Talk at the IEEE International Conference on Services Computing (SCC 2008)

1. Formalising Message Exchange Patterns
using BPELlight
Tammo van Lessen, Jörg Nitzsche, Frank Leymann
Institute of Architecture of Application Systems
(IAAS)
University of Stuttgart
Universitätsstr. 38
70569 Stuttgart
Germany
tammo.van.lessen@iaas.uni-stuttgart.de

2. The Talk Today
Motivation and Introduction
Web Services
Message Exchange Patterns
Problem Statement
WSDL 2.0 Message Exchange Patterns
BPEL / BPELlight
Abstract Profile for MEPs
Describing MEPs with Abstract BPELlight
Conclusions
© Tammo van Lessen, Jörg Nitzsche, Frank Leymann 2

3. Problem Statement
WSDL / WS‐I breaks loose coupling in WS‐*
WSDL 1.1 defines 4 operation types:
solicit-response + request-response notification + one-way
© Tammo van Lessen, Jörg Nitzsche, Frank Leymann 3

4. Problem Statement
WSDL / WS‐I breaks loose coupling in WS‐*
WSDL 1.1 defines 4 operation types:
request-response one-way
Operational semantics of notification and solicit-response
can be interpreted differently
WS-I Basic Profile restricts set of WSDL operations to request-
response and one-way
Service Requesters bind against these operations
Against the principle of loose coupling
© Tammo van Lessen, Jörg Nitzsche, Frank Leymann 4

5. Problem Statement (2)
WSDL does not support interactions beyond req‐resp
WSDL 1.1 defines 4 operation types:
request-response one-way
They map nicely to imperative programming languages…
…but not to business processes (e.g. BPEL processes),
where business partner exchange messages, e.g. a
purchase order, in order to achieve a business goal
Operations that are restricted to request-response at
most are not sufficient
© Tammo van Lessen, Jörg Nitzsche, Frank Leymann 5

6. Our Approach
Associate complex
message exchange patterns
with WSDL operations
Message exchange pattern describe
sequence, direction, cardinality and
senders and receivers of messages
© Tammo van Lessen, Jörg Nitzsche, Frank Leymann 6

7. WSDL 2.0
Operations are defined using MEPs which are
identified by a URI
MEPs are defined using templates
Multiple inputs, outputs, infaults and outfaults can be
defined Complex MEPs are possible
Explicitly introduces the notion of compatibility (via
dual MEPs), e.g. a service implementing an out-in is
compatible with a service implementing an in-out.
There is a lot of potential!
<operation name=quot;xs:NCNamequot;
pattern=quot;xs:anyURIquot;?
style=quot;list of xs:anyURIquot;? >
<documentation />*
[<input/>|<output/>|<infault/>|<outfault/>]*
</operation>
© Tammo van Lessen, Jörg Nitzsche, Frank Leymann 7

8. Example: out-optional-in (WSDL 2.0 MEP)
The out-optional-in message exchange pattern consists of
one or two messages, in order, as follows:
A message:
indicated by a Interface Message Reference component whose
{message label} is “Outquot; and {direction} is “outquot;
Sent to some node N
An optional message:
indicated by a Interface Message Reference component whose
{message label} is “Inquot; and {direction} is “inquot;
Received from node N
…
An operation using this message exchange pattern has a
{message exchange pattern} property with the value
quot;http://www.w3.org/ns/wsdl/out-opt-inquot;.
© Tammo van Lessen, Jörg Nitzsche, Frank Leymann 8

9. Deficiencies of the W3C proposed template
Weak control flow
No parallelism
No conditional flow
No notion of cardinality
Weak definition of what a node is (node type vs. node
instance)
Multi-casts not supported
Imprecise
Weak definition of “optional” (how can a service
decide whether an optional message will arrive or
not?)
© Tammo van Lessen, Jörg Nitzsche, Frank Leymann 9

10. BPEL + BPEL light
BPEL defines flow between WS operations
receive, reply, invoke, pick, eventHandler
BPEL and WSDL define the message exchange
of (multiple) parties from the point of view of one
participant
A BPEL that is decoupled & independent of
WSDL (BPEL light) is an eligible candidate for
defining the flow within WS operations
Nitzsche, J.; Lessen, T. van; Karastoyanova, D.; Leymann, F.: BPELlight.
5th International Conference on Business Process Management (BPM 2007).
Brisbane, Australia, 24-28 September 2007.
© Tammo van Lessen, Jörg Nitzsche, Frank Leymann 10

11. BPELlight
Extends BPEL with a WSDL-less interaction model:
BPEL BPELlight
<receive>, <reply>, <invoke> <interactionActivity>
(configurable)
<onMessage> (in pick) WSDL-less <onMessage>
<onEvent> (in EventHandler) WSDL-less <onEvent>
<partnerLink> referenced by <conversation> referenced by
<receive>, <reply>, <invoke>, <interactionActivity>,
<onMessage>, <onEvent> <onMessage>, <onEvent>
BPEL 1.1 <partner> groups BPELlight <partner> referenced by
partnerLinks messages
© Tammo van Lessen, Jörg Nitzsche, Frank Leymann 11

