=Paper=
{{Paper
|id=Vol-494/paper-9
|storemode=property
|title=WADE: An Open Source Platform for Workflows and Agents (Extended Abstract)
|pdfUrl=https://ceur-ws.org/Vol-494/mallowawesomepaper9.pdf
|volume=Vol-494
|dblpUrl=https://dblp.org/rec/conf/mallow/CaireQS09
}}
==WADE: An Open Source Platform for Workflows and Agents (Extended Abstract)==
https://ceur-ws.org/Vol-494/mallowawesomepaper9.pdf
WADE – An Open Source Platform for Workflows and
Agents
G. Caire, E. Quarantotto, G. Sacchi
Telecom Italia
Via Reiss Romoli 274
10148 Torino - Italy
Categories and Subject Descriptors 2. WADE OVERVIEW
I.2.11 {Artificial Intelligence]: Distributed Artificial Intelligence WADE (Workflow and Agent Development Environment) is a
- Multiagent systems; C.2.4 {Computer Communication domain independent platform, built on top of JADE 5, an open
Systems]: Distributed systems; D.2.11 [Software Engineering]: source middleware for the development of distributed applications
Software architecture based on the agent-oriented paradigm. The distribution of JADE
includes a runtime environment, a library of classes that
programmers can use to develop their application and some
General Terms graphical tools for administration and monitoring purposes.
Management, Performance, Languages.
Each running instance of the JADE runtime environment is called
Container and a set of containers is called Platform. In a JADE
Keywords Platform a single special Main Container must always be active
Software Agent, workflow, JADE, Open Source, XPDL, and the other containers register with it at startup.
Scalability, Flexibility.
One or more application agents can be started into a Container.
The actual job of an Agent is to perform some tasks assigned to it.
1. INTRODUCTION In JADE, a “Behavior” represents a task to be performed by an
WADE is the main evolution of JADE and adds to it the ability to Agent and it is implemented as an object of a class that extends
define system logics according to the workflow metaphor. the class Behaviour of the JADE library.
WADE is not just an add-on, but a complete platform (built on An Agent to perform its tasks may need to communicate with
top of JADE) providing advanced administration and fault other Agents in the Platform. JADE provides the agents with the
tolerance mechanisms and enabling a group of agents to ability to communicate. The communication model adopted is the
cooperatively execute complex tasks defined as workflow. “Asynchronous Message Passing” and the format of the messages
Nowadays workflows are mostly adopted in BPM (Business is the ACL (Agent Communication Language) defined by FIPA 5.
Process Management) environments where they are used to WADE adds to JADE the support to the workflow execution and
represent business processes and orchestrate existing systems a few mechanisms to manage the complexity of the distribution, in
typically (but not necessarily) accessible by means of Web terms of administration and fault tolerance. It should be noticed
Services-based interfaces. that a WADE-based application may even not use Workflow
The main challenge in WADE is to bring the workflow approach Engine agents at all and just exploit the administration and fault
from the business process level to the level of system internal tolerance features. In that case we end up with a WADE-based
logics. That is, even if in principle it could be used for that application that does not use workflows. On the other side, it is
purpose too, WADE does not target high level orchestration of also possible to use the workflow metaphor inside of a Jade
services provided by different systems, but the implementation of platform.
the internal behaviour of each single system. Each agent embeds a Wade specific components are:
micro-workflow engine and a complex process can be carried out
by a set of cooperating agents each one executing a piece of the • BootDaemon processes: there is a bootDaemon process
process. for each host in the platform and it is in charge of the
Containers activation in its local host.
The approach followed by WADE is to provide a workflow view
on top of a normal Java class. That is a workflow is implemented • Configuration Agent (CFA): the configuration agent
as a Java class with a well defined structure. A key element in this always runs in the Main Container and is responsible
approach is WOLF (WOrkflow LiFe cycle management for interacting with the boot daemons and controlling
environment), the graphical development environment for WADE the application life cycle.
based applications. • Controller Agents (CA): there is a controller agent for
The presentation will provide an overview of the WADE each container in the platform and they are responsible
platform, will highlight its most distinguishing features such as for supervising activities in the local container and for
workflow inheritance, support for web services and delegation. all the fault tolerance mechanisms provided by WADE.
