Virtual Yahoo!
An Example of Virtual Access to Remote Web Catalogues


M. Angelaccio, S. Passiglia
Dip. di Ingegneria Informatica dei Sistemi e Produzione Universita' di Roma "Tor Vergata"
Via della Ricerca Scientifica 00133 - ROMA (Italy)
angelaccio@utovrm.it

Abstract

We present the description of a WWW Information Retrieval system called Virtual Yahoo! based on a Yahoo mirroring, i.e. a local copy of a subset of the remote Yahoo hypertext. The aim is to access the well known but overloaded WWW Catalogue in a virtual context, by extending the memory virtual system organization to the Yahoo hypertext. The result is a tool that may be considered intermediate between a mirroring system and caching system able to reduce the number of remote accesses. The key issue is a page prefetching mechanism in cascade connection with Search Engine answers. In particular the Virtual Yahoo! access depends on a category fault event (analogous to the page fault event in Virtual Memory Systems) that may occur in search engine answers. This allows a virtual search process that simulates the hand procedure of "loading a remote page-while browsing the local one".


1. Introduction

Subject tree catalogues (or A2Z lists) are an important alternative to database search engines because categories are explicit. The user can integrate local search engine queries with browsing steps. The main disadvantage is the network traffic overload that in most cases degrades the browsing efficiency. For example Yahoo, which is considered the most diffuse Subject Tree on the Web (see [Bray95]), combines a wide thematic description with query capability and as consequence it often registers high workload at the server site. By remarking that (as pointed out by Denning [Den96]) the Web extends virtual memory to the world, it seems clear the analogy with the inefficiency due to memory hierarchies. In this case the slow-access level corresponds to the remote site, whereas the local site might be considered as a fast-access level. One of the most common internet solutions is mirroring (e.g. sites mirroring internet services like software ftp and Usenet news). However these mirroring sites are not based on a virtual schema because they do not introduce a virtual level. The local reference is explicit at every time for the user. In addition, because new link are frequently added, it holds that implementing a Yahoo mirroring like ftp mirroring is practically impossible due to the growing size of the Catalogue. Another solution is given by the caching techniques that locally save the most recently visited pages to improve browsing efficiency. However, the caching approach gives fast access only for a very small subset of the remote hypertext.

To better motivate the implementation of Subject Trees as a virtual system, we show two examples of remote Yahoo access each giving evidence of a strong relationship between network traffic conditions and time intervals.

Netscape session example
Table 1 shows the varying time accesses registered at different intervals by a Netscape browsing session. It plots the remote page loading time values that correspond to three different accesses to Yahoo (home, search and one remote category). The measures are taken during three different periods from an Ethernet-connected workstation of our university.

day period Yahoo home (sec)search (sec) remote category (sec)
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4.6		28.230.5
12.00-12.30 am27.653.6 123.5
7.00-7.30 pm 2.628.110.4

Table 1. Access time values for three Yahoo pages taken during different periods


Partial mirroring script example
The second set of data has been obtained by taking the time values registered from the execution of a mirroring script applied to a category subtree composed of 20 remote pages. This shows that subcategory mirroring is effective and it depends on a careful choice of an uncongestioned period too.

Figure 1. Access Time Values for a Sequence of 20 Yahoo pages taken during different periods

The proposed solution, named VirtualYahoo!,  aims to maintain a local copy by offering the capability to virtually access such a well known (but overloaded) Catalogue. The main issues are the following ones:

2. Virtual Yahoo Organization

Fig. 1 shows the architecture of the system with its components. The goal is to furnish a tool that allows the user to access Yahoo without taking care of network traffic conditions. This has been obtained by distinguishing between local hypertext searching and mirroring operations that automatically update a part of the local hypertext.

It is composed as follows:

Figure 2. Virtual Yahoo! Organization

To force the user to keep his/her searching close to local copy, a frame based interface has been adopted. The idea is to furnish a separated index frame for the hypertext documents showing a tree-oriented view of local categories by using a Java applet communicating with the search engine and mirroring agent.

Figure 3. Example of Java-Frame Based HTML Interface

3 Virtual Subject Tree Searching

Differently from database search engines, Subject Tree searching is characterized by a mixture of query submission and browsing through category links, with the aim of refining previous answers. This type of searching is a trial and error process. For instance let us consider the following example of a Yahoo searching session. The user submits a query given by the keyword "ATM" or "Hardware", and the returned answers are the following (we restrict answers to the first two):

Found 1 Category and 33 Site Matches for ATM Hardware.

