Problems with reincarnation
Author: Bela Ban
The identity of a JGroups member is always the IP address and a port. The port is usually chosen by the OS, unless
bind_port is set (not set by default).
Let's say a member's address is hostA:5000. When that member dies and is restarted, the OS will likely assign a
higher port, say 5002. This depends on how many other processes requested a port in between the start and restart
of the member.
JGroups relies on the fact that the assignment of ports by the OS is always (not necessarily monotonically)
increasing across a single machine. If this is not the case, then the following problems can occur:
When a member P crashes and then is restarted, if FD is used and P is restarted before it is excluded,
then we have a new member under the same old address ! Since it lost all of its state (e.g. retransmission table),
retransmission requests sent to the new P will fail.
Regarding shunning: a member keeps its last N (default is 100) ports used, and makes sure it doesn't reuse one of
those already-used ports when it is shunned. However, this is process-wide and not machine-wide, e.g. when we have
processes P1 on A:5000 and P2 on A:5002 (on machine A), and both of them are shunned at the same time,
when they rejoin, P1 does not use port 5000, but might use port 5002, and P2 doesn't use 5002, but might use 5000, so
they could assume each other's identity !
Both problems cannot be solved by remembering the last 100 ports: in case #1, this list is lost because we start a
new process and in case #2, the list is process-wide, but not machine-wide.
Again, these problems occur only when the OS reuses previously assigned ports.
A: Use temporary storage (per host) to store the last N addresses assigned on a given host. This makes sure we
don't reuse previous addresses
B: Use logical addresses, such as java.rmi.VMID or java.rmi.server.UID, which are unique over time for a given host.
Then, it doesn't matter what ports we use because the ports are not used to determine a member's identity.
The JIRA task for logical addresses is http://jira.jboss.com/jira/browse/JGRP-129.