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From: vixie@pa.dec.com (Paul Vixie)
Newsgroups: comp.mail.sendmail
Subject: Re: Idempotence of S3 (Was Re: IBM Do It Again)
Message-ID: <1991Apr16.065813.24921@pa.dec.com>
Date: 16 Apr 91 06:58:13 GMT
References: <1991Apr12.164151.8459@csl.dl.nec.com> <1991Apr12.192026.11296@csl.dl.nec.com> <1991Apr12.213421.22457@decuac.dec.com>
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In article <1991Apr12.213421.22457@decuac.dec.com> Fred said:
# Shoot.  I have never even heard or read the word "idempotent" until
# just now.

Idempotent means, roughly, that doing it N times is the same as doing it
1 time, for all N>1.  In the case of S3, this is usually accomplished by
bailing out near the top if the <brackets> are already there, as in:

S3
...
R$+<@$+>$+		$@$1<@$2>$3		bail out
...

I have rarely encountered S3's that hurt you if you ran them N>1 times.
Where I have encountered them, I have changed them.  I'm not sure where
I read that S3 was supposed to be idempotent, but I did read it somewhere
and it seems intuitively correct that it should work that way.  

It also lets me call S3 on my way out of other rulesets, even in cases
where I don't know quite what the hell I'm sending in.

Btw, my Webster server didn't know about idempotency either.

Cheers,
-- 
Paul Vixie
DEC Western Research Lab	<vixie@pa.dec.com>	<paul@vixie.sf.ca.us>
Palo Alto, California, USA	...!decwrl!vixie	...!vixie!paul