Abstract

BPEL's processing model is based on the assumption that all import and export of data is done by using Web Services. BPEL differentiates two kinds of processes, executable processes and business protocols (abstract processes). While executable processes can be executed by a BPEL engine, business protocols use process descriptions that specify the mutually visible message exchange behavior of each of the parties involved in the protocol, without revealing their internal behavior. For the executable processes, BPEL provides a number of constructs for describing the behavior of such a process.

Web Service Integration

• BPEL is designed to integrate Web Services
  • Web Services are components described by interfaces (WSDL)
  • WSDL does not make statements about message sequences
• Implementing a service may build on other services
  • programs (services) can use other services through APIs (SOAP messaging)
  • if possible, a more declarative way of combining services is preferable
• BPEL processes can be used to model and execute compound services
  • components are combined using basic control primitives
  • BPEL provides fault handlers for exception handling

XML Technologies

• BPEL is part of the Web Services activities
  • the next version (BPEL 2.0) will be called WS-BPEL
• BPEL is built on top of a number of XML-related specifications
  • XML is used as the syntax for BPEL
  • WSDL is used as the interface description of Web Services
  • XML Schema is used to describe the types used by BPEL processes
  • XPath is used to extract parts of data in a BPEL process

Notation

• There is not standardized graphical notation for BPEL
  • XML is used as the standardized syntax
  • the BPEL syntax is defined by an XML Schema
• Products providing BPEL support provide graphical notations
  • different products use different notations
  • Business Process Modeling Notation (BPMN) has been proposed as a graphical notation

BPMN Example

Extending WSDL

• Web Services are described using WSDL interface definitions
• BPEL uses Web Services (BPEL is orchestrating these services)
• The BPEL process itself is a Web Service (it has interfaces)
  • when defining a BPEL process, it also is described by a WSDL (its interface)
• WSDL Port Types are named sets of abstract operations
• WSDL extensions are used to identify which port types are used to link services

WSDL Combination

Basic WSDL

• The loan approver BPEL process provides a service
• Users may invoke the service which then starts running

<definitions targetNamespace="http://tempuri.org/services/loanapprover" xmlns:tns="http://tempuri.org/services/loanapprover" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:loandef="http://tempuri.org/services/loandefinitions" xmlns="http://schemas.xmlsoap.org/wsdl/">
 <import namespace="http://tempuri.org/services/loandefinitions" location="http://localhost:8080/bpws-samples/loanapproval/loandefinitions.wsdl"/>
 <message name="approvalMessage">
  <part name="accept" type="xsd:string"/>
 </message>
 <portType name="loanApprovalPT">
  <operation name="approve">
   <input message="loandef:creditInformationMessage"/>
   <output message="tns:approvalMessage"/>
   <fault name="loanProcessFault" message="loandef:loanRequestErrorMessage"/>
  </operation>
 </portType>
 <binding ...> ... </binding>
 <service name="LoanApprover">....</service>
</definitions>

WSDL Example

 <portType name="riskAssessmentPT">
  <operation name="check">
   <input message="loandef:creditInformationMessage"/>
   <output message="tns:riskAssessmentMessage"/>
   <fault name="loanProcessFault" message="loandef:loanRequestErrorMessage"/>
  </operation>
 </portType>

• This service must be invoked from within the new service
• The relationship is captured in the new service's WSDL

<plnk:partnerLinkType name="riskAssessmentLinkType"> 
    <plnk:role name="assessor">  
      <portType name="asns:riskAssessmentPT"/>  
    </plnk:role>  
</plnk:partnerLinkType>

Orchestration vs. Choreography

• BPEL is mainly known as orchestration language
  • Executable Processes define a process that can be executed
  • orchestration is based on a runtime engine which executes the process
• BPEL also supports Business Protocols (Abstract Processes)
  • this description is a choreography, describing the message exchange patterns
  • such a description cannot be executed, there is no workflow definition
  • choreographies can be used to better understand message exchange patterns
  • choreographies can be used to monitor message exchanges

Executable Processes

• Most BPEL applications are executable processes
  • describing the interfaces to external data sources
  • describing the control flow for orchestrating these data sources
• BPEL is used for defining a new Web Service
  • the process is invoked through one of its partner links
  • it starts executing by following the process description
  • it may contact other partners through other partner links
  • as the final result, it may send back a response to the initial caller

Business Protocols (Abstract Processes)

• The least popular feature of BPEL
  • all BPEL products focus on the Executable Processes and the BPEL engine
  • business protocols are not executable
• Business protocols describe the message exchanges
  • a good complement to WSDL, which does not describe message exchanges
  • BPEL abstract processes could be used to guide executable processes
  • depending on the abstract process definition, the executable process could behave differently
  • abstract processes could avoid out-of-sequence messages with partners

Combining Services

• BPEL processes communicate with external partners
  • receive waits for incoming messages from clients
  • invoke invokes other services by sending a message
• BPEL processes can be synchronous or asynchronous
  • synchronous BPEL processes block the client when being called
  • reply is used for the response of a synchronous BPEL process
  • asynchronous BPEL processes are not blocking and may use callbacks

