Path: utzoo!utgpu!jarvis.csri.toronto.edu!clyde.concordia.ca!uunet!cs.utexas.edu!rutgers!rochester!cornell!wood
From: wood@hermod.cs.cornell.edu (Mark D. Wood)
Newsgroups: comp.sys.isis
Subject: Re: A technical question
Summary: proposed BY-ABCAST impact on Meta
Keywords: Bypass protocols
Message-ID: <35593@cornell.UUCP>
Date: 30 Dec 89 21:28:06 GMT
References: <35521@cornell.UUCP>
Sender: nobody@cornell.UUCP
Reply-To: wood@cs.cornell.edu (Mark D. Wood)
Distribution: comp
Organization: Cornell Univ. CS Dept, Ithaca NY
Lines: 28


The proposed change to the abcast protocol could present some problems
for Meta, depending on its exact semantics.  Meta currently guarantees
that all clients are notified of events in the same order.  This
guarantee is trivially provided by the current abcast semantics.  The
proposed BY-ABCAST protocol provides total ordering for messages sent
to the same logical address; this will be enough for Meta as long as
it holds for process addresses too, not just group addresses.

In particular, when a sensor perceives that one or more conditions are
satisfied, it sends a broadcast to the list of addresses {S,a,b,...}
where S is the sensor process group (useful if the sensor is
replicated) and {a,b,...} are the addresses of the processes (not
groups) who are being notified.  The guarantee that Meta provides is
that if processes a and b receive notifies from two sensors, they
receive the notifies in the same order.

This seems like a useful property, but it often may not be necessary
and so perhaps we are wasting effort enforcing it those times when it
is not needed.  What thoughts do others have on this subject?  Another
approach would be to have the programmer explicitly inform Meta of
those processes that need to see things in a mutually consistent
ordering; these processes could be made to join a single process
group.  Carrying this to the extreme, all clients of a sensor would
join a process group; the sensor would broadcast notifies to this
single group.  The problem with this approach is that it results in
extra network traffic and requires clients to filter out unwanted
information.