composition • Requirements for Automated Web Service Composition • Classification and Comparison of Existing Approaches • QoS-Awareness, Dynamicity and Scalability in AI Planning techniques Research Plan & ConclusionsTowards Realizing Dynamic QoS-aware Web Service George Baryannis 23Composition
  24. 24. Research Plan (1)• We have identified two major goals for our research plan• The first goal: define and formalize a novel specification language for Web services and service compositions – Taking into account the frame, ramification and qualification problems, similarly to the way Kakas et al. provided Modular-E, an action language that solves these problems by definition – Providing support for the derivation of composite specifications based on a composition schema• Two initial milestones are identified 1. Explore the challenge of automatically deriving composite specifications 2. Identify the cases where the frame, ramification and qualification problems appear and examine the effects they may haveTowards Realizing Dynamic QoS-aware Web Service Composition George Baryannis 24
  25. 25. Research Plan (2)• The second goal: design and implement a dynamic, QoS-aware automated service composition framework – This framework will rely on the results of the first goal • Services will be specified using the language produced in the first goal • The same language will be used to describe the resulting composition• A tentative roadmap for this goal is the following: – Introduce QoS characteristics in the specification language by exploring the work of Kritikos and Plexousakis on OWL- Q – Based on the literature review, select the most successful planning technique (e.g. planning as model checking) and attempt to introduce dynamicity by applying run-time replanning methods.Towards Realizing Dynamic QoS-aware Web Service Composition George Baryannis 25
  26. 26. Conclusions• Two major problems in the fields of service description and composition were identified – The inability of current service description and composition frameworks to provide complete formal service specifications – The lack of an effective Web service composition approach that combines most desired requirements, such as QoS-awareness and dynamicity• Addressing these major issues is crucial for the realization of primary principles of Service-Oriented Architecture such as service reusability, discoverability and composability• Realizing scalability, dynamicity and QoS-awareness in automated Web service composition may prove to be the necessary incentive for the wider adoption of automated techniques – Of course, the eventual adoption of such approaches depends on the availability of semantically-enabled services and effective discovery techniquesTowards Realizing Dynamic QoS-aware Web Service Composition George Baryannis 26
  27. 27. QuestionsTowards Realizing Dynamic QoS-aware Web Service Composition George Baryannis 27
  28. 28. BACKUP SLIDESTowards Realizing DynamicQoS-aware Web Service George Baryannis 28Composition
  29. 29. Composition Models (1)• Orchestration: – a description of how the services that participate in a composition interact at the message level, including the business logic and the order in which the interactions should be executed. – An orchestration should define which message is sent when and by which participating service.• Choreography: – associated with the globally visible message exchanges, rules of interaction and agreements that occur between multiple business processes. – Partners are in full control of their internal business processes and do not expose them to other partners, unless they are essential to the communication• Coordination: – involves temporarily grouping a set of service instances following a coordination protocol. This protocol dictates the way participating services interact and also the outcome of the interaction, whether it was successful or not. – All communication is done through a third party, called the coordinator, who ultimately is responsible for the upholding of the protocol and the decision and dissemination of the outcome, once the activity ends.• Component Model: – involves the actual linking between inputs and outputs when services are being composed together.Towards Realizing Dynamic QoS-aware Web Service Composition George Baryannis 29
  30. 30. Composition Models (2)• Orchestration vs. Composition Synthesis – Composition synthesis concerns synthesizing a specification of how to coordinate the component services to fulfill the client request, generating a plan that dictates how to achieve a desired behavior by combining the abilities of multiple services. – Orchestration is about executing the result of composition synthesis by coordinating the control and data flow among the participating services and also about supervising and monitoring that execution.• Orchestration vs. Choreography: – Multi-party business process execution (in choreographies) vs. creation of a business process executed by a single party (in orchestrations). – In choreographies, the internal processes of partners are not exposed – Although orchestration and choreography can be used separately to implement a service composition, their different viewpoints can be combined for a more complete representation. – Orchestration can be used to describe participating services in the lower abstraction level and choreography can give a higher level description of how these orchestrations interact with each other and capture the complex conversations between them in order to realize the goal set by the requester.Towards Realizing Dynamic QoS-aware Web Service Composition George Baryannis 30
