A. Aldeeb, K. Crockett and M. J. Stanton on
Department of Computing and Mathmatics
Manchester Metropolitan University, Chester Street, Manchester, M1 5GD, U.K.
Keywords: Dynamic agents, Peer-to-Peer, Exception handling, Workflow adaptability.
Abstract: Peer-to-Peer (P2P) technology is being recognized as a new approach to decentralized workflow
management systems to overcome the limitation of the current centralized Client/Server workflow
management systems. However, the lack of supporting adaptability and exception handling at instance level
of this approach seems to be responsible for the weakness of the P2P workflow management systems.
Dynamic agents can be used within P2P workflow management systems architecture to facilitate
adaptability and exception handling. This paper presents a novel dynamic-agent P2P workflow management
system which integrates three major technologies: software agents, P2P networking and workflow systems.
The adoption of dynamic agents within P2P network can help in overcoming the adaptability problem,
reducing the need for human involvement in exception handling and improves the effectiveness of the P2P
workflow management system.
Conventional workflow management systems are
based on centralized client/server architecture. This
requires a centralized database to store the workflow
process definition and a centralized workflow engine
to manage activities such as coordination and
monitoring process execution (Montagut, 2008; Yan
, 2006). The main disadvantages of any such
architecture are the potential bottleneck that can
arise during process execution, and that the central
database can become single point of fault.
In recent years, the integration of workflow and
agent technology has attracted the attention of many
researchers as a means to support distributed
business processes in a dynamic and unpredictable
environment (Mo, 2006; Buhler, 2005; Zhang, 2004;
Müller, 2004; Wang, 2005; Cao, 2005). Agents are
persistent active entities that have the properties of
autonomy, reactivity, and pro-activity and can
perceive, reason, and communicate with other agents
(Singh, 1999). In addition, agents have the capability
to dynamically form social structures through which
they share commitments to the common goal of
workflow enactment by forming a collective entity
called Multi-agent systems (Buhler, 2005). Recent
research has shown an increased interest in P2P
based workflow systems (Aldeeb, 2007; Yan, 2006)
to decentralize workflow systems. P2P is a means of
developing distributed applications where different
nodes, or peers, share resources and have symmetric
roles of either server or client. P2P workflow
management systems (WFMSs) are proposed to
avoid the bottleneck and the central point of fault
caused by centralized client/server workflow
systems. P2P based workflow can also be used to
improve scalability, system openness and support
incompletely specified processes (Yan, 2006). In
P2P based WFMS peers join “virtual communities”
according to their capabilities and discover each
other using the services provided by an open P2P
network. The coordination between peers is
performed by exchange of notification messages.
However, the lack of supporting adaptability and
exception handling at instance level of this approach
seems to be responsible for the weakness of the P2P
WFMS. This paper presents a novel dynamic-agent
P2P WFMS system which integrates three major
technologies: software agents, P2P networking and
workflow systems. The adoption of dynamic agents
within P2P network can help in overcoming the
adaptability problem, reducing the need for human
involvement in exception handling and improves the
effectiveness of the P2P WFMS.
Aldeeb A., Crockett K. and J. Stanton M. (2009).
In Proceedings of the 11th International Conference on Enterprise Information Systems - Software Agents and Internet Computing, pages 167-170
DOI: 10.5220/0001999101670170
The rest of this paper is organized as follows.
Section 2 describes the problem and states the
solution adopted. Section 3 describes the proposed
dynamic-agents P2P WFMS. Section 4 illustrates the
adaptability and exception handling process. Section
5 introduces a case study. Finally in section 6, some
concluding remarks are given.
In the centralized Client/Server WFMS the
exceptions are handled and managed by the server.
