An Effective Search Method for Distributed Information
Systems Using a Self-Organizing Information Retrieval Network
Kouichi Abe and Toshihiro Taketa
Department of Electrical and Information Engineering, Faculty of Engineering,
Yamagata University, Yonezawa, Japan 992-8510
Hiroshi Nunokawa
Faculty of Software and Information Science, Iwate Prefectural University, Morioka, Japa n 020-0173
Norio Shiratori
Research Institute of Electrical Communication, Tohoku University, Sendai, Japan 980-8 577
SUMMARY
In this paper the authors propose a search system
composed of a self-organizing information retrieval net-
work as a method to effectively search for information
managed in a distributed fashion. An information retrieval
network is a virtual network consisting of World Wide Web
(WWW) server hosts that have search functions. A search
system making use of the features of the authors� method
can provide the following benefits: (1) information searches
on a newly added WWW server host; (2) information
searches that avoid a problem host; (3) information searches
from any host. In this paper the authors demonstrate the
validity of their idea by creating an information search
system based on their idea and performing experiments on
a university LAN. © 2000 Scripta Technica, Electron
Comm Jpn Pt 1, 84(3): 29�37, 2001
Key words: WWW; information retrieval network;
self-organization; agent; information search.
1. Introduction
Currently a significant amount of research is being
performed on information searches for information stored
in a distributed fashion on the World Wide Web (WWW).
Retrieval methods that use conventional information re-
trieval networks can only designate static paths to the host.
As a result, such methods are deficient in that they cannot
respond to information searches on a newly added host or
to problems on a network. Therefore, in this paper the
authors propose a search method composed of a self-organ-
izing information retrieval network in order to resolve the
above problems. Here, an information retrieval network
shall refer to a virtual network composed of various WWW
servers and hosts.* A search system for WWW information
created with this search method as its foundation will be
able to offer the following benefits as its self-organizing
features are used.
© 2000 Scripta Technica
Electronics and Communications in Japan, Part 1, Vol. 84, No. 3, 2001Translated from Denshi Joho Tsushin Gakkai Ronbunshi, Vol. J82-B, No. 5, May 1999, pp. 809�817
*A host running a WWW server.
29
(1) Searching for information newly added to a
WWW server host
A newly created WWW server host will automat-
ically be included into the information retrieval
n e t work by virtue of the composition of the self-
organizing information retrieval network, and the in-
formation on that WWW server host can then be
searched.
(2) Searching for information while avoiding
hosts with problems
In a WWW server host on a self-organizing in-
formation retrieval network, each WWW server host
itself can control the path information for the host. As
a result, even when a problem (e.g., host fails) arises
in the WWW server host in the system, an effective
information retrieval path can be determined by using
the estimates obtained from the search results (referred
to as the expected search value), and the information
search can be continued.
(3) Searching for information from any host
In a self-organizing information retrieval net-
work, each WWW server host manages by itself only
the WWW information in its own host in order to take
advantage of the self-organizing features. A database
is then created based on this information, and search
services are provided for the WWW information. Con-
sequently, the user can search for information on any
WWW server host on the system.
In this paper, the authors set up an Agent-based
Total Resource Access System (ATRAS) information
retrieval system designed based on search methods
composed of a self-organizing information retrieval
system in a university Local Area Network (LAN),
perform experiments to show the advantages obtained
using the search system for WWW information using
this method, and demonstrate the validity of this
system.
In Section 2 the basic technology for the authors�
method, including agents, self-management of distrib-
uted information, and migration searches, is de-
scribed, and the information retrieval mechanism
using a self-organizing information retrieval system
and the authors� method is explained. In Section 3 an
ATRAS information retrieval system designed to
prove the validity of the search method consisting of
a self-organizing information retrieval system is de-
scribed. In Section 4 information search experiments
using an ATRAS prototype system are performed in
order to demonstrate the validity of the proposed
method, and the results are described. In Section 5 the
authors consider the validity of the search system for
WWW information introduced by this method based
on the ATRAS experimental results. Section 6 offers a
conclusion and identifies future topics of study.