• Workflow Engine Agents (WEA): Workflow Engine Additionally, when processes have to be executed within the
Agents embed an instance of the micro workflow engine scope of a transaction, semi-automatic rollback procedures can be
and therefore they are able to execute workflows. activated in case of unexpected fault. Finally, since workflows are
fully self-documented, workflow-based development releases the
development team of the burden of keeping documentation
aligned each time design choices must be revisited to face
implementation details or evolving requirements.
Nowadays the workflow metaphor is mostly used in BPM
environments where a workflow represents a business process and
orchestrates a number of existing systems typically (but not
necessarily) accessible by means of Web Services based
interfaces.
The main challenge in WADE is to bring the workflow approach
from the business process level to the level of system internal
logics. That is, even if it could be used for that purpose too,
WADE does not target high level orchestration of services
provided by different systems, but the implementation of the
internal behaviour of each single system.
First of all it should be noticed that WADE does not include a
Figure 1 – a WADE platform single powerful workflow engine as the majority of BPM oriented
Figure 1 shows the topology of a WADE-based application. tools. On the contrary WADE provides an extension of the basic
WADE specific components are highlighted in blue. Agent class of the JADE library called
WorkflowEngineAgent that embeds a small and lightweight
As mentioned, workflows are represented in Wade as Java classes,
workflow engine (we talk about "micro-workflow engine"). As a
so, in principle, Wade supports “notepad-programming” in the
consequence, besides normal JADE behaviours, all Workflow-
sense that there is not hidden stuff that developers can’t control.
Engine agents active in a WADE-based multi-agent applications
However, expecially considering that one of the main advantages
are able to execute workflows represented according to a WADE
of the workflow approach is the possibility of representing
specific formalism.
processes in a friendly graphical form, Wade comes with a
development environment called Wolf that facilitates the creation The second important point to highlight is that, in order to allow
of Wade-based applications. Wolf is an Eclipse plug-in and as a developers to exploit the workflow metaphor to define system
consequence allows Wade developers to exploit the full power of internal logics and, at the same time, to give them the same power
the Eclipse IDE plus additional Wade specific features. of a software programming language and a comparable execution
efficiency, the WADE workflow representation formalism is
3. Workflow approach based on the Java language. That is, a workflow that can be
A workflow is a formal definition of a process in terms of executed by WADE Workflow-Engine agents is expressed as a
activities to be executed, relations between them, criteria that Java class with a well defined structure (as it will be explained in
specify the activation and termination and additional information the following). As such WADE workflows can be edited,
such as the participants, the software tools to be invoked, required refactored, debugged and in general managed as all Java classes
inputs and expected outputs and internal data manipulated during and can include all pieces of code (methods, fields of whatever
the execution. types, inner classes, references to external classes and so on)
needed to implement the process details. In addition, of course,
The key aspect of the workflow metaphor is the fact that the
the execution flow they specify can be presented and modified in
execution steps as well as their sequencing are made explicit. This
a friendly, graphical way. More in details WOLF (the
makes it possible to give a graphical representation of a process
development environment for WADE based applications) is an
defined as a workflow. Such representation is clearly extremely
Eclipse plugin and allows developers to work with a graphical
more intuitive with respect to a piece of software code and in
view (suitable to manage the process flow) and a code view (the
general is understandable by domain experts as well as by
usual Eclipse Java editor suitable to define execution details) that
programmers.
are kept in synch.
Domain experts can therefore validate system logics directly and
Finally it must be noticed that WADE does not impose that all
not only on documents that most of the time are not perfectly up
system logics are defined as workflows. Developers are free to
to date. In some cases they could even contribute to the actual
exploit the workflow metaphor to describe those tasks for which
development of the system without the need for any programming
they think it is appropriate and use normal JADE behaviours (or
skill.
other purely Java patterns) elsewhere.