  31. 31. Workflow-based Service Composition: The PAWS framework (Ardagna et al.) Faulty services are replacedConstraints usually by other candidate servicesrefer to QoS aspects and recovery actions areand are expressed as SLAs enabled Towards Realizing Dynamic QoS-aware Web Service George Baryannis 31 Composition
  32. 32. Model-based Service Composition:The SAM system (Brogi and Corfini)• Web services are represented as CPR Nets, a Petri Net variant – OWLS2PNML translates OWL-S service descriptions to CPR Nets described using Petri Net Markup Language (PNML)• A two-step analysis is performed: – Functional: the requester expresses the inputs and outputs of the desired service and SAM attempts to incrementally produce the outputs, starting from the inputs, resulting in a minimal set of participating services – Behavioral: if the request is purely functional, SAM attempts to create a CPR Net that represents the behavior of the composite service and checks if it is correct. If the request contains a behavior represented as a CPR Net, the bisimilarity of the two Petri Nets is checked.Towards Realizing DynamicQoS-aware Web Service George Baryannis 32Composition
  33. 33. Mathematics-based Service Composition:The SPICE ACE engine (Lecue et al.) • Requests are expressed in a formal manner, containing IOPEs, non- functional properties, related ontological schemas and so on. • The composite services returned by the Composition Factory are ranked based on aggregate non-functional property values and/or semantic similarity between the requested and the produced composite serviceTowards Realizing DynamicQoS-aware Web Service George Baryannis 33Composition
  34. 34. Mathematics-based Service Composition:The SPICE ACE engine (Lecue et al.) • The authors exploit the fact that OWL-S is based on Description Logics and attempt to compute similarities between such DL descriptions • Similarities are expressed as causal links (exact, plugin, subsumption, intersection and disjoint, and a matrix of all possible links is constructed • The causal link matrix is extended with non-functional properties (CLM+) • Using the CLM+ the output is attempted to be achieved incrementally starting from the input.Towards Realizing DynamicQoS-aware Web Service George Baryannis 34Composition
  35. 35. Service Composition as AI Planning Messages Initial set sent and of inputs • Given: received – S, the set of all possible world states, with s0 the initial state set of Final Available – G, the set of goal states we are attempting to reach outputs serviceoperations – A, the set of all possible actions that can be performed – Γ, a transition relation that describes the resulting state, when an Internal process action is executed at a given statedescription • Then AI planning (in the classical sense) is the problem of reaching G, starting from s0 by executing actions ∈ A that lead to transitions ∈ Γ from one state ∈ S to another • AI Planning techniques categorization 1. Classical Planning: state-space, or plan-space planning 2. Neoclassical Planning: graph-based planning, propositional satisfiability and constraint satisfaction 3. Heuristics and Control Strategies: Partial Order Refinement, Hierarchical Task Networks 4. Other planning techniques: abductive planning, planning based on model checking Towards Realizing Dynamic QoS-aware Web Service Composition George Baryannis 35
  36. 36. Service Composition as AI Planning: OWLS-XPLAN (Klusch et al.)OWL-S descriptions translated to PDDXML,an XML variant of PDDL The planner combines graph- planning, to ensure that a plan is always found, and HTN, to take advantage of decompo- sition Towards Realizing Dynamic QoS-aware Web Service George Baryannis 36 Composition
  37. 37. Service Composition as AI Planning: The ASTRO project (Pistore et al.)State transition systems describe all possiblebehaviors of a BPEL process and support non- EaGLe is a formal language for thedeterminism and partial observability representation of planning goals This formula represents all the actions of the component services as they are controlled by the composite service Towards Realizing Dynamic QoS-aware Web Service George Baryannis 37 Composition
  38. 38. More on the ASTRO project (1) BPEL-to-STS transformationTowards Realizing Dynamic QoS-aware Web Service Composition George Baryannis 38
  39. 39. More on the ASTRO project (2)Expressing requirements in EaGLeTowards Realizing Dynamic QoS-aware Web Service Composition George Baryannis 39

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