At 9:22 PM -0500 12/22/05, David Magda wrote:
>On Dec 22, 2005, at 19:34, David Harnick-Shapiro wrote:
>
>>> Indiana University NOC Tools
>>> Luke Fowler paper
>>
>>There was also a reference made to Trey Harris' paper,
>>probably from the previous LISA, which discussed a different
>>way to look at configuration, with validation.
>
>Can anyone shed light on these two papers? Given Mr. Harris'
>involvement in USENIX and SAGE his name appears all over the place,
>so narrowing things down is a bit tricky.
Trey's talk was an invited talk at LISA 2004 in Atlanta; it was about
the power of "Guarded Commands", which he implemented through a Perl
module named "Commands::Guarded"
(http://www.annocpan.org/~TREY/Commands-Guarded-0.01/Guarded.pm).
Here's a good discussion of the problem and solution from the module docs:
http://www.annocpan.org/~TREY/Commands-Guarded-0.01/Guarded.pm#an_example
Essentially, the idea is that lots of folks write configuration
scripts with lots of steps in them that make two unwarranted
assumptions:
1) The step needs to be done (i.e., it hasn't been done already)
2) The step succeeds (so you simply proceed to the next step without checking)
One consequence of this style of scripting is that, if such a script
fails part way through, you can't simply fix the problem and rerun
the script because you might inadvertently and incorrectly repeat
work that's already been done.
Commands::Guarded makes it easy for you to write scripts as a series
of steps, where each step includes a condition (a test to see whether
the step has succeeded) and a set of commands that (if executed)
should cause the condition to become true. Each step composed of
CONDITION and COMMANDS is then executed according to the following
pseudo-code:
unless (CONDITION) {
COMMANDS;
die unless (CONDITION);
}
The Perl module also includes facilities for rollbacks, sanity
checks, and so forth, but that's the idea in a nutshell. It's a very
powerful idea to apply in a configuration management context, whether
of servers or network devices.
-Brent
--
Brent Chapman <brent@greatcircle.com> -- Great Circle Associates, Inc.
Specializing in network infrastructure for Silicon Valley since 1989
For info about us and our services, please see http://www.greatcircle.com/
Great Circle Waypoints Blog: http://www.greatcircle.com/blog
References:
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