Dont go with the flow : Web services composition standards exposed

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Dont go with the flow Web services
composition standards exposed

• W.M.P. van der Aalst
• Presented By Prachi Jain

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The Problem

• Many web services composition languages
• Remarkable how much attention these different
  standards receive
• Fundamental issues semantics, expressiveness
  and adequacy not addressed
• Having standards is good, but too many and most
  of them die before becoming mature
• These languages have no clear semantics
• PDL, XPDL, BPSS, EDOC, BPML, ebXML, BPEL4WS

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The Goal

• Overcome the problems
• Critically evaluate the so-called standards for
  web services composition i.e. Dont go with the
  flow

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The Trends

• 2 trends in the world of E-business
• Technology push technologies taking XML-based
  standards and Internet as starting point
• Need to improve efficiency of processes from a
  business perspective
• Need to utilize potential of Internet Technology
  by automating business processes across
  enterprise boundaries

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Goal of Web Services

• Goal of web services
• Exploit XML technology and Internet
• Integrate applications that can be published,
  located and invoked on the web

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The Need for Web Services Composition Languages

• Integrate business processes across enterprise
  boundaries
• Simple interactions using standard messages and
  protocols not enough
• Require long running interactions driven by
  explicit process model
• Hence the need for Web Services Composition
  Language

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Web Services Composition Languages

• BPEL4WS, WSFL, XLANG, WSCI and BPML
• Also known as
• Web Services Flow Languages
• Web Services Execution Languages
• Web Services Orchestration Languages
• Web Enabled Workflow Languages

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Overview of Web Services Technology
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SOAP

• Simple Object Access Protocol
• Protocol for exchange of information
• In decentralized, distributed environment
• Using typed message exchange and remote
  invocation
• XML-based Protocol. 3 parts
• Envelope that defines framework for describing
  what is a message and how to process it
• Set of encoding rules for expressing instances of
  application defined datatypes
• Convention for representing Remote Procedure
  calls and responses

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WSDL

• Web Services Description Language
• XML format for describing network services based
  on standard messaging layer like SOAP
• Defines services as a collection of network
  endpoints or ports
• Abstract definition of endpoints and messages
• Separated from concrete network deployment or
  data format bindings

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WSDL

• Allows reuse of abstract definitions
• Messages abstract descriptions of data being
  exchanged
• Port types abstract collection of operations
• Concrete Protocol and data format specifications
  for a particular port type constitute reusable
  binding
• Port Network Address Reusable Binding
• Service Collection of Ports

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UDDI

• Universal Description Discovery Integration
• Set of services supporting description and
  discovery of
• Businesses, Organizations and other web service
  providers
• The web services they make available
• Technical interfaces which may be used to access
  those services
• Can be used to build yellow pages for web
  services

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Web Services Composition Languages

• Build directly on top of WSDL
• Provides and/or uses one or more WSDL services
• WSDL service ports that provide operations
• Each operation
• One-way receives a message
• Request-Response receives and sends a message
• Solicit-Response sends and receives a message
• Notification Sends a message

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Web Services Composition Languages

• WSDL services and corresponding operations glued
  together to provide composed services
• Process model needed to glue such services
• Process model specifies order of execution of
  operations
• Web services composition language provides the
  means to specify such a process model
• Example BPEL4WS Business Process Execution
  Language for Web Services

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Difference between WSDL and Composition Language

• WSDL is stateless language not aware of states
  in-between operations
• WSDL only state notion supported is state in
  between sending and receiving a message in
  request-response or solicit-response operation
• Technology supporting web service composition
  language will have to record states more complex
  than simple request-response
• Triggered development of languages like BPEL4WS,
  WSFL, XLANG, etc

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Overview of so-called standards

• BPEL4WS builds on
• IBMs WSFL ( Web Services Flow Language)
• Microsofts XLANG ( Web Services for Business
  Process Design)
• XLANG
• Block-structured language
• Basic control flow structures
• Sequence, switch Conditional Routing
• While Looping
• All Parallel Routing
• Pick Race Conditions based on timing or
  external triggers