2. Self-Organization of an Information
Retrieval Network
In this section the authors first explain agents, self-
management of distributed information, and migration
searches, the basic technology necessary to effectively use
the self-organizing information retrieval network. Next the
authors describe a self-organizing information retrieval net-
work. Finally, the authors explain the information retrieval
mechanism using their method.
2.1. Agents
Considerable research is being done on information
searches using agents. However, the definition of an agent
varies with each different research project [3�10]. In Ref.
11, an agent is described as a computer system based in
hardware or software that is autonomous, social, adaptable,
and self-motivated. In Ref. 12 Internet agents are explained,
with autonomous, permanent, individual, cooperative,
adaptive, and dynamic mentioned as characteristics of an
agent.
In this paper, an agent shall be defined as a program
(or process) that is autonomous and social. Here, autono-
mous means that the agent determines its next action in
accordance with its own decisions. In other words, it oper-
ates independently of the user. Social means that the agents
can exchange data among themselves based on the particu-
lar communications protocols each agent uses (or via an
agent communications language). In addition, agents may
be equipped with features other than autonomous or social
behavior (for instance, dynamism).
In the authors� method five types of agents are used,
as shown in Fig. 1. Each agent is explained below.
(i) InfoManager
There is one InfoManager on each WWW server host.
It manages the WWW information on the host and provides
search services for WWW information (refer to Section
Fig. 1. Agents.
30
2.2). It creates an information search catalog (WWW infor-
mation database) from existing WWW search systems and
in the same fashion from HyperText Markup Language
(HTML) documents and then manages it. The information
catalog (WWW Database) is created and managed for each
system page and user page.
(ii) InfoSeeker
InfoSeeker performs information searches by mov-
ing between WWW server hosts on an information retrieval
network and then gathers together the results (refer to
Section 2.3). Internally it has a search history table that
stores the WWW server hosts that have been searched. The
maximum number of retrieved hosts, the maximum number
of retrieved hits, the maximum retrieval time, the maximum
number of migration errors, and the maximum number of
search results are also stored as information retrieval pa-
rameters. Here, the maximum number of retrieved hosts is
the maximum value for the number of WWW server hosts
that can be moved by InfoSeeker. The maximum number of
retrieved hits is the maximum number of search results that
can be collected. The maximum retrieval time is the time
limit for InfoSeeker to perform a retrieval. The maximum
number of migration errors is the allowed number of fail-
ures for movement between WWW server hosts. The maxi-
mum number of search results is a value that restricts the
number of search results returned from a single information
catalog to InfoManager. Among these parameters, all ex-
cept the maximum number of search results are used as
conditions for the termination of information retrieval.
(iii) SeekersManager
SeekersManager manages the InfoSeeker informa-
tion retrieval parameters and verification information.
(iv) GateKeeper
GateKeeper makes decisions regarding security man-
agement and host destinations. It also manages the list of
WWW server hosts (Hosts List) that InfoSeeker can move
to next. It creates the information retrieval network using
this host list (refer to Section 2.4). The host list is obtained
from the default GateKeeper only when GateKeeper is
launched for the very first time. There are several default
GateKeepers on the information retrieval network stored
for each individual GateKeeper. In addition, the informa-
tion retrieval network consisting of one GateKeeper as its
foundation is referred to as a domain. An information
retrieval framework (the foundation of an information re-
trieval environment using the authors� method) can be
expanded by connecting the GateKeepers in several do-
mains (refer to Fig. 2).
(v) Communicator
Communicator provides the user interface. It receives
search requests from the user, then issues search requests to
the SeekersManager and creates a list of the search results
for the user.
2.2. Self-management of distributed
information
Self-management of distributed information refers
to an information management method in which each
WWW server host manages only the WWW information
in that host, creates the information catalog, and provides
search functions for WWW information. This method
forms the foundation for the creation of the information
retrieval system that can make use of a distributed environ-
ment (load distribution, resource distribution, and so on).
This is realized through the management of WWW server
information and agents (InfoManager) that have search
functions.
