Chapter 12: Coordination Frameworks for Web Services

1

• Chapter 12Coordination Frameworks for Web
  Services

Service-Oriented Computing Semantics, Processes,
Agents Munindar P. Singh and Michael N. Huhns,
Wiley, 2005
2
Highlights of this Chapter

• WSCL Web Services Conversation Language
• WSCI Web Service Choreography Interface
• WS-Coordination
• Web service transaction types
• BTP Business Transaction Protocol

3
Achieving Business Interoperation

• The parties must
• Know each others identity and location (presumes
  suitable directories)
• Agree on the low-level transport protocols and
  encoding formats
• Agree on the syntax and semantics of documents to
  be exchanged
• Agree on their expectations about when different
  documents will be sent and received
• This specification is termed a business protocol
• An instance of a business protocol is a
  conversation (but sometimes the term is used to
  mean protocol watch out!)

4
Process Abstractions

• Orchestration A process is a partial order of
  actions under the control of a central conductor
  akin to a workflow Global central
• Choreography A process is an exchange of
  messages among participants akin to a
  conversation as described by WSCL and WSCI
  Global distributed
• Collaboration A process is a joint set of
  activities among business partners Local
  distributed
• Workflow narrower concept than process, which
  emphasizes control and data flows from a central
  perspective original idea behind modern
  orchestration

5
Parts of a WSCL Specification

• WSCL Web Services Conversation Language
• Document Type Definitions specify what types of
  XML documents will be exchanged
• Interaction Types Send, Receive, SendReceive,
  ReceiveSend, Empty
• ltInteraction interactionType"SendReceive"
  id"Payment"gt
• ltOutboundXMLDocument id"Invoice"
• hrefSchema"http//sc.edu/InvoiceRS.xsd"/gt
• ltInboundXMLDocument id"Payment"
• hrefSchema"http//ncsu.edu/Payment.xsd"gt
• lt/InboundXMLDocumentgt
• lt/Interactiongt
• Transitions order of the interactions
• ltTransitiongt
• ltSourceInteraction href"Quote"/gt
• ltDestinationInteraction href"Purchase"/gt
• lt/Transitiongt
• ltTransitiongt
• ltSourceInteraction href"Quote"/gt
• ltDestinationInteraction href"CatalogInquiry"/gt
  
• lt/Transitiongt

6
Concepts in WSCL
                                               
                                                  
                                              
7
Well-Formed Conversations

• All interactions are reachable from the initial
  interaction
• The final interaction is reachable from all
  interactions
• If a transition from interaction A to interaction
  B specifies a SourceInteractionCondition, then
  all transitions from A to B do so
• The final interaction and transitions to the
  final interaction unambiguously clarify for each
  participant when a conversation is finished

8
Example Conversation Definition
9
Example WSCL Specification 1

• ltConversation name"StoreFrontServiceConversation"
  xmlns"http//www.w3.org/2002/02/wscl10"
  initialInteraction"Start" finalInteraction"End"
  gt
• ltConversationInteractionsgt
• ltInteraction interactionType"ReceiveSen
  d" id"Login"gt
• ltInboundXMLDocument
  hrefSchemahttp//conv1.org/LoginRQ.xsd
• id"LoginRQ"/gt
• ltOutboundXMLDocument
  hrefSchemahttp//conv1.org/ValidLoginRS.xsd
• id"ValidLoginRS"/gt
• ltOutboundXMLDocument
  id"InvalidLoginRS" hrefSchemahttp//co
  nv1.org/InvalidLoginRS.xsd/gt
• lt/Interactiongt
• ltInteraction interactionType"Empty"
  id"Start" /gt
• ltInteraction interactionType"Empty"
  id"End" /gt
• lt/ConversationInteractionsgt

