1 WS Technologies III Orchestration Roberto Bruni Dipartimento di Informatica Università di Pisa Models and Languages for Coordination and Orchestration IMT- Institutions Markets Technologies - Alti Studi Lucca

Roberto IMT Lucca 22 March 2005 Institutions Markets Technologies IMT 2 Models and Languages for Coordination and Orchestration 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)

Roberto IMT Lucca 22 March 2005 Institutions Markets Technologies IMT 3 Models and Languages for Coordination and Orchestration 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)

Roberto IMT Lucca 22 March 2005 Institutions Markets Technologies IMT 4 Models and Languages for Coordination and Orchestration 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

Roberto IMT Lucca 22 March 2005 Institutions Markets Technologies IMT 5 Models and Languages for Coordination and Orchestration Is the Whole more than just the Sum of its Parts? Or less? area: 10*12/2 = 60 area: 60-2 =

Roberto IMT Lucca 22 March 2005 Institutions Markets Technologies IMT 6 Models and Languages for Coordination and Orchestration Dominoes and a Chessboard: a Tiling Problem 64-2 = 62 cells 31 dominoes

Roberto IMT Lucca 22 March 2005 Institutions Markets Technologies IMT 7 Models and Languages for Coordination and Orchestration Dominoes and a Chessboard: a Tiling Problem 64-2 = 62 cells 31 dominoes

Roberto IMT Lucca 22 March 2005 Institutions Markets Technologies IMT 8 Models and Languages for Coordination and Orchestration Contents Orchestration and Choreography WSFL Transactions and Compensations BPEL4WS

Roberto IMT Lucca 22 March 2005 Institutions Markets Technologies IMT 9 Models and Languages for Coordination and Orchestration Contents Orchestration and Choreography WSFL Transactions and Compensations BPEL4WS

Roberto IMT Lucca 22 March 2005 Institutions Markets Technologies IMT 10 Models and Languages for Coordination and Orchestration 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

Roberto IMT Lucca 22 March 2005 Institutions Markets Technologies IMT 11 Models and Languages for Coordination and Orchestration 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

Roberto IMT Lucca 22 March 2005 Institutions Markets Technologies IMT 12 Models and Languages for Coordination and Orchestration 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

Roberto IMT Lucca 22 March 2005 Institutions Markets Technologies IMT 13 Models and Languages for Coordination and Orchestration 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

Roberto IMT Lucca 22 March 2005 Institutions Markets Technologies IMT 14 Models and Languages for Coordination and Orchestration 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

Roberto IMT Lucca 22 March 2005 Institutions Markets Technologies IMT 15 Models and Languages for Coordination and Orchestration 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

Roberto IMT Lucca 22 March 2005 Institutions Markets Technologies IMT 16 Models and Languages for Coordination and Orchestration 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

Roberto IMT Lucca 22 March 2005 Institutions Markets Technologies IMT 17 Models and Languages for Coordination and Orchestration 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

Roberto IMT Lucca 22 March 2005 Institutions Markets Technologies IMT 18 Models and Languages for Coordination and Orchestration 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

Roberto IMT Lucca 22 March 2005 Institutions Markets Technologies IMT 19 Models and Languages for Coordination and Orchestration 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 Orchestration vs Choreography II

Roberto IMT Lucca 22 March 2005 Institutions Markets Technologies IMT 20 Models and Languages for Coordination and Orchestration 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

Roberto IMT Lucca 22 March 2005 Institutions Markets Technologies IMT 21 Models and Languages for Coordination and Orchestration 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

Roberto IMT Lucca 22 March 2005 Institutions Markets Technologies IMT 22 Models and Languages for Coordination and Orchestration 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

Roberto IMT Lucca 22 March 2005 Institutions Markets Technologies IMT 23 Models and Languages for Coordination and Orchestration 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

Roberto IMT Lucca 22 March 2005 Institutions Markets Technologies IMT 24 Models and Languages for Coordination and Orchestration 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

Roberto IMT Lucca 22 March 2005 Institutions Markets Technologies IMT 25 Models and Languages for Coordination and Orchestration 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