2.3. Migration searches
Migration refers to mechanism in which an agent
itself moves to a remote host and then operates at that
destination. A migration search is a search method in which
an agent (InfoSeeker) moves between each WWW server
host on the information retrieval network while at the same
time performing information retrieval on each host and
collecting the search results. In addition, information re-
trieval using migration searches can be made parallel by
using more than one agent (InfoSeeker) during one search.
2.4. Self-organizing information retrieval
network
In this paper the information retrieval network is a
virtual network set up between each WWW server host
based on the self-management of distributed information.
Fig. 2. Information retrieval framework.
31
The agent (InfoSeeker) has a function that automatically
records the WWW server host at the migration point of
origin when it migrates from a WWW server host not in the
host list (the path table to the WWW server host that
InfoSeeker will move to next) in a WWW server host at
the migration destination. In this paper, this is referred
to as the self-organization of an information retrieval
network. This is realized through the following mecha-
nism in the agent (GateKeeper) that manages the host
list. If the host list managed by the agent (GateKeeper)
of host A is Alist and the host list managed by the agent
(GateKeeper) of host B is Blist, then Alist and Blist can be
represented as follows:
Alist = {�a1, w1�, . . . , �am, wm�} (m: the number of hosts)
Blist = {�b1, w1�, . . . , �bn, wn�} (n: the number of hosts)
Here, ai �i 1, . . . , m� and bj �i 1, . . . , n� are the WWW
server host names to which the agent (InfoSeeker) perform-
ing the migration search can move. wi �i 1, . . . , m� and
wj � j 1, . . . , n� are the estimates (hereafter referred to as
the expected search values) for which the search results are
obtained using the WWW server hosts ai and bj. If we set
the total amount of information that matches all of the
search requests up to the present time in the WWW server
host a to Ra, and the total amount of migration performed
to the WWW server host a from the current host to Ma, then
the expected search value wa for the migration destination
WWW server host a can be obtained from wa Ra /Ma. As
a result, the expected search value varies depending on the
results of the migration search. The number of hosts (m and
n) for which migration can occur from the current host is
referred to as the degree of self-organization. As the degree
increases, the number of candidates that can be selected as
the next host rises, and the possibility of finding the search
information grows larger.
If the next destination host Bout from the host B is the
host A, then Bout can be expressed as
Furthermore, if the partner host connected to host A (in this
instance host B) is Ain, then Ain can be expressed as
Here,
(�:=� means substitution)
refers to the self-organization of the information retrieval
network. In other words, the self-organization of the infor-
mation retrieval network means that the connections be-
tween each host are set up automatically. The expected
search value for a newly added host uses the average value
for the expected search values for the total number of hosts
already listed in the host table. Furthermore, when the
number of hosts in the host table exceeds the maximum
allowed number of records, hosts are erased in sequence
from the host with the smallest expected search value until
the maximum allowed number of records is no longer
exceeded. The decision mechanism for the next migration
destination host is as follows:
(1) The expected search value for each host in the host
table is calculated.
(2) The selection probability S for each host is calcu-
lated from the expected search value using the following
equation.
(3) The next migration destination host is determined
by making the rate at which each host is selected equal to
the selection probability.
This method avoids concentrating the migration des-
tination hosts in one area. In addition, compared to a deci-
sion method using simple random numbers, this method
can choose as the next host one with a high probability of
yielding search results. The next topic involves proving the
validity of this decision method.
2.5. Information retrieval mechanism using a
self-organizing information retrieval
network
Figure 3 shows the framework for information
retrieval using the self-organizing information retrieval
network described in Section 2.4. The operation of the
network will be explained step by step (numbers in
parentheses correspond to the numbers in Fig. 3). Note
also that the explanation uses the agent names given in
Section 2.1.
(1) The user requests information retrieval from
Communicator.
(2) Communicator passes the information request to
SeekersManager.
(3) SeekersManager gives instructions for the infor-
mation retrieval to InfoSeeker.
(4) InfoSeeker sends the search request to the In-
foManager in the HomeHost (the WWW server host with
InfoSeeker) and then receives the results.
