In this work we present a research line that focus on the design and implementation of a Directory Facilitator for a FIPA multi-agent platform. We will introduce three different approaches: a centralized version, a distributed one, and a hybrid solution. As we will explain below, the hybrid approach will have several advantages and it will be our choice for future implementations.Eje: Inteligencia artificialRed de Universidades con Carreras en Informática (RedUNCI

García, Alejandro Javier

Martínez, F. A.

English

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A Hybrid Approach for Directory Facilitators
in a FIPA Multi-agent Platform
Fernando A. Martin
fam@cs.uns.edu.ar
Alejandro J. Garc´ıa
ajg@cs.uns.edu.ar
Grupo de Robo´tica Cognitiva
Laboratorio de Investigacio´n y Desarrollo en Inteligencia Artificial (LIDIA)
Departamento de Ciencias e Ingenier´ıa de la Computacio´n
Universidad Nacional del Sur
Avda. Alem 1253 - (B8000CPB) Bah´ıa Blanca
Tel: ++54 291 4595135 - Fax: ++54 291 4595136
In this work we present a research line that focus on the design and implementation of
a Directory Facilitator for a FIPA multi-agent platform. We will introduce three different
approaches: a centralized version, a distributed one, and a hybrid solution. As we will explain
below, the hybrid approach will have several advantages and it will be our choice for future
implementations.
1 The FIPA Agent Platform
This investigation is based on the standards provided by the Foundation for Intelligent Physical
Agents (FIPA) [4]. According to FIPA, an Agent Platform (AP) should provide the physical
infrastructure in which agents can be deployed. This infrastructure should contain, besides the
hardware, three main logical component: A Message Transport System, an Agent Management
System, and the Directory Facilitator (see Figure 1). We will briefly describe them next. More
details can be found in the FIPA Agent Management Specification [3].
The Message Transport System (MTS) is the default low-level communication between
agents, not only in the same AP, but between agents in different AP’s. The Agent Management
System (AMS) is a mandatory component of the AP. Each AP will have only one AMS and its
primary objective is to offer white pages services to the agents in the AP. The AMS has the
location of every agent in the system, and his function is to provide to authorized agents the
location of anyone in the platform. The Directory Facilitator (DF) provides yellow pages to
other agents. Agents may register their services with the DF or query the DF to find out what
services are offered by other agents.
Partially supported by SGCyT Universidad Nacional del Sur, and Agencia Nacional de Promocio´n Cient´ıfica
y Tecnolo´gica (PICT 2002 Nro 13096)
Figure 1: FIPA Agent Management System
In this research line we will consider the design and implementation of DF. Next we will
introduce the main features that FIPA propose, and in the following sections three different
approaches for its implementation will be described.
The primary objective of a DF is to provide an environment where agents may register
their services or query for the services that are offered by other agents in the AP. The DF, like
any other agent in a FIPA AP, uses the FIPA ACL Message Structure [2] as standard way of
communication. Agents may interact with a DF with four different communicative acts:
Register: The execution of this function has the effect of registering a new object into the
knowledge base of the DF. The DF cannot ensure the accuracy of the information that
the agent is registering.
Deregister: An agent may request to remove an item of the knowledge base of the DF. This
act liberates the obligation of the DF to broker the information related to the agent.
Modify: In this case, the agent may make a modification of an object that in the past has
been inserted in the knowledge base.
Search: An agent may ask to the DF to find one or more agents that can satisfy a particular
service. Again, the DF cannot guarantee the accuracy of the information provided in
response of the search act.
Many DFs can exist in one AP, and DFs may register with each other to maintain a global
consistency of the knowledge base. Though the a facilitator is responsible for granting (or deny-
ing) access to the information, it has no responsibility whether the content is either accurately
or valid. The decision of the distribution of de DFs is mandatory, and has a direct influence on
the overall performance of the AP. Our research will focus on different possible implementations
for a Directory Facilitator.
In the following sections two possible approaches for DFs are described and analyzed. A
brief explanation of how this two possibilities can be merged to fit the requirements of an Agent
Platform will be also introduced.
2 A centralized approach
The simplest way to implement the Directory Facilitator is with only one agent in the entire
Platform. Furthermore, possibly the Agent Management System (the one who is unique and
has the information about the location of all agent in the AP) and the Directory Facilitator
can be merged into one agent. This alternative has two main advantages: simplicity, and the
problem of maintaining consistency is solved.
Like any other centralized approach, leaving the responsibility in only one agent has many
drawbacks, including, amongst others, poor fault tolerance, scalability related issues, and,
depending on the size of the AP, the DF may become a bottleneck in the system. Depending
