July 1999 Troubleshooting Professional Magazine: Troubleshooting CGI
Copyright (C) 1999 by Steve Litt. All rights reserved.
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Using Semantic Nets to Model Troubleshooting's Knowledge,
By Marc-Henri Poget. Published with permission of the author.
As mentioned in The History of Troubleshooting (January 1999 TPM
by Steve Litt), today's systems are highly complex and tough to troubleshoot.
There is a strong need for Technologically Enhanced Troubleshooting tools
(Era 4), especially when dealing with computer systems. Oriented towards
the realization of such a tool, this article discusses semantic nets as
a way to model the troubleshooting's knowledge.
EDITORS NOTE: Era 4 tools are expert systems whose foundation
and work flow are based primarily on a valid troubleshooting process.
Semantic nets, also called mind maps by some authors, are graphs used to
represent concepts and their relationships. The term mind maps,
assumes that this model is quite similar to how we represent things in
our minds. Concepts are the nodes of the net, graphically represented as
labeled boxes. Relationships are links between the boxes, represented as
For example, imagine typical concepts related to birds, these concepts
can be: Birds, Feathers, Wings, Worms, Trees. These concepts by
themselves don't give so much information. Additional information is contained
in the following sentences:
All these sentences express a relationship between concepts. They can be
graphically depicted by the semantic net below.
Birds fly using their Wings.
Their Wings are made of Feathers.
Birds eat Worms.
Birds build their nests in the Trees.
figure 1: Birds related concepts
Applying Semantic Nets to Troubleshooting
As an example, consider a typical Web application as depicted by the figure
2 system diagram. This application is accessed from a Web browser running
on a client computer, communicating with the server using HTML and HTTP.
On the server side, there are a Web server software, a CGI (Common Gateway
Interface) program and a database. Since it contains many components, coming
from different vendors, a broad knowledge is required to troubleshoot such
an architecture. Problems may occur at the network, database, CGI program,
Web server and client levels, therefore you need to know how these components
are linked together and how to setup them to narrow down the problems.
figure 2: Web application with database.
Although the figure 2 presents the architecture of a typical CGI application,
it lacks some information that may be useful when troubleshooting it. This
figure doesn't show that the Web browser requires some dynamic link libraries
(DLL) or that the CGI program is written in Perl. Semantic nets aren't
restricted to an architectural view of the system, but they can show all
the Web application related concepts and the relationships which express
either dependancies between these concepts or the action of a component
on another. Therefore, semantic nets allow to build better models than
system diagrams do.
Semantic nets creation is an iterative process. First create a very
simple semantic net that can be easily extended when new information about
the system is gained. To create the semantic net for the Web application
example depicted in figure 3, I followed these steps:
Identify the concepts. They may be either physical component (as the Web
browser) or some more abstract entities (as the TCP / IP protocol).
Identify the relationships between these concepts. In the Web application
example, one can identify the following relationships between the concepts:
The Web application is accessed through the Web browser.
The Web application runs on the Web server.
The Web browser communicates using HTTP and TCP / IP.
The Web browser needs Dynamic link libraries.
The Web server communicates using HTTP and TCP / IP.
When using HTTP, TCP / IP, one gets host names using the Domain Name Server.
The Web server is configured by Configuration files.
The Web server launches the CGI program.
The CGI program is written in Perl.
The CGI program gets results from the database.
The CGI program formats the results in HTML.
The Database is configured by Configuration files.
Draw the semantic net. Each concept is mapped to a box and each of the
sentences above is mapped to an arrow linking the related boxes.
When new information is gained, new concepts and relationships can be added
to refine the semantic net.
figure 3: Semantic net for troubleshooting the Web application.
As a troubleshooting's guide, this semantic net helps to see how things
that may fail are related. Thus, it is a good tool to support the 6th step
of the Universal Troubleshooting Process (Narrow it down to the root cause).
To navigate from one concept to another, the Troubleshooter must know
which tests to do. For example, starting from Web application, one
needs to know how to test whether the problem is on the Web browser
or on the Web server side. This testing information may be associated
to concepts and typical Web browser related tests may be: try accessing
other pages, try using another browser. Assuming that all these tests run
perfectly, one will conclude that the problem is located in the Web
EDITORS NOTE: The test described above is impossible given
the preceding Semantic Net because the Web browser and Web server share
http and TCP/IP on the preceding Semantic Net. Fortunately all that's required
to make the test possible is to refine the preceding Semantic Net to break
TCP/IP into components that can be put squarely on either the browser or
the server side, probably with relationships connecting them. Those relationships
can then be strongarmed to narrow the scope. This is very much like troubleshooting
an audio amplifier with a feedback loop from output to input. That
feedback loop must be temporarily broken to draw meaningful information
from strongarming the input and observing the output.
This illustrates the real power of Semantic Nets. It provides the Troubleshooter
with a straightforward tool to improve insufficient Mental Models during
the hot persuit of a solution.
To go further from the Web server concept, the semantic net shows
that the problem may be related to the Web server configuration (Web
server is configured by Configuration files), the CGI program
(Web server launches
CGI program) or the communication protocols
(Web server communicates using HTTP. TCP/IP). A simple test
like trying to access other static pages stored on the Web server immediately
rules out a communication protocols problem. It also partially eliminates
a configuration problem, since the Web server handles the static
pages. Thus the Troubleshooter will have to perform additional checks in
the Configuration files to make sure that the CGI program
can be launched.
Depending on the problem description he's received, the Troubleshooter
can start directly from the appropriate concept. Imagine that the problem
description is: "I can't access the Web application from my browser, but
I'm able to access any other page on this server.". Given this description,
an experienced Troubleshooter will immediately suspect the CGI program
and navigate in the semantic net from this concept. A less experienced
Troubleshooter will have to start from another concept, like for instance
the Web server . Navigation through the semantic net is the Troubleshooter's
choice, he also can explore several paths in parallel. Thus the Troubleshooter's
efficiency higly depends on the semantic net navigation capabilities. The
navigation software shall provide this flexibility.
This article briefly introduced semantic nets, how to create them and how
to use them for troubleshooting. Semantic nets are superior to system diagrams,
because they can describe more than just an architectural view of the system.
This make semantic nets good tools to support the troubleshooting process.
The next articles will describe creation and navigation in more details.
EDITORS NOTE: Marc needs to go broader and deeper to fulfill
the promise of his article. I have issued Marc a important
series of challenges in the next article.
References Mentioned in this Article
Links to these resources can be found in the URL's section of this Troubleshooting
http://www.cm.cf.ac.uk/Dave/AI2/AI_notes.html provides a very good introduction
to knowledge representation formalisms, including semantic nets.
http://www.troubleshooters.com/tpromag/199901/199901.htm january 1999 issue
of the Troubleshooting Professionals Magazine.
former author's article about decision graphs.
Marc-Henri Poget is a Swiss software engineer, working in a hospital computer
department. His main interests are: network and Web applications, network
management, and troubleshooting. He can be reached at email@example.com.