]
Bela Ban commented on JGRP-1742:
--------------------------------
Hmm, perhaps we go back to queueing messages during a closed BARRIER. The advantage is
that we don't have to rely on stability to trigger retransmission on a last-msg-lost
problem, but the messages in the queue will trigger retransmission.
Even better, we could only store the *last message sent per sender* (separate for unicast
and multicast messages): this would reduce space requirements and it would also trigger
retransmission, and we wouldn't have to wait for stability to kick in...
BARRIER: minimize closing time
------------------------------
Key: JGRP-1742
URL:
https://issues.jboss.org/browse/JGRP-1742
Project: JGroups
Issue Type: Enhancement
Reporter: Bela Ban
Assignee: Bela Ban
Fix For: 3.5
During a state transfer, BARRIER.up() waits until all incoming threads (delivering
messages to the application) are done, and blocks further incoming messages. This is done
to get the digest and the state.
However, duing the block, the following messages are not sent up:
* Views !
* STABLE messages, triggering retransmissions
This is bad, so we should try to minimize the time BARRIER is closed. This can be done
with JGRP-1352.
However, we could also do the following:
* A state request is received
* Close BARRIER and flush all pending threads. This ensures that any message which
updated the *digest* also updated the *application state*
* Get the digest D
* *Open* BARRIER. Messages will now be delivered and thus applied to the state
* Get the application state S
* When done, return D and S to the state requester
The difference to JGRP-1352 is that we don't queue messages during state transfer.
How does this work ? It is critical to ensure that all mesages which updated the digest D
also updated the state S, or else messages present in D but not in S would not be
retransmitted. However, if there are more messages in S than in D, this is not an issue as
they will be retransmitted again.
Example:
* BARRIER is closed and pending threads are flushed
* Digest D is (only for a given member P) 5, state S is 5 as well
* Now we open BARRIER
* P sends a few more messages (6, 7 and 8)
* The digest is now 8, but the copy we have is still 5
* State S is 8
* We return D=5 and S=8
* The state requester closes BARRIER and sets its digest to 5 and its state to 8
* Since the digest is only 5 for P, the state requester asks P for retransmission of
messages 6, 7 and 8
* Messages 6, 7 and 8 from P are received and applied to the state
* The assumption here is that if messages 6, 7 and 8 are applied twice, the state
doesn't change (idempotency). This should be the case with Infinispan.
The advantage of this issue over JGRP-1352 is that we don't need to queue messages
for a long time if the state is large.
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