Another important characteristic is that, being the execution steps
As mentioned the approach followed by WADE is to provide a
explicitly identified, the workflow engine (i.e. a system able to
workflow view on top of a normal Java class.
automatically execute a process defined as a workflow) can trace
them. This makes it possible to create automatic mechanisms to As a consequence, a workflow is implemented as a Java class and
facilitate system monitoring and problem investigation. no standard workflow definition language is used. However, Wolf
adopts the workflow meta-model defined in the XPDL [2],
standard specified by the Workflow Management Consortium. the agent executing the subflow acts as responder. This
The XPDL meta-model has been chosen, because the XPDL protocol allows managing, when required, a process
language has been conceived as interchange formalism between carried out by a set of cooperating agents as a single
different systems. WADE supports the import of XPDL files and transaction.
the adoption of this meta-model facilitates these operations.
• Route Activity: a route activity is an activity which
Moreover, the XPDL meta-model is based on a Finite State
performs no work processing, but simply supports
Machine computational model that is the same model supported
routing decisions among its incoming and out coming
by the WADE agents.
transitions.
In the XPDL meta-model a process is represented as a workflow,
Finally, WADE introduces a few types of Activity, among
consisting of one or more activities that can be thought as tasks to
them the CodeActivity and the Web Services Activity.
be executed.
In a Code Activity the operations are specified directly by a
In a workflow, the execution entry point is defined, specifying the
piece of Java code embedded in the workflow process
first activity to be performed; this activity is called Start Activity.
definition.
On the other hand, a workflow must have one or more termination
points, named Final Activities. A Web Service Activity, instead, provides an easy way to
invoke Web Services from a workflow. Two kinds of Web
The execution flow is defined by means of transitions. A Services invocations are allowed: static and dynamic.
transition is an oriented connection between two activities and
may have a condition associated. Regular or exception transitions In the first case, it is necessary to import the WSDL
can be defined. Exception Transitions allow specifying branches describing the Web Service using WOLF. This operation
that are taken only when an Exception is raised in the source generates a set of classes that will be used at workflow
activity. execution time to actually invoke the web service described
Excluding the final ones, each activity may have one or more by the imported WSDL. These classes can be extended by
outgoing transitions. When the execution of an activity is users with development skills, in order to satisfy some
terminated, the conditions associated to its outgoing transitions specific requirements (e.g.: logging some information before
are evaluated. As soon as a condition is verified the corresponding and after the invocation of the Web Service).
transition is activated and the execution flow proceeds towards the In the second case, the dynamic invocation is performed
destination activity. without the need of generating new classes. This approach is
Normally a process execution uses some internal data, for useful if the user doesn’t need to customise these classes.
instance, to pass intermediate results between activities and/or for As mentioned a workflow is implemented by a Java class and it
evaluation of conditional expressions. In the XPDL meta-model extends directly or indirectly the WADE class
internal data are modeled by Data Fields. WorkflowBehaviour.
A process can have one or more inputs to be provided and one or The WorkflowBehaviour class provides a set of APIs,
more outputs expected at the end of its execution. Inputs and consistent with the XPDL meta-model described in the previous
outputs of a process can be formalized in the XPDL meta-model section, which can be used by developers to implement their own
by means of the workflow Formal Parameters. workflows.
The XPDL meta-model defines some predefined types of activity. The mapping between the meta-model objects and the workflow
The most important ones are: implementation is shown in the example depicted in Figure 2.
• Tool Activity: a tool activity is an activity that is
implemented by means of the invocation of one or more
software tools, named Applications.
• Subflow Activity: a subflow activity is an activity that
requires the execution of another workflow. When a
workflow is called by a subflow activity, the workflow
formal parameters permit the exchange of necessary
data between calling and called process. A
distinguishing characteristic of the WADE workflow
engine is the Delegation mechanism that allows a set of
agents to cooperatively execute a complex process.
More in details the agent executing the calling workflow
can decide to delegate the subflow to another agent on
the basis of conditions evaluated at runtime. Such
conditions may be related for instance to the current
load (thus supporting the implementation of a GRID-
like system) or to specific abilities required to carry out
a portion of the whole process. The delegation
mechanism is implemented by means of a an extension
of the fipa-contract-net protocol (5) where the agent
executing the calling workflow acts as initiator while
the possibility of modeling the agents’ behaviours following the
public class SendMail extends WorkflowBehaviour {...
workflow metaphor. Moreover some administration and fault
protected void defineActivities() { SEND MAIL PROCESS tolerance features are provided too.
