> REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) <
3
WofBPEL to support three types of analysis, i.e., reachability
analysis, competing message-consuming activities and garbage
collection of queued messages by generating the full state space.
They adopt Petri net reduction rules to reduce a model before
generating its state space [26]. Lohmann et al. adopt open
workflow nets (oWFN)[27-29] for modeling BPEL processes
and automatically analyze the interactional behavior of a given
oWFN.
Hinz
et al. propose a parser to automatically transform a
given BPEL process into a Petri net [30] and perform model
checking.
Martens [31-32] proposes a BPEL annotated Petri nets (BPN)
and presents a decision algorithm for the controllability of a
BPN model based on a communication graph (c-graph). A
c-graph is a directed, bipartite graph in which nodes denote
reachable states of the BPN and edges denote messages that the
BPN is able to send or receive. Martens transforms the check of
interaction between the composed BPEL processes into the
verification of deadlock-freeness of a BPN. After all parts that
yield a deadlock are cut off, the remaining part of BPN is proven
to be controllable. As shown in Fig. 3, Martens’ system starts
from a given set of BPEL processes. The next steps are: (1)
Transform each BPEL process into a BPN. (2) Compose all
BPN models and reduce them to speed up analysis. (3)
Generate and analyze the c-graph of the composed BPN. (4)
Based on the related states and traces in case of incompatibility,
re-design the composition.
König et al. [29] point out that the BPEL specification
distinguishes two kinds of business processes, i.e., executable
and abstract. Most of the previous work [24-32] focuses on
abstract ones. Their work permits the occurrence of
incompatible cases. When it happens, it is detected by the
analysis method based on the Petri net-based model, and then,
two BPEL profiles for composition re-design can be applied,
i.e., the abstract process profile for observable behavior from
bottom-up viewpoint and the abstract process one for templates
from top-down viewpoint. Their main idea is to substitute an
erroneous service with a correct one. The efficiency of this
“substitute” based approach depends upon how to find out the
exactly right web service from thousands of candidate web
services with the exact behavior that conforms to the other web
services. In general, these methods may become complex,
considering that a single error of a certain web service may
collapse the compatibility of the whole BPEL process.
The detail of our proposed system flow is shown in Fig. 4. We
address the web service compatibility issues by concentrating
on a) modeling and definition of the compatibility, b)
compatibility analysis, and c) a policy for resolving
incompatibility. We model the BPEL in terms of Petri nets
called workflow module net (WMN) and Composition net
(C-net). These models are different from the existing Petri net
ones since we can characterize the compatibility in terms of
structural Petri net objects, i.e., siphons. Finally, we formulate a
policy by appending additional possible information channels in
terms of Petri net elements.
. . .
. . .
Fig. 3. Illustration of the flow through Martens’ system [31]
. . .
. . .
Fig. 4. Illustration of the flow through the proposed system
The main contribution of our paper is that as we analyze the
model through structural properties instead of the reachable
states, we can impose constraints on the model to prevent it from
reaching incompatible states. Since we do not need to search
other replaceable web services and the new model can be
transformed back to code in BPEL in order to guarantee the
compatibility automatically, this modification based approach is
more efficient than the existing “substitute” based ones.
Although the compatibility problem for web service
interaction is similar in some way to the traditional deadlock
problem in flexible manufacturing systems (FMS) [33-34], they
are not totally the same. Firstly, the pre-conditions that must be
held for a deadlock to occur are totally different. Four
conditions [35], i.e., mutual exclusion, no preemption, wait for
condition and circular wait, must be held for a deadlock in FMS
to occur. However, the web service incompatibility is mainly
attributed to the erroneous web service behaviors. Secondly,
since deadlock in FMS is caused by an inappropriate allocation
of resources to concurrently executing processes, the deadlock
problem can be solved in many cases by increasing the number
of shared resources. Web services can be made to be compatible
by adding information channels, or they are just incompatible at
all. Thirdly, compared with the relatively fixed working
processes in FMS, the routes in BPEL are much more flexible.
There are a large number of choices of services with different
behaviors that can provide the same syntactic interfaces.
III. M
ODELING WEB SERVICES INTERACTION WITH
P
ETRI NETS
We assume that a reader is familiar with basic Petri nets
[33-37]. The following definition is used.
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