Don’t go with the flow : Web services composition standards exposed

1. Don’t go with the flow : Web services composition standards exposed W.M.P. van der Aalst Presented By – Prachi Jain

2. 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

3. The Goal • Overcome the problems • Critically evaluate the so-called standards for web services composition i.e. Don’t go with the flow

4. 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

5. 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

6. 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

7. 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

8. Overview of Web Services Technology

9. 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

10. 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

11. 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

12. 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

13. 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

14. 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

15. 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

16. Overview of so-called standards • BPEL4WS builds on • IBM’s WSFL ( Web Services Flow Language) • Microsoft’s 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

17. 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 IBM’s 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

18. 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)

19. 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

20. 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 IBM’s 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

21. 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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23. 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

24. 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

25. 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

26. 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)

27. Questions ?