Yahoo! Category Matches   (1 - 1 of 1)

Computers and Internet: Communications and Networking: ATM

Yahoo! Site Matches   (1 - 19 of 33)

Computers and Internet: Communication and Networking: Intranet

  • ATM.... .... ATM .....hardware.

The displayed answers correspond to

  1. a link matching ATM as a subcategory in category \Communication&Networking
  2. a link matching ATM and Hardware as a site in category \Communication&Networking\Intranet
After visiting link1 (subcategory match) and link2 (site match), the user can decide whether to contact the Yahoo "\Communication&Networking\Intranet" category again for better answers.

This can be regarded as a category completion browsing process carried out from the user in order to better satisfy her/his needs. The problem is the cost due to network congestion because proxy caching servers are not able to support such user actions.
The virtual approach gives the opportunity to organize a form of prefetching for all pages that could be visited in the future. The pages that must be loaded can be chosen in correspondence with the type of resulting answers, thus automatically starting remote page loading independently from user actions. Note that a practical example of such behaviour is given by the well-known technique of "opening a new browser-window on the remote site while continuing to browse".

The main question is now:

how to utilize the answers returned from search engine for choosing the pages to be prefetched?
To this purpose, in analogy with virtual memory systems, the notion of Category Fault event is introduced. This acts as the Page Fault event for virtual memory systems. Instead of an "out-of-cache" reference, there are a set of ("potential") out-of-LAN references. To be more precise, recall the (ATM,Hardware) query example. From the answer given by the link1, a Category Fault event associated with the "Communication&Networking" category may be derived. In fact this answer has no matches for the keyword "hardware". In this case to support future references to those categories that are missing in the local hypertext, a category completion process for the "Communication&Networking" category occurs. Basically, such a category completion process allows remote access avoidance from the user, by accomplishing the following actions:

4. The Current Virtual Yahoo Effort

The Virtual Yahoo! system is being developed at the site http://www.ce.utovrm.it/mirror. A prototype with user interface and an example of Yahoo mirroring has been implemented. For the creation of the local hypertext, a software module of the BOTH project [Ang96] has been used. The implementation of the search engine and mirroring agent is still in progress but a demo exists at (http://www.ce.utovrm.it/mirror/w6demo.html) that shows the layout of the html interface with the Java class that implements the dynamic tree index visualization.

Another issue to be developed is the introduction of a network congestion analysis tool able to support remote accesses. The purpose is to estimate the best network bandwidth before accessing the remote site. There exists a great deal of effort for network bandwith estimations. However, Virtual Yahoo!  is characterized by a sequence of remote accesses carried out from the mirroring agent. Hence, a good level of efficiency requires the ability to predict the available bandwith in the future. As pointed out by ([Car96]) this question remains open and it will be a topic of future investigations.

5. Other Sources

There is an increasing interest on the Web for caching/mirroring tools.

References

[Ang96] M. Angelaccio, L. Zamburru, D. Genovese, BOTH:Cooperative Automatic Web Navigation with Hierarchical Filtering, AusWeb96 conference July 1996 http://elmo.scu.edu.au/sponsored/ausweb/ausweb96/tech/

[Bray95] T. Bray, Measuring the Web, Fifth International World Wide Web Conference, May 6-10, 1996, Paris, France http://www5conf.inria.fr/fich_html/papers/P9/Overview.html

[Car96] Robert L. Carter, Mark Crovella Measuring Bottleneck Link Speed in Packed Switching Network, Technical Report CS Dept. Boston University, BU-CS-96-0006, March 15 1996

[Den96] Peter J. Denning, Virtual Memory, ACM Computing Surveys , Vol. 28, No. 1, March 1996

[Li95] H. Lieberman, Letizia: An Agent That Assists WebBrowsing, Proceedings of AAAI 95 - AI Applications in Knowledge Navigation and Retrieval, MIT Cambridge MA, USA, 1995, pp. 97 - 103

[Kov96] L. Kovacs Caching and Mirroring Techniques in WWW and Digital Library Architectures, ERCIM news 96, No.27, October 1996 http://www-ercim.inria.fr/www-ercim.inria.fr/publication/Ercim_News/enw27/kovacs.html


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