//ACTIVITY Prepare Mail … As mentioned, in WADE a workflow is represented as a java class
registerActivity(prepareMail, INITIAL); and WOLF provides a graphical view of it, making available to
Prepare Mail the developers both the expressiveness of a visual representation
//ACTIVITY Verify Mail…
and the power of usual programming languages.
registerActivity(verifyMail);
As well known, the workflow metaphor is traditionally used in the
…} Send Verify Mail BPM context for web service orchestration. Because WADE
provides support for Web Service invocation, it can be used even
protected void defineTransitions() { in this context, but its actual challenge is to bring the workflow
approach from the business process level to the level of system
registerTransition(new Transition(), PREPARE_MAIL, internal logics. A direct consequence of the approach described is
VERIFY_MAIL); Modify Mail that the full power of WADE can be exploited for applications
that imply the execution of possibly long and fairly complex tasks.
registerTransition(newTransition("IsCorrect", this),
VERIFY_MAIL, SEND_MAIL);} Furthermore, unlike the majority of existing workflow systems
that provide a powerful centralized engine, in WADE each agent
protected void executePrepareMail (final ApplicationList can embed a “micro workflow engine” and therefore a complex
applications) throws Exception {… process can be carried out by a set of cooperating agents through
the delegation mechanism.
}
protected boolean checkIsCorrect() {... From an industrial point of view WADE can be particularly useful
to develop applications with strong requirements of both
} performance and scalability and high flexibility in defining the
systems’ logics.
Figure 2 – Mapping between meta – model objects and
workflow implementation
5. REFERENCES
The methods registerActivity() and [1] JADE - Java Agent Development framework.
registerTransition() must be used respectively to add an http://jade.tilab.com
activity and a transition.
[2] FIPA – Foundation for Intelligent Physical Agents
When the registerActivity() method is called, its first
parameter is an instance of the activity to be added. http://www.fipa.org
The actual tasks to be performed by an activity (no matter of its [3] FIPA – The FIPA Contract Net interaction protocol
type) are specified in a void method of the workflow class; this
[4] XPDL XML Process Definition Language,
method must have the same name of the activity, preceeded by the http://www.wfmc.org/standards/xpdl.htm
prefix “execute” (execute). The
workflow engine is in charge of invoking that method when the [5] Shapiro, R. 2002. A comparison of XPDL, BPML and
activity is visited. BPEL4WS (Rough Draft), Cape Vision
In the same way, the registerTransition() method takes [6] BPMN Business Process Modeling Notation
a transition instance as parameter. For transitions with conditions, http://www.bpmn.org/
the boolean expression to be evaluated by the workflow engine
[7] G. Caire “WADE: An Open Source Platform for Workflows
is specified in a boolean method that has same name of the
and Agents”
condition, preceeded by the prefix “check”
(check). http://jade.tilab.com/wade/doc/tutorial-aamas2008.zip
An important feature of the object-oriented programming [8] Caire G., Gotta D., Banzi “WADE: A software platform to
languages is the possibility to reuse the code by means of develop mission critical applications exploiting agents and
inheritance mechanisms. In order to bring the programming workflows”, M., Proc. of 7th Int. Conf. on Autonomous
languages power also to the workflow representation, it has been Agents and Multiagent Systems (AAMAS 2008) – Industry
choosen to provide the workflow with the inerithance mechanism and Applications Track, Berger, Burg, Nishiyama (eds.),
too. Therefore it is possible to define a new workflow extending May, 12-16., 2008, Estoril, Portugal, pp. 29-36.
an old one, and then adding/removing activities and transitions. [9] “WADE User Guide”
The overriding of methods associated to the activities and to the
http://jade.tilab.com/wade/doc/WADE-User-Guide.pdf
conditions of transitions is also permitted.
[10] G. Caire, M. Porta, E. Quarantotto, G. Sacchi “ Wolf – An
4. CONCLUSIONS Eclipse Plug-in for WADE”
In this paper we described WADE, an open source framework to
develop distributed applications based on the agent programming
paradigm. WADE is based on JADE and in particular adds to it