Process Example

Partners

• BPEL supports different relationships with partners
  1. partners may invoke the BPEL process
  2. the BPEL process may invoke partners
  3. partners and the BPEL process play both roles
• Partner Links define different relationships to partner
  • invoked partner links are links to partners called by the BPEL process
  • client partner links are links to partner which invoke the BPEL process
• BPEL processes will have at least one client (the partner activating the process)

Invoked by a Partner

Invoking a Partner

Interaction with a Partner

Defining Partner Links

• Partners are defined in BPEL definitions
  • they identify the interface (the WSDL port type) of the participating partner
  • they define the roles that the partners play
  • for the BPEL process itself, they define its role

<partners>
    <partner name="customer"
             partnerLinkType="lns:loanApproveLinkType"
             myRole="approver"/>
    <partner name="approver"
             partnerLinkType="lns:loanApprovalLinkType"
             partnerRole="approver"/>
    <partner name="assessor" 
             partnerLinkType="lns:riskAssessmentLinkType" 
             partnerRole="assessor"/>
</partners>

Correlation Sets

• Messages to a BPEL service must be delivered
  • it may be the case that an incoming message creates a new instance
  • but it may also belong to a conversation with an existing process instance
• Correlation sets define the properties that identify correlated messages
  • it may be the case that an incoming message creates a new instance
  • but it may also belong to a conversation with an existing process instance
• Correlating properties can vary between messages
  • properties are defined to identify data used for correlation
  • property aliases bind properties to concrete parts of a message
  • correlation sets group properties for correlating messages

Executable Processes

• BPEL executable processes are defined by control flow
  • interactions with Partners are inputs or outputs
  • BPEL processes are stepwise definitions of algorithms
  • steps of this algorithm are called BPEL Activities
• BPEL orchestration allows the construction of complex algorithms
  • conditional behavior (depending on the outcome of earlier steps)
  • loops (repeating a set of activities)
  • declaring variables, copying and assigning values
  • fault handlers for catching exceptions in the BPEL process

Language Constructs

• BPEL can be regarded as a programming language
  • it interacts with its environment through Web Service Interfaces
  • its control structures are activities which are used to compose processes
• Activities have some common attributes
  • they have a name
  • a join condition which specifies how to handle concurrency
  • an indication how to handle join faults
• Activities have some common elements
  • source and target for synchronization relationships

Activity Categories

Invoking Web Service Operations

• Web Services are called by using invoke
  • Web Service invocations can be request/response
  • Web Service invocations can be asynchronous messages
• invoke need variables for input and output
  • output variables are not required for asynchronous invocations
• fault handling can be defined to handle WSDL fault messages
  • resilient BPEL processes should always handle fault messages
• compensation handlers can be defined
  • compensation is a mechanism to handle transactions in BPEL

Providing Web Service Operations

• receive specifies the ability to receive an incoming message
  • receive specifies the partner link, the port type, and the operation
• Special activities can be set to cause process instantiation
  • only activities with no prior activities are allowed to create instances
  • it is possible to have multiple instantiation activities (multiple initial inputs)
• reply sends a response to a previous receive

Variable Assignments

• Variables are used to pass data between activities
  • they are declared with a name within a scope
  • variables may hold messages, XML Schema simple type values, or XML Schema elements
• Variables can be assigned within a business process
  • very simple computations can be performed on a variable
  • for more complex operations, XSLT can be used as part of the process

Waiting

• Processes may wait before continuing
  • wait time may be specified as a duration
  • wait time may be specified for a deadline to be reached

BPEL Control Flow

• Basic activities are the building blocks of a BPEL process
• Structured activities are used for composing basic activities
  • sequence, switch, and while support sequential control
  • flow allows concurrency and synchronization
  • pick allows choices based on external events

Sequential Control

• sequence packages sequential control flow activities
  • the contained activities can be basic or structures activities
• switch operates based on some condition
  • the condition contains an expression operating on a variable
  • there may be any number of case branches
  • there may be an otherwise branch
• while conditionally executes activities

Flows

• flow specifies concurrent activities
• The flow completes when all activities did complete
• Flows may contain synchronization dependencies
  • this is the construct that enables BPEL processes to be graphs rather than trees
• Synchronization uses link elements to connect activities

Pick

• pick waits for events
  • one possibility for an event is message arrival (by a partner)
  • the other possibility is the expiration of a timer
• Pick chooses exactly one of the specified activities
• Pick can also be used for instance creation
  • a business process specifies that it starts upon arrival of a set of messages
  • only one of these messages is sufficient to start the process
  • another message will then start a new instance of the business process

BPEL is Programming

• BPEL is a language for creating executable code
• BPEL processes can be executed and thus are programs
• BPEL is a specialized and dedicated programming language
• BPEL combines two tasks when working with Web Services
  • it creates a new Web Service which is described by a WSDL interface
  • it implements the Web Service by orchestrating a number of partners