Thanks for the reply, Risto. Allow me to make the following remarks:

1. I won't argue with the argument about simplicity and efficiency!
2. I would argue with the argument about ambiguity because my understanding of 
things is/was that when the threshold number of messages didn't arrive during 
the time window there would be no event. It seems very strange that when an 
event is detected the information you have available refers to a message that 
had nothing to do with the event!
3. I can do the tail thing via shell commands or by using perl so that is quite 
neat!

thanks, Allen.

PS I have other "fundamental" questions but I'll ask them elsewhere. Thanks 
again.

-----Original Message-----
From: Risto Vaarandi [mailto:[email protected]]
Sent: Tuesday, March 10, 2009 6:16 PM
To: Conway Allen
Cc: [email protected]
Subject: Re: [Simple-evcorr-users] SingleWith2Thresholds question

Conway Allen wrote:
> [---cut---]
> The action associated with the lower threshold of 3 is executed when
> xyz/7 arrives but if the action involves writing to a file $0, for
> instance, what I find in the file is xyz/1 and not the xyz/5 as I would
> expect.
> [---cut---]

Allen,
IMHO, the use of $0 variable makes in most cases sense only if you are
correlating messages with a rule where the action is executed
immediately after a match has been found. In other words, the action
gets executed on one match only when it is clear that $0 is set by this
particular match.
If the action is triggered by the multiple matches, the $0 variable is
inherently ambiguous. SEC always sets $0 and other variables when the
event correlation operation starts which is the most simple and
efficient way for handling match values (otherwise we would have to
store the pattern match values from all matches, and even then it is not
clear which particular value to prefer for $0).
If you would like to fetch an N-th matching line when action is
executed, I'd recommend to employ a separate context for keeping
matching lines in memory. When an action is triggered from
SingleWith2Thresholds, you can use 'report' to access the content of the
context and extract the N-th line.
[---cut---]
br,
risto




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