32
(5) InfoSeeker queries the WWW server host to
which GateKeeper next moves. GateKeeper decides on the
WWW server host that InfoSeeker moves to next.
At this point,
(6) The GateKeeper at the destination WWW serv-
er host requests InfoSeeker verification from the In-
foSeeker HomeHost and confirms that InfoSeeker is
valid.
(7) InfoSeeker moves to the next WWW server
host.
(8) In the destination WWW server host, Info-
Manager searches for information that matches the condi-
tions requested by InfoSeeker and returns the results to
InfoSeeker.
(9) InfoSeeker repeats the information retrieval cycle
until the termination conditions for the information re-
trieval are satisfied.
(10) When the termination conditions are satisfied,
InfoSeeker returns to the HomeHost and then returns the
search results to Communicator.
(11) Communicator reports the search results to the
user after processing them to make them easy to view.
Note that if the search termination conditions are sat-
isfied at (4), the process may move to (10). Termina-
tion conditions like this can occur in the following
instances.
x When the maximum number of search hits is
exceeded in the local host alone.
x When there is no host to migrate to from the local
host and the maximum number of migration errors
is exceeded.
x When the maximum retrieval time is exceeded
in the local host.
In such instances, the search results are returned
and displayed when the termination conditions are
satisfied. The user can decide to perform the search
again after looking at the displayed search results.
3. ATRAS
The authors designed the ATRAS information re-
trieval system based on the method described in Section
2 as a way to demonstrate the validity of their proposed
method. Here a host with the agents described in Section
2.1 installed in a WWW server host shall be referred to
as an ATRAS host. The features and benefits of ATRAS
obtained by using the search method consisting of a
self-organizing information retrieval network are inher-
ited as is.
First, because the ATRAS host itself is a search
server host, it can update the information catalog using
only the information on the host itself as well as control
and search the most recent information. In addition, even
when a new ATRAS host is set up, it can be connected to
the information retrieval network automatically through
the GateKeeper�s functionality, and a retrieval path to the
new ATRAS host can be created on the information
retrieval network. As a result, the user can search for
information on the newest ATRAS host (information
retrieval on the newly added WWW server host). This
shows that ATRAS is capable of handling the daily ex-
pansion of information on the Web flexibly.
Next, through migration searches and the mutual
use of the information retrieval network, ATRAS can also
provide to the user stable information search services for
which host problems do not affect the ATRAS informa-
tion retrieval framework (information searches that avoid
host problems). This allows InfoSeeker to find a retrieval
path to a separate ATRAS host from GateKeeper even
when a problem occurs in the ATRAS host it is to move
to next, as well as being able to avoid an ATRAS host in
which problems have occurred.
Finally, as can be seen in Fig. 2, the user can
perform information searches from any ATRAS host
(information searches from any host). This is a result of
the agent (Communicator) that receives requests for in-
formation searches from each ATRAS host.
Fig. 3. Information retrieval mechanism using
self-organizing information retrieval network.
33
4. Experiments
4.1. Purpose of the experiments and
experimental methods
The purpose of the experiments is to show the validity
of the search system for WWW information using a search
method consisting of a self-organizing information retrieval
network. This will be demonstrated by verifying the
ATRAS functions as used in the proposed method. In the
experiments, the authors investigate whether or not the
ATRAS information retrieval framework has the following
features: (1) information searches on newly added WWW
server hosts, (2) information searches that avoid problem
hosts, and (3) information searches from any host. These
features represent the advantages obtained in a search sys-
tem based on the authors� proposed method. By investigat-
ing these features in ATRAS, the authors can demonstrate
the validity of search methods that consist of a self-
organizing information retrieval network.
The experiments were performed by installing
each agent was installed onto eight WWW server hosts
operating on an academic LAN and then creating the
ATRAS information retrieval framework. The host
agents were created using C++.
Next the methods used to verify each function on
the experimental system will be described.
Operation of the migration searches was confirmed
by verifying that the host name used for the searches in
the search history table in InfoSeeker were recorded.
