Problem with MERGE2 when not using multicast
--------------------------------------------
Key: JGRP-31
URL: http://jira.jboss.com/jira/browse/JGRP-31
Project: JGroups
Type: Bug
Versions: 2.2.8
Environment: SUN Java 1.4.2_05
Reporter: B.S.Navin
Assigned to: Bela Ban
Hi,
There is one case in which MERGE2 will fail while using TCPPING/UDP(without
mcast):
========================================
The initial_hosts property is "abc.com[7800];xyz.com[7801]".
These 2 machines are permanent group members (if they are up, they will be
members of the group).
Now there are numerous other programs on different machines that may
dynamically join and leave the group. These members are not known before hand
and cannot be specified in the initial_hosts property.
The members on abc.com and xyz.com are started and they join the same group.
Now another member from "mnop.com" starts and joins the group. The co-ordinator
will be abc.com
A network problem occurs and mnop.com is separated from abc.com and xyz.com
mnop.com forms its own single-member group with itself as the co-ordinator.
Now suppose the network problem is fixed. The MERGE2 protocol on mnop.com
periodically checks on the initial_hosts list to see if they are up. It now
finds that abc.com is reachable and decides that abc.com is the leader (by
lexical sorting) and will take care of the merging. So it does not go ahead
with the merge.
On the other hand, the MERGE2 protocol on both abc.com and xyz.com just check
if they can find members on the initial_hosts with a different co-ordinator.
Both of them never consider mnop.com as it is not in the initial_hosts list
So mnop.com will never be merged with the group.
========================================
Even the new MERGE3 & MERGEFAST protocols don't seem to help here. I checked
them out and found the following:
MERGEFAST works only if multicast is used, which is not my case.
MERGE3 just sends "I am co-ordinator" messages to a null destination. So in the
case where multicast is enabled, it goes to all possible members. But in my
case, with multicast disabled, the message will only be unicast to each member
of the current group. So in the above example, the "I am co-ordinator" messages
will never go to mnop.com after the network problem.
So even MERGE3 does not work in this case.
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