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Overview of so-called standards

• WSFL
• Not limited to block structure
• Allows directed graphs
• Graphs can be nested but need to be acyclic
• Iteration only supported through exit conditions
• Activity/subprocess iterated until exit condition
  is met
• Control flow part identical to workflow language
  used by IBMs MQ Series Workflow
• Workflow language very different from most
  languages
• Correspondence between WSFL and MQ series
  workflow defined by same set of people
• Similarly, XLANG completely based on current
  Microsoft middleware solution
• Therefore hardly qualifies as a standard

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Other so-called standards

• Sun, BEA, SAP and Intalio introduced WSCI (Web
  Service Choreography Interface)
• Intalio initiated Business Process Management
  Initiative (BPMI.org) which developed BPML
• ( Business Process Markup Language)
• OASIS and UN/CEFACT support ebXML
• ( Electronic Business using eXtensible Markup
  Language)
• Abundance of overlapping standards for web
  services composition
• These competing standards without clear added
  value referred as Web Services Acronym Hell (WSAH)

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Other so-called standards

• Workflow Management Coalition (WfMC) released
  XPDL (XML Process Definition Language)
• Support exchange of workflow specifications
  between different workflow products
• Standards of WfMC not adopted by workflow vendors
• Some systems can export to XPDL
• None of the systems can import XPDL from another
  system and still produce meaningful results
• Still no consensus on the workflow constructs
  that need to be supported and their semantics

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Comparing BPEL4WS, XLANG, WSFL, XPDL and WFM
products

• Development of web services composition language
  driven by software vendors IBM, Microsoft, Sun,
  BEA, SAP and Intalio
• Standards often based on existing products
• WSFL copy of IBMs FlowMark/MQ Series Workflow
  Language
• Standards involving multiple software vendors,
  often a compromise between competing viewpoints
• Tend to be imprecise or unnecessarily complex
• XPDL imprecise standard therefore allowing
  vendors to have their own interpretation

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Comparing BPEL4WS, XLANG, WSFL, XPDL and WFM
products

• Look for objective methods for comparing
• Use set of workflow patterns for comparing
• Patterns correspond to a routing construct often
  required when designing a workflow
• Patterns have been used for comparing about 20
  workflow management systems
• Author shows comparison for BPEL4WS, XLANG, WSFL,
  XPDL and four concrete workflow management
  systems.

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t
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Comparing BPEL4WS, XLANG, WSFL, XPDL and WFM
products

• First 5 patterns basic routing constructs e.g.
  sequence, one can find in any language
• Other patterns refer to more advanced constructs
  not supported by most standards
• refers to direct support
• Construct in the language that directly supports
  the pattern
• - refers to no direct support
• Does not mean that it is not possible to realize
  the pattern through some workaround
• /- represents feature that only partially
  supports a pattern

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Observations

• BPEL4WS indeed combination of XLANG and WSFL
• WSFL and MQ Series Workflow are identical
• XPDL is less expensive than BPEL4WS
• XPDL can be seen as Greatest Common Denominator
  of exisiting workflow languages rather than Least
  Common Multiple
• Relevant differences between Web Services
  Composition Languages and Workflow Management
  Systems for routing constructs
• FLOWer is block structured like XLANG
• Other 3 are graph based like WSFL and XPDL

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Lessons Learned

• Well established process modeling techniques
  combining expressiveness, simplicity and formal
  semantics exist
• Software industry has chosen to ignore these
• Hence, too many standards
• Driven by concrete products and/or commercial
  interests
• Users need to ignore standardization proposals
  that are not using well established process
  modeling techniques
• Force vendors to address real problems

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What has been done since then ?

• BPEL4WS has been renamed by the OASIS WS-BPEL
  technical committee to WS-BPEL
• Truly, Web Services Acronym Hell (WSAH)

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Questions

• ?