Self-organization of the information retrieval network
was confirmed by verifying that the content of the host
table for each GateKeeper was updated. In order to verify
the capacity of an information search that avoids problem
hosts, the functions of GateKeeper in a particular ATRAS
host were stopped, and a situation in which a problem
occurred in that ATRAS host was created artificially. The
authors confirmed that at that time the information re-
trieval was performed while avoiding the ATRAS host in
which the problem arose, and they also confirmed that
information searches unaffected by the host�s problems
are possible. Furthermore, the authors confirmed that the
same kind of information search could be performed
from any ATRAS host on the ATRAS information re-
trieval framework for information searches from any
host.
4.2. Experimental results
Results are shown for experiments on the following
features in an information retrieval system using the
authors� method: (1) information retrieval for a newly
added WWW server host, (2) information retrieval that
avoids a problem host, and (3) information retrieval from
any host. The arrows in Figures 4 through 6 represent the
connections to the host in which the host table is recorded.
Bidirectional arrows refer to mutual connections.
First the authors confirmed that the basic migration
searches and the self-organization of the information
retrieval network were performed correctly. For instance,
Fig. 4 shows that when an information search is re-
quested from host A, a migration search is performed in
Fig. 4. Expanding information retrieval network.
Fig. 5. Information retrieval network after the new
server has connected.
34
the sequence A o D o E o G o B o A. This is one
path for the migration search as selected by the decision
mechanism described in Section 2.4. Therefore, the path
of the migration search varies with each search. Figure 4
shows the information retrieval network after the search
experiment was run five times.
4.2.1. Information retrieval on a newly
added WWW server host
When a new ATRAS host H is added to an infor-
mation retrieval network, H automatically requests the
host list table from the ATRAS host A where the default
GateKeeper resides. Here, if the host table for hosts A
and H are designated Alist and Hlist, host H takes Hlist =
{A, B, C, D, E, F, G} in order to obtain a copy of the host
table for host A at the present point in time. At this point,
connections from host H to other hosts are unidirec-
tional. Moreover, because host A records host H in the
host table as a new host, it takes Alist = {A, B, C, D,
E, F, G, H}. At this point, hosts A and H have bidirec-
tional connections. Figure 5 shows the information re-
trieval network as changed by the addition of ATRAS
host H. Next an information request search is made to
host C. The results show that a migration search is
per formed us ing the pa th host C o E o D o
A o H o G o C. This indicates that in a search system
created using the authors� method, information on the
newly added WWW server host can also be searched.
4.2.2. Information searches that avoid a
problem host
The GateKeeper on host F was stopped, and a
pseudo-problem state was created on the host. At this time,
a request for an information search was issued to host A. As
a result, the authors confirmed that the migration search was
performed using the path host A o C o E o D o A.
Here, the authors were able to confirm that when In-
foSeeker failed to be able to move to host F once during its
move from host E to the next host, it then moved to the next
candidate, host D. In this fashion, even when a problem
occurs in an ATRAS host, it is clear that an information
search that avoids the problem can be performed. This
demonstrates that in an information system using the pro-
posed method, stable information search services in which
problems that occur on a host do not affect the use can be
provided.
4.2.3. Information searches from any host
The authors confirmed that the same information
search could be performed from any ATRAS host when a
search request was made to each ATRAS host. Here an
example for a search request from hosts B, D, and G is
shown. The migration search was performed using the path
host B o A o E o D o B for a search request on host B.
For host D, the migration search was performed using the
path host D o E o G o B o A o C o D. For host G,
the migration search was performed using the path host
G o B o A o C o E o D o G. This shows that in a
search system that incorporates the authors� method, the
user can perform an information search from any ATRAS
host (even a WWW server host). Figure 6 shows the status
of the information retrieval network at the end of the
experiments.
5. Considerations
In this section the authors consider the validity of the
search method composed of a self-organizing information
retrieval network by using an ATRAS information retrieval
system with their method as its foundation. Section 4
showed that an ATRAS information retrieval framework
has the following characteristics.
(1) Information searches on a newly added WWW
server host
(2) Information searches that avoid a problem host
(3) Information searches from any host
This is demonstrated by the experimental results on a
small-scale, local LAN. However, this suggests that an
information retrieval system using as its foundation this
Fig. 6. Information retrieval network at the end of
experiments.