on the population of the platform, and where the agents are physically located, the performance
of the centralized approach can drop to unacceptable levels.
This approach can be useful for a platform for a multi-agent system with a small number
of agents. This kind of systems avoid the bottleneck issue, and have the advantage of the
consistency of the knowledge base, making this implementation feasible. For a system which has
no bounds on the number of agents, and furthermore, in an environment where the population
can grow (or diminish) dynamically, this approach is not viable.
3 A distributed approach
The other possibility is having several DFs in the system. For every agent in the AP, there is a
daemon running on the same machine where the agent is. In this approach, the primary goal
is to provide a simple and fast communication between the agent and the facilitator (in this
section we use daemon or DF indistinctly). The daemon has two main responsibilities:
• The first is maintaining his knowledge base as complete as possible. The agent depends
entirely on the DF, and the only way for the agent to know the services of other agents in
the AP is through it. In order to achieve simplicity and velocity (main goals of the this
approach), the agent shouldn’t wait while the daemon broadcasts the search to the other
DFs in the AP, instead, the daemon has to respond the act immediately. To accomplish
this, the DF periodically asks to other facilitators about the services of their agents.
• The DF has the responsibility of publishing the services of the agent to other facilitators.
The success of the agent relies on that. Any kind of malfunctioning of a daemon (external,
just as message problems, or internal, i.e. bad coding of the DF) could affect a group of
agents, isolating them.
The benefit of this approach is the simplicity for the agent to gather information of the
environment: the agent only needs to communicate with one daemon, and he gets te response
immediately. Another advantage is that a failure on one daemon do not affect the entire
platform, giving to the system a very high degree of fault tolerance.
Although this approach solves the bottleneck problem stated above, an even worse problem
could arise: every daemon has to continuously communicate with every other daemon in the
entire AP generating overload. Like the centralized approach, the performance of this system
diminishes as the number of DF grows.
The number of DF in a distributed approach should be balanced: if there are only a few,
each DF inherits the problems of the centralized approach; if there are too many, the overload
can be unacceptable. The number of DF could be fixed, or decided dynamically by some agent.
4 A hybrid approach
The performance of both approaches described above relies on the number of agents in the AP.
A fixed number of DF is not a feasible solution for an AP where the population of agents can
grow dynamically.
Instead of having a fixed number of DF, our proposal is letting the AMS (Agent Management
System) decide when the AP needs another Directory Facilitator. When an agent wants to join
an AP, he must send a proper message to the AMS. At this point the AMS can decide whether
to create or not a new facilitator in order to balance the number of DF. The AMS will respond
to the agent with the address of a facilitator (either is new or not).
In this approach an agent will interact (register, deregister, modify or search for services)
with only one DF. Thus, the AMS will know the number of agents associated with each DF. It
is important to note, that the AMS only recommends one DF for the agent. The agent is free
to ask for the location of any other DF in the AP to interact with. However, the AMS may
refuse to associate an agent with an overloaded DF.
The decision process of the AMS can be implemented in many ways. One possibility is
having one DF in each subnet. This grants fast communication with entities in the same
physical area. When an agent request a search, the DF can try to satisfy the service required
with an agent of the same subnet, if the service cannot be satisfied in the area, the facilitator
may ask for the service to the other DFs in the AP. This approach solves, in part, the bottleneck
issue, and makes the system easier to extend. In case that one subnet is highly overloaded, the
AMS may decide to create another DF to diminish the overload.
We are planning to implement this hybrid approach using the framework reported in [5].
References
[1] Agust´ın Alejandro Denegri. “Comunicacio´n en Sistemas Multiagentes“. Te´sis de Magister
en Ciencias de la Computacio´n. Computer Science Department, Universidad Nacional del
Sur, Bah´ıa Blanca, Argentina, July 2002.
[2] FIPA. FIPA ACL Message Structure Specification. http://www.fipa.org/specs/fipa00061/.
[3] FIPA. FIPA Agent Management Specification. http://www.fipa.org/specs/fipa00023/.
[4] FIPA. Foundation for Intelligent Physical Agents. http://www.fipa.org.
[5] Mariano Tucat. “Primitivas de Interaccio´n para el Desarrollo de Sistemas Multi-agente“.
Proyecto Final de Ingenier´ıa en Sistemas de Computacio´n. Computer Science Department,
Universidad Nacional del Sur, Bah´ıa Blanca, Argentina, March 2005.


A hybrid approach for directory facilitators in a FIPA multi-agent platform

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A hybrid approach for directory facilitators in a FIPA multi-agent platform

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