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Bela Ban updated JGRP-1401:
---------------------------
Fix Version/s: 3.2
(was: 3.1)
RELAY: messages lost when relay coordinator crashes
---------------------------------------------------
Key: JGRP-1401
URL:
https://issues.jboss.org/browse/JGRP-1401
Project: JGroups
Issue Type: Feature Request
Reporter: Bela Ban
Assignee: Bela Ban
Fix For: 3.2
When we have sites {A,B,C} and {X,Y,Z} (with relay coords A and X), during the time X
leaves (or crashes) and Y taking over, all messages sent by the first site are not relayed
to the second site.
Because the sites are autonomous, there won't be any retransmission of the dropped
messages.
This can have an adverse affect, e.g. in Infinispan:
- Say key K is stored on A, B and Z
- Now we're updating K, on A and B, but before the change is relayed to the other
site, X crashes
- If there is no rebalancing, e.g. because K is still to be stored on A, B and Z, since
the update on Z was dropped, Z has a stale value !
SOLUTION 1:
- Have a backup coordinator B cache the last N messages in memory (with overflow to
disk)
- A numbers relayed messages
- As soon as A has relayed message #50, it sends this info to B. Or, alternatively, this
could be done periodically, or based on the number of relayed messages (e.g. every 10
messages)
- B can then purge those messages
- When A crashes, B runs a reconciliation protocol with X to determine whether to relay
some backed up messages
- C now starts acting as backup relay to B
This solution is probably the simplest to implement, and doesn't require any code
changes in Infinispan. However, there is still a chance of message loss if both the relay
*and* the backup relay crash at the same time.
SOLUTION 2:
- After a crash (not a graceful leave !) of a relay coordinator, there has to be a full
rebalancing of all keys
- This is wasteful though
- May not be needed, perhaps Infinispan could check whether a full rebalancing is
required ?
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