Path: utzoo!utgpu!news-server.csri.toronto.edu!bonnie.concordia.ca!uunet!mcsun!unido!logixwi!rcr
From: rcr@logixwi.uucp (Rainer Ruppert)
Newsgroups: comp.databases
Subject: two phase commit ???
Summary: two phase commit not secure
Keywords: commit in distributed systems
Message-ID: <1991Jun23.191959.29921@logixwi.uucp>
Date: 23 Jun 91 19:19:59 GMT
Distribution: comp
Organization: Logix GmbH, Wiesbaden, W-Germany
Lines: 39

Hi Networkers,

yesterday a friend of mine, edp-manager, said: "The last week the salesman
of all the big database companys joined me. Most of them wanted to sell the
*two phase commit* for our new distributed system, but I could not believe that 
the tpc is really able to make distributed systems recoverable."

At first, I remebered Ceri & Pelagatti and Date and told him, be sure, its save.
But seconds later I wasn't quite that sure then before. Let us suppose following
scenario:

	Three systems a,b,c where a is the coordinator b,c are the agents
	managing two tables which are under update operation.

	The coordinator is doing the phase one and after completing it, phase 
	two. Phase two is completed on b and c, but the final quit (ack) from
	c could not received by the coordinator because the network dropped it
	while a millisecond of blabla... .
	
	System c is releasing the locks and the local actions on that system 
	are able to manipulate the c-local table, while the coordinator a
	is rejecting the transaction on a and b.

	The database is corrupted-or ???

Ok, the possibility to loose an ack, because of a millisecond network failure isnot very high, and normal database systems are trying to get the ack again, but suppose it is a problem to transmit the ack, then you have that situation.

In my opinion CP and Date discussed only the situation for the distributed
database without discussing problems with the underlying network.

Does anyone else have thought about that problem ? Is there a solution
with which the boy could be satisfied ? 

	thanks, rcr


-- 
Rainer Ruppert, Logix GmbH				    rcr@logixwi.UUCP
Moritzstr. 50, D-6200 Wiesbaden            ...!uunet!mcsun!unido!logixwi!rcr