Tecnologie per servizi web: WSDL, WSFL

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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
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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)

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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)

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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

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Is the Whole more than just the Sum of its Parts?
Or less?
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Dominoes and a Chessboarda Tiling Problem
31 dominoes
64-2 62 cells
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Dominoes and a Chessboarda Tiling Problem
31 dominoes
64-2 62 cells
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Contents

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

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Contents

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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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