35
search method, comprised of a self-organizing information
retrieval network, could be effective for a university with a
campus split up physically over various areas and with WWW
server hosts set up in different laboratories.
For instance, even if WWW server hosts are set up in
each laboratory at a university, their information cannot be
used unless it is made available throughout the university. In
ATRAS, when the WWW server host is an ATRAS host, an
information retrieval framework is automatically incorpo-
rated, and as a result, users in other laboratories can do searches
for WWW information in newly set-up labs. Furthermore,
there is also the benefit of users being able to perform searches
via the WWW server host (an ATRAS host) in their own
laboratory without having to make special search queries.
In this fashion, in information spaces in which when a
WWW server is set up or taken down is unclear, each WWW
server host itself controls its own host information. The
method of generating retrieval paths automatically through
self-organization is then applied flexibly to the WWW infor-
mation space as it expands daily, and the need for maintenance
by people is reduced.
However, determining the migration destination host
using the expected wait value uses migration to the host that
is most likely to be found. Consequently, when information is
required from a host with a low expected wait value (for
instance, a host with information on a particular game only)
(here, when searching for something related to the game),
information can be retrieved while avoiding that host. This
problem represents a topic for further study.
Nevertheless, making the most of performance on a
gigantic network like the Internet is a difficult problem. In
particular, the effects on the network during a migration search
must be sufficiently small compared to a search engine using
current Web robot methods. Moreover, information retrieval
from a newly added WWW server host, information retrieval
from any host, and other functions are limited to search systems
based on a search method composed of a self-organizing infor-
mation retrieval network. Applying this system as is to a massive
computer network such as the Internet will not be easy.
6. Conclusions
In this paper the authors proposed a search method using
a self-organizing information retrieval network as a method to
search effectively for WWW information managed in a dis-
tributed fashion. Moreover, the authors created an ATRAS
information retrieval system and then ran experiments on a
university LAN to demonstrate the effectiveness of their idea.
The authors found the following:
(1) Information searches on a newly added WWW
server host
(2) Information searches that avoid a problem host
(3) Information searches from any host
The authors plan to run experiments on the Internet (or a
large-scale network) in the future.
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36
AUTHORS (from left to right)
Kouichi Abe graduated from the Department of Electrical and Information Engineering of Yamagata University in 1994.
He completed the first part of his doctoral studies in 1996 and currently is working on the latter half of his doctorate. His research
interests are agents, computers, and the Internet. He was the recipient of the Excellence in Research Papers Award at the 12th
International Conference on Information Networks (ICOIN). He is a member of the Information Processing Council and the
Software Council.
Toshihiro Taketa (member) graduated from the Department of Electrical and Information Engineering of Yamagata
University in 1989 and completed his master�s program in 1991. He then became a lecturer on the Faculty of Engineering and
is pursuing research on computers, networks, parallel algorithms, and distributed systems. He is a member of the Information
Processing Council, the Artificial Intelligence Association, and the Software Council.
Hiroshi Nunokawa (member) graduated from the Department of Electrical Engineering of Yamagata University in 1983.
He completed the latter half of his doctoral studies in engineering research at Tohoku University in 1988. He became a lecturer
at the Research Institute for Electrical Communication at Tohoku University and is currently a professor at Iwate Prefectural
University. He holds a D.Eng. degree. He is interested in systems that join software systems, distributed systems, and human
beings. He is a member of the Information Processing Council, the Software Council, the Recognition Council, and ACM.
Norio Shiratori (member) completed his doctorate at Tohoku University in 1977. He became an assistant professor at
the Research Institute for Electrical Communication at Tohoku University in 1984, a professor of information engineering in
1990, and a professor at the Research Institute for Electrical Communication in 1993. He is pursuing research on information
communications systems, software development environments, and human interface. He was the recipient of the Information
Processing Council�s 25th Anniversary Concept Award and the 1996 Research Paper Prize. He is an IEEE Fellow and a member
of the Information Processing Council and the Artificial Intelligence Council.
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