Coda File System

Re: Ubik and PTS issues

From: Brian Bartholomew <>
Date: Mon, 16 Feb 1998 00:52:45 -0500
> What does "distributed simultaneous group broadcast code" mean?  It
> sounds like it means "multicast".  I assume, though, it provides
> some facilities beyond simple IP multicast.

Oops, sorry, one more time in English.  Mark and others, feel free to
jump in and correct me.  Ensemble is based on IP multicast.  However,
under the covers it is a multi-phase commit protocol.  It gives you
primitives like the following:

	Message A is guaranteed to be delivered to every recipient, or
	none of them.  You know which happened.

	All recipients can find out who all the other recipients are,
	with no race conditions relative to sent messages.

	Messages A, B, and C are guaranteed to be delivered in ABC
	order, no matter how the network reorders or drops them.

	It is multicast, so applications that want to be multicast are
	efficient.  Wouldn't you like to have redundant Coda servers
	on your LAN, in a self-correcting/voting arrangement?  Or
	distributed not across the LAN, but around the world?

	(I believe this is true) Recipients listen to the protocol
	details of a sender.  If the sender makes a mistake (because
	that machine is crashing), one or more of the recipients takes
	over, declares the sender to be busted, kicks it out of the
	group, and finishes the protocol to a consistant state.

Another member of the League for Programming Freedom (LPF)
Brian Bartholomew - - - Working Version, Cambridge, MA
Received on 1998-02-16 00:50:23