[infinispan-dev] JGroups 2.7 and 2.8 work with Infinispan now

[Bela Ban]

Vladimir Blagojevic wrote:
> On 6/2/09 2:48 PM, Bela Ban wrote:
>>>   Initially we thought FLUSH will work around this problem, and 
>>> while it reduces the window of occurence, the problem still exists 
>>> since there is a race between A & B receiving unblock() and then 
>>> broadcasting queued messages, and C receiving these queued messages 
>>> and receiving unblock()
>>
>> No, that should *not* occur because messages will always be received 
>> by C even though it is still blocked. The blocking only affects the 
>> *sending* of messages. Second, C will have view {A,B,