However, in a P2P WFMS that is fully distributed,
there is no central server and so the exception
handling task poses a major challenge. The main
shortcoming of the proposed P2P WFMS is that it
does not fully support adaptability and exception
handling at instance level. The reason for this
limitation is that peers are bound to a specific
limited set of actions, protocols or messages
specified in the protocols defined for workflow
application (Willmott, 2004). In addition, rational
and cooperative behaviour cannot be guaranteed,
because workflow peers are attached to human
workflow participants. In (Aldeeb et al., 2007) a
mechanism for adaptability and exception handling
in P2P WFMS is proposed. This mechanism is based
on separating the business logic and exception
handling logic using a super-node with exception
handling capabilities (Exception Handling Peer -
EHP) to make it easy to keep track of both. The EHP
captures exceptions, from the workflow peers,
characterises the exceptions and applies a recovery
policy. However, from the initial findings of this
research, extra traffic overhead is generated through
the P2P network due to the P2P conversation
between the Exception Handling Peer and the other
peers which have been affected by an exception.
This can lead to an exponential growth of messaging
load. As a result, the peer’s bandwidth connection
may be exceeded which will affect the overall
system performance. Chen (2000) and Cao (2005)
show that software agents can help in conducting the
run-time function of WFMS, monitoring,
adaptability and exception handling. In addition,
they can move to the appropriate location for high-
bandwidth conversation. This was the motivation of
this research to superimpose the software agents
paradigm on the P2P architecture, where agents can
reside in different peers and perform workflow tasks
utilizing their decision support capabilities, the
collective behaviour, and the interaction protocols of
the P2P system. Furthermore, P2P infrastructure
supports dynamic service construction, modification
and movement, and allows a dynamic agent to
participate in exception handling procedures.
The dynamic agent can be defined as the
software agent that supports dynamic behaviour
modification and does not have fixed set of
predefined functions, however, it carries application-
specific actions, which can be loaded and modified
on the fly (Chen, 2000). Dynamic agents can,
therefore, be used to mediate between the workflow
peers and the exception handling peer to automate
the exception handling process in P2P WFMS. Thus
using dynamic agents within a P2P WFMS can
facilitate system optimization; reducing the
communication overhead, reducing the human
involvement in exception handling and improving
the effectiveness of the P2P workflow management
A high level architecture of the dynamic-agents
based P2P WFMS is shown in Figure 1. This system
is upgraded from previously designed P2P WFMS
(Aldeeb, 2007). The detailed mechanism and the
functionalities of the peer-to-peer workflow system
can be found in (Yan, 2006; Aldeeb, 2007).
Figure 1: Dynamic-Agent based P2P WFMS.
The new dynamic-agent version of the system is
proposed to take the exception handling function out
of the P2P system level to a dynamic-agent level.
The exception handling function includes: exception
ICEIS 2009 - International Conference on Enterprise Information Systems
monitoring, exception detection and exception
handling. The build-time function is conducted at
P2P level which includes workflow process
modelling, storing process definitions and
distributing the process to workflow peers. At run-
time, one of the workflow peers will be responsible
for Instantiating a workflow instance which includes
several tasks. The coordination of tasks will be
achieved by message exchange between peers.
Dynamic agents reside at workflow peers to conduct
exception handling function of the system. As
shown in figure 1, the P2P network provides
services that include advertisement services, group
services, peer services, pipe services, and discovery
services. In addition, it facilitates a user interface
with human workflow participant. The workflow
peers can be classified into three types based on their
capabilities: two super-nodes: Workflow Definition
Peer (WFDP) and Exception Handling Peer (EHP).
The third type is an ordinary Workflow Peer (WFP).
The WFDP facilitates the design and the storage
of the whole workflow schema at build-time. The
workflow process is partitioned to separate tasks
according to the roles of the workflow participants
and the organizational structure. These tasks are
distributed to associated peers.
Figure 2: The workflow exception handling peer.
The EHP has the capability to deal with various
types of exceptions. Figure 2 shows the internal
structure of the exception handling peer. The
presence of this peer is essential for the P2P
workflow management system and if this peer
became unavailable, the system must wait for
another peer with exception handling capabilities to
join the group. The EHP generates and deploys
dynamic agents corresponding to workflow peers.