12. BPELlight Abstract Profile for MEPs
Allows definition of MEPs in terms of message
exchanges
Unifies timing expressions
Allows modelling of sending and receiving faults
<mep:timingExpression
expressionLanguage=quot;anyURIquot;?
type=quot;for|until|repeatEveryquot;
expression=quot;exprquot;/>
© Tammo van Lessen, Jörg Nitzsche, Frank Leymann 12

13. Example: out-optional-in (BPELlight)
<bpel:process <bl:pick>
xmlns:bpel=quot;http://.../wsbpel/ <bpel:targets>
2.0/process/abstractquot; <bpel:target linkName=quot;L1quot;/>
suppressJoinFailure=quot;yesquot;
abstractProcessProfile=quot;http:// </bpel:targets>
.../BPELlight/abstract/mep/2008/quot; <bl:onMessage
targetNamespace=quot;http://.../mep-in-bpelquot; name=quot;Inquot;
outputVariable=quot;##opaquequot;
name=quot;out-optional-inquot;> conversation=quot;out-optional-inquot;>
<bl:conversations> <bpel:empty/>
<bl:conversation </bl:onMessage>
name=quot;out-optional-inquot;/> <bl:onMessage
</bl:conversations> name=quot;InFaultquot;
<bpel:flow> faultName=quot;##opaquequot;
<bpel:links> outputVariable=quot;##opaquequot;
<bpel:link name=quot;L1quot;/> conversation=quot;out-optional-inquot;>
</bpel:links> <bpel:empty/>
<bl:interactionActivity </bl:onMessage>
name=quot;Outquot; <bpel:onAlarm>
outputVariable=quot;##opaquequot; <mep:timingExpression
conversation=quot;out-optional-inquot;> name=quot;timeoutquot;
<bpel:sources> type=quot;##opaquequot;
<bpel:source linkName=quot;L1quot;/> expression=quot;##opaquequot;/>
</bpel:sources> <bpel:empty/>
</bl:interactionActivity> </bpel:onAlarm>
</bl:pick>
</bpel:flow>
</bpel:process>
© Tammo van Lessen, Jörg Nitzsche, Frank Leymann 13

14. Referencing BPELlight MEPs in WSDL Operations
Standard WSDL 2.0 operations
Reference to BPEL MEP via pattern attribute
Parameterisation of the MEP
Messages are linked via name
(message label == interactionActivitiy/@name)
Timing expressions, variable names/types, arbitrary
elements via extension element (mep:configure) in
WSDL.
© Tammo van Lessen, Jörg Nitzsche, Frank Leymann 14

15. Referencing BPELlight MEPs in WSDL Operations
<interface name=quot;myInterfacequot;>
<fault name=quot;inFaultquot; element=quot;types:aFaultquot; />
<operation name=quot;myOperationquot;
pattern=quot;http://.../mep-in-bpel#out-optional-inquot; >
<input messageLabel=quot;Inquot;
element=quot;types:inTypequot; />
<output messageLabel=quot;Outquot;
element=quot;types:outTypequot; />
<infault ref=quot;tns:inFaultquot;
messageLabel=quot;InFaultquot; />
<mep:configure
xpath=quot;//mep:timingExpression[@name='timeout']/@typequot;>
for
</mep:configure>
<mep:configure
xpath=quot;//mep:timingExpression[@name='timeout‘]/@expressionquot;>
PT5M
</mep:configure>
</operation>
</interface>
© Tammo van Lessen, Jörg Nitzsche, Frank Leymann 15

16. Conclusion
Abstract BPELlight process models can be used
to describe complex MEPs
Complex MEPs can be attached to WSDL 2.0
operations (with in-house means)
Allows to describe protocol and message format
independent message exchanges.
MEP operations can improve modelling tools
MEP operations foster message-oriented Web
Services.
© Tammo van Lessen, Jörg Nitzsche, Frank Leymann 16

17. End of Document

18. Selected References
Nitzsche, J., van Lessen, T., Karastoyanova, D., Leymann, F.: BPELlight. In:
5th International Conference on Business Process Management (BPM).
(2007) Brisbane, Australia. http://tinyurl.com/ypzahn
Nitzsche, J.; van Lessen, T.; Leymann, F.: WSDL 2.0 Message Exchange
Patterns: Limitations and Opportunities. In Proceedings of the 3rd
International Conference on Internet and Web Applications and Services
(ICIW) 2008. Athen, Greece, June 8-13, 2008.
van Lessen, T.; Nitzsche, J.; Leymann, F.: Formalising Message Exchange
Patterns using BPELlight. In Proceedings of the IEEE 2008 International
Conference on Services Computing (SCC). Honolulu, Hawaii, USA, July 8-
11, 2008.
Nitzsche, J.; van Lessen, T.; Leymann, F.: Extending BPELlight for Expressing
Multi-Partner Message Exchange Patterns. In Proceedings of the 12th IEEE
International EDOC Conference (EDOC 2008). München, Germany,
September 15-19, 2008.
© Tammo van Lessen, Jörg Nitzsche, Frank Leymann 18