Debugging & Troubleshooting

	“To err is human, but it takes a computer to really foul things up”

Using Computers...

	In IT, stuff goes wrong … debugging is the process of finding the error
		Term coined by Grace Murray Hopper
	Best solution … make no mistakes!
		Be accurate … get it right the 1st time
		In most cases computers can’t recover for our errors

When You Debug...

	There are guidelines for debugging…
		Rather than trying things aimlessly and becoming frustrated, think of yourself as solving a mystery
			Be objective: What are my clues? What is my hypothesis? Do I need more data?
			Consciously ‘watch’ yourself debug -- its an out-of-body experience
			When stumped, don’t become frustrated, but ask, “What am I misunderstanding?”

Debugging Guidelines

		Verify that the error is reproducible
		Determine exactly what the problem is
		Eliminate the “obvious” causes
		Divide process into working/faulty parts
		On reaching a dead end, reassess the information you have, trying to identify the mistake you are making
		Work through process making predictions and checking they’re fulfilled

Reproducibility

	First step: verify the error is reproducible
		Transient errors are very rare, but they do happen … try again
		Rebooting the operating system is advisable, especially for errors involving peripheral devices (printers, modems)

Determine the Problem

	Second step: figure out what’s wrong
		Often there is a sequence of steps following an error and propagating it … work backwards looking to see where the error first occurred

Eliminate the Obvious

	Third step: eliminate obvious causes
		“If the cause were obvious, the problem would have been fixed!”
		There are standard things to check:
			Inputs
			Connections
			“Permissions”
			Physical connectivity

Isolate the Problem

	Try to “partition” the situation into  working and non-working parts
			Form a hypothesis of what’s wrong
			Make as few assumptions as possible
			Take nothing for granted

At a Dead End, Reassess

	When everything seems to check out, don’t get frustrated ... ask, “What am I misunderstanding?”
		Your goal is to see the situation as it is, not as you think it should be
			Are you assuming too much?
			Are you mis-reading the clues?

Make Predication/Check

	Beginning with the isolated part, step through the process, predicting the outcome and verifying it
		A prediction that is not fulfilled shows…
			A possible bug
			A possible misunderstanding
			A chance to narrow the search

A Debugging Example

	After building a class web page, we find it is wrong

Debugging Demo

Summary

	Debugging is not algorithmic, but there are guidelines to follow
		It probably pays to memorize them so they come to mind while debugging
		Watch yourself debug -- assess how you are doing, what you need to know
		Being accurate -- avoiding textual mistakes at all -- saves frustration