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Tecnologie per servizi web: WSDL, WSFL

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Models and Languages for Coordination and Orchestration ... some event sequences are disallowed. it does not make sense for a party to outdo its own offer ... – PowerPoint PPT presentation

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Title: Tecnologie per servizi web: WSDL, WSFL


1
Models and Languages for Coordination and
Orchestration IMT- Institutions Markets
Technologies - Alti Studi Lucca
WS Technologies III Orchestration
Roberto Bruni Dipartimento di Informatica
Università di Pisa
2
Web Services to Bits
  • Lingua Franca XML
  • firmly established, platform independent
    information and data encoding
  • Communication Protocol SOAP
  • high flexibility, works on different transport
    protocols (HTTP, SMTP)
  • Service Discovery UDDI
  • browser-accessible service registry, white /
    yellow / green pages
  • Service Descriptions WSDL
  • unambiguous, machine-readable, tells how
    (interfaces), what (transports protocols) and
    where (endpoints)

3
The Car Analogy
  • WS is primarily an integration technology
  • WS composition MUST BRIDGE THE GAP between
    business people and technologists in an
    organization
  • WS are units of work, each handling a specific
    functional task
  • designed and built by technical people
  • like a whole car engine, a car frame,
    transmission
  • Tasks ARE TO BE COMBINED into more complex
    business-oriented tasks
  • business process architects can then aggregate WS
    in solving business level problems
  • like assembling the engine, the frame and the
    transmission without having to look at the many
    pieces within them
  • and the manufacturer does not matter
    (interoperability)

4
The "Coffee Bar" Problem
  • The description of WS interactions is an
    important problem, in particular with regard to
  • the exchange of messages,
  • their composition,
  • and the sequences in which they are transmitted
    and received
  • these interactions may take place among composite
    services, which span and interact across
    organizational boundaries
  • Think of a coffee bar service that let you
  • order an espresso
  • pay for the espresso and get the receipt
  • but you don't know what to do first
  • it depends from the coffee bar

5
Is the Whole more than just the Sum of its Parts?
Or less?
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area 1012/2 60
area 60-2 58
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Dominoes and a Chessboarda Tiling Problem
31 dominoes
64-2 62 cells
7
Dominoes and a Chessboarda Tiling Problem
31 dominoes
64-2 62 cells
8
Contents
  • Orchestration and Choreography
  • WSFL
  • Transactions and Compensations
  • BPEL4WS

9
Contents
  • Orchestration and Choreography
  • WSFL
  • Transactions and Compensations
  • BPEL4WS

10
Web Service Composition
  • WS composition is an emerging paradigm for
    application integration
  • within and across organization boundaries
  • A landscape of languages and techniques for WS
    composition has emerged
  • and is continuously being enriched with new
    proposals from different vendor coalitions
  • Business collaborations require long-running
    interactions driven by explicit process models
  • Accordingly, it is a natural choice to capture
    the logic of a composite web service using
    business process modeling languages tailored for
    web services
  • Many such languages have recently emerged WSCI,
    XLANG, BPML, WSFL, BPEL4WS, BPSS, XPDL

11
Some Scenarios
  • Web orders
  • at least three entities are involved
  • customer
  • provider
  • carrier
  • Web auctions
  • multiple iterations are required
  • some event sequences are disallowed
  • it does not make sense for a party to outdo its
    own offer

12
Service Composition
  • To build novel functionalities by assembling
    available services in a proper way
  • both as a new web service
  • and as an interaction model among distributed
    services
  • Two main approaches
  • a centralized flow invokes the services according
    to some internal logic / policy
  • separately designed services interact according
    to a suitable conversation pattern to reach their
    goals
  • intrinsically distributed perspective

13
Flows
  • Operational description of control logic
  • to explain how to use the service in the proper
    way
  • invocation order of certain methods
  • elaboration strategy of requests
  • Single activities composing the flow can be
    realized by different entities
  • Certain flows can become services on their own

14
Challenges
  • Traditional workflow models rely on message-based
    computing methods that are tightly coupled to
    specific B2B protocols
  • exploit controllable environment (not for the
    web)
  • Parties are normally known in advance
  • contrast with dynamic discovery
  • Traditional models normally call for centralized
    engines
  • but the nature of the web is widely distributed