When an agent is deployed, it registers its symbolic
name and address at its destination node and keeps
an address book for other agents. The dynamic
agents delegate the exception handling functions of
correspondent workflow peers. The exception
handling procedure is discussed in the next section.
The WFP can reside on any machine on the P2P
network enabling direct communication with other
workflow peers to enact the workflow process. Each
WFP performs one task or more of the whole
workflow process. The WFP is associated with a
human workflow participant involved in the
workflow process enactment. Process co-ordination
is achieved by the exchange of messages between
Workflow adaptability is the ability of the workflow
processes to react to exceptional circumstances
(Sadiq, 2005). Exceptions that occur during
workflow execution have been divided into: basic
failures, application failures, expected exceptions
and unexpected exceptions (Casati, 1998). Basic
failure is related to failures at system level (e.g.,
DBMS, operating systems, or network failure).
Application failure corresponds to failures of any
applications invoked by the WFMS. In the proposed
Dynamic-agent based P2P WFMS, two workflow
exceptions can be identified according to their
location: local workflow exception and global
workflow exception. Local workflow exception
affects the task of one workflow peer. The workflow
peer can handle this exception using its local
dynamic-agent and by applying one of two possible
self-recovery policies; forward recovery or
backward recovery. If the local workflow exception
can not be handled within the affected workflow
peer, its effect may propagate to the other peers
leading to a global workflow exception. These types
of exceptions will, of course, affect more than one
node and a coordinating node is required to deal
with this exception. The coordinating node which
has the capability to deal with the exceptions is the
exception handling peer. This peer generates and
deploys dynamic agents loaded with updated
exception handlers based on prior knowledge. Once
these dynamic agents are deployed to the workflow
peers, they communicate with each other to capture
exceptions, characterize the exceptions and apply a
recovery policy. In addition, these dynamic agents
can be mobile agents which have the capability to
move between the EHP and the exception raising
peers. This can lead to the reduction of the amount
of the communication between nodes as the
interaction will take place locally at the exception
raising node (Cao, 2005). In addition, The EHP will
acquire knowledge from previous exceptions and
updates its exception handling knowledge base to
deal with similar exceptions in future. The EHP is
provided with a Case Based Reasoning (CBR) unit
to handle exceptions which need to be managed in
similar way, but may occur in different instances.
The EHP reloads the dynamic agents with the
acquired exception handling logic in order for these
agents to react to similar exceptions.
To better illustrate how the proposed Dynamic-agent
based P2P WFMS works, an example of a motor
insurance claim process is used. The workflow tasks
are distributed over the workflow peers, based on the
roles of the workflow participants and structure of
the organization. After the tasks are distributed, each
workflow peer is aware of its own tasks and the
input and output workflow attributes. The Workflow
tasks in this case study are modelled in Petri Net.
Java has been chosen as the programming language
for this prototype. The P2P network environment is
based on the Java coded Sun Microsystems JXTA.
The workflow peers engines are based on Bossa
workflow engine. Bossa is an open source and
lightweight workflow engine written in Java. In
addition it does not require a RDBMS and is very
simple to use and to integrate with java applications.
For these reasons Bossa is chosen for the prototype
implementation and four Java classes; Place,
Transition, Edge and PetriNet can be adapted for the
P2P environment. To examine the system, build-
time, run-time and exception handling functions are
being implemented.
This paper has proposed a Dynamic-agent based P2P
Workflow Management System. This builds on our
previous work on P2P WFMS. P2P systems and
software agents can fit together for workflow
application. We believe that these two technologies
will play a key role in the future of workflow
systems design and implementation. Dynamic agents
can be used to mediate the workflow peers and the
exception handling peer to automate the exception
handling process. Future work includes: completion
of the different components of prototype and
evaluation of the effectiveness of the system.
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