10
Example WSCL Specification 2

• ltConversationTransitionsgt
• ltTransitiongt
• ltSourceInteraction href"Start"/gt
• ltDestinationInteraction
  href"Login"/gt
• lt/Transitiongt
• ltTransitiongt
• ltSourceInteraction href"Login"/gt
• ltDestinationInteraction
  href"Registration"/gt
• ltSourceInteractionCondition
  href"InvalidLoginRS"/gt
• lt/Transitiongt
• ltTransitiongt
• ltSourceInteraction href"Logout"/gt
• ltDestinationInteraction
  href"End"/gt
• lt/Transitiongt
• lt/ConversationTransitionsgt
• lt/Conversationgt

11
WSCI Web Service Choreography Interface

• Choreography refers to a global view of how
  messages are exchanged
• WSCI describes the flow of messages exchanged by
  a Web service that is interacting with other
  services
• WSCI is viewed as an enhancement to WSDL
  (inserted within a WSDL description)

12
Example WSCI Add-In to WSDL

• ltcorrelation name"quotationCorrelation
  property"tnsquotationID/gt
• ltinterface name"StockQuoteWS"gt
• ltprocess name"ProvideStockQuote"
  instantiation"message"gt
• ltsequencegt
• ltaction name"ReceiveLogin
  role"tnsStockQuoteWS
• operation"tnsQuoteToUser/L
  ogIn"/gt
• ltaction name"ReceiveStockQuoteReque
  st" role"tnsStockQuoteWS"
• operation"tnsQuoteToUser/P
  rovideQuote"gt
• ltcorrelate correlation"tnsqu
  otationCorrelation"/gt
• ltcall process"tnsLookupPrice
  "/gt
• lt/actiongt
• ltaction name"ReceiveLogout
  role"tnsStockQuoteWS
• operation"tnsQuoteToUser/L
  ogOut"/gt
• lt/sequencegt
• lt/processgt
• ltprocess name"LookupPrice"
  instantiation"other"gt
• ltaction name"QueryNYSE"
  role"tnsStockQuoteWS
• operation"tnsQuoteToUser/Quer
  yNYSE"/gt

13
WSCI Example for Transaction Compensation

• ltsequencegt
• ltcontextgt
• lttransaction name"buyStock" type"atomic"gt
• ltcompensationgt
• ltaction name"NotifyUnavailable"
  role"NYSE operation"tnsNYSEtoBroker/NotifyUna
  vailable"/gt
• lt/compensationgt
• lt/transactiongt
• lt/contextgt
• ltaction name"BuyShare" role "Broker
  operation"tnsBrokerToNYSE/BuyShare"/gt
• ltwhile name"BuyShares"gt
• ltconditiongtdefsfundsRemainlt/conditiongt
• ltaction name"BuyShare" role "Broker
  operation"tnsBrokerToNYSE/BuyShare"gt
• ltcorrelate correlation"defsbuyingCorrela
  tion"/gt
• lt/actiongt
• lt/whilegt
• lt/sequencegt
• lt!-- Compensating Behavior for the Above
  Transaction --gt
• ltexceptiongt
• ltonTimeout property"tnsexpiryTime"
  type"duration" reference"tnsBuyShares_at_end"gt

14
WS-Coordination

• WS-Coordination is the specification for a
  service whose job is to coordinate the activities
  of the Web services that are part of a business
  process
• A coordination service is an aggregation of three
  services
• Registration for parties to register with the
  coordinator
• Activation to create an instance of a
  coordination context
• Coordination protocol to define the specific
  protocol to be followed

15
WS-Coordination Service
16
WS-Coordination Service
17
WS-AtomicTransactionProtocol for an Atomic
Transaction
The travel agency and the airline can perform
2PC, but the museum can perform only a simple,
zero-phase update
18
Business Transaction Protocol (BTP)
19
Chapter 12 Summary

• SOC needs an ability to specify interactions
  among business partners in a manner that respects
  the partners autonomy and heterogeneity
  conversations are a natural way to do so
• SOC needs support for transactions
• The more relaxed varieties are preferable, but
  there is a place for traditional transactions as
  well
• The transactions are based on a small set of
  primitives