Path: utzoo!attcan!uunet!samsung!usc!snorkelwacker!mit-eddie!uw-beaver!cornell!ken
From: ken@gvax.cs.cornell.edu (Ken Birman)
Newsgroups: comp.sys.isis
Subject: Detecting client failures
Message-ID: <38955@cornell.UUCP>
Date: 22 Mar 90 15:50:46 GMT
References: <38866@cornell.UUCP>
Sender: nobody@cornell.UUCP
Reply-To: ken@gvax.cs.cornell.edu (Ken Birman)
Distribution: comp
Organization: Cornell Univ. CS Dept, Ithaca NY
Lines: 39

As part of the isis_remote() mechanism I am implementing a facility
by which ISIS will periodically poke a client program to see if it
is alive.

Here's the proposed interface; I am interested in comments or feedback:

1) Initializing the mechanism.
	application calls	isis_probe(freq, timeout)
        where
				freq = frequency to probe, in seconds
				timeout = speed of expected reply, in sec
2) Defaults:
	disabled for "local" clients of ISIS
	freq = 60, timeout = 30 for "remote" clients
3) Implementation:
        client starts sending a HELLO message every freq. seconds
        ISIS has a timer; if it doesn't get a HELLO within freq+timeout secs
		it "kills off" the client
        killed client that was really alive calls isis_failed, then panics
		with message "killed by ISIS" unless isis_failed traps the
		failure (i.e. by reconnecting).

In addition, a remote client monitors its mother machine and calls
isis_failed if the mother machine dies; the isis_probe() constants
are used for this purpose, too.

Robert Cooper has an idea for a very fancy mechanism that would let
a client (any client) drop "offline" for a while and then come back.
For example, say that you are doing a stock trading system and the
broker presses "analyze".  You might want to stop monitoring stocks,
crunch hard for a few seconds and do some fancy display, then show
all the updates that arrived while you were crunching as a batch and
go back to monitoring passively.

We are thinking about how this could be added to ISIS.  It wouldn't be
trivial, but could probably be done.  Also being considered are ways to
connect remote non-UNIX clients (PC's, etc).  I doubt that either of these
features will ever be in the public version of ISIS, but my guess is that
IDS will have products extending ISIS in these ways late this year...