15
Goals
  • The challenge will be to describe WS
  • from an external point of view
  • without knowing how internally operate
  • independently of any particular integration model
  • precisely enough to allow interaction
  • other components must understand how to properly
    interact with them
  • (how they fit) in the context of each specific
    message exchange in which they can participate

16
Orchestration and Choreography
  • Orchestration
  • describes how WS can interact with each other at
    the message level, including the business logic
    and execution order of the interactions
  • Choreography
  • tracks the sequence of messages that may involve
    multiple parties and multiple sources, including
    customers, suppliers and partners
  • The entirely deliberate analogy is with
  • the way in which skilled musicians and dancers
    co-ordinate their actions while carrying out
    predetermined instructions

17
The Ballet Analogy
  • Consider a dance with more than one dancer
  • each dancer has a set of steps to perform
  • dancers orchestrate their own steps because they
    are in complete control of their domain (their
    body)
  • a choreographer ensures that the steps all of the
    dancers make is according to some overall scheme
    (the choreography)
  • the dancers have a single view point of the dance
  • choreography is the multi-party or global view
    point of the dance

18
Orchestration vs Choreography I
  • Orchestration is about describing and executing a
    single view point model
  • Traditional Workflow Management Systems
  • centralised schedulers of (distributed)
    activities
  • Choreography is about describing and guiding a
    global model
  • The single-view point model can be derived from
    the global-model
  • by projecting based on participant
  • But the distinction can blur in many occasions

19
Orchestration vs Choreography II
  • Orchestration implies a centralized control
    mechanism
  • closer to an executable framework
  • Choreography has no centralized control
  • an abstract discipline
  • control is shared between domains
  • that are responsible for their tasks
  • describing peer to peer interaction in a global
    model by means of a choreography description
    language

20
Web Services Choreography
  • Web Services Choreography concerns the observable
    interactions of services with their users
  • users may, in turn, be other WS, applications or
    human beings
  • The problems of WS choreography are largely
    focused around
  • message exchange
  • and sequencing these messages in time to the
    appropriate destinations
  • To fulfil the needs of the Web Services
    community, these aspects of Web Services must be
    developed and standardized in an interoperable
    manner

21
Choreography Description
  • A choreography description is
  • a multi-party contract
  • that describes from global view point the
    external observable behaviour across multiple
    clients
  • (which are generally Web Services but not
    exclusively so)
  • in which external observable behaviour is defined
    as
  • the presence
  • or absence
  • of messages that are exchanged between a Web
    Service and its clients

22
How is a Choreography Used?
  • The main use of a choreography description is
  • to precisely define the sequence of interactions
    between a set of cooperating web services
  • in order to promote a common understanding
    between parties
  • and to make it as easy as possible to
  • promote a common understanding between WS
    participants
  • automatically validate conformance
  • ensure interoperability
  • increase robustness and to
  • generate code skeletons

23
How is a Choreography Used?Examples
  • To aid the testing of WS through the generation
    of test messages that could be sent to parties
  • by means of an appropriate vendor specific tool
    that reads the choreography description and
    manages the test interaction according to it
  • To validate the multi-party observable
    interactions amongst a collection of WS
  • ex. a hotel may load a choreography description
    into a vendor specific tool which informs them of
    any breaches of the choreography
  • To show the presence of useful properties such as
    lock freedom and leak freedom in the behavioural
    contract
  • in this sense a choreography description acts as
    a model of the behaviour across a number of Web
    Services
  • which in turn can be subject to static analysis
    to show that if and only if the underlying Web
    Services behave according to the contract that
    the interaction between the Web Services will
    exhibit these properties

24
Benefit of Choreography
  • A standardized language for the description of
    choreographies offers many benefit
  • the construction of more robust Web Services
  • the enabling of more effective interoperability
    of WS through behavioural multi-party contracts,
    which are choreography descriptions
  • the reduction of the cost of implementing Web
    Services by ensuring conformance to expected
    behaviour
  • the increasing of the utility of WS as they will
    be able to be shown to meet contractual behaviour

25
Model-Driven Architecture
  • At the highest conceptual level (of SOA)
  • service components as building blocks
  • written in many different languages and platforms
  • assembled without really writing any line of code
  • blocks composed in workflows
  • tools can monitor the execution of (group of)
    services
  • developers can work in a Model-Driven
    Architecture and put away the use of regular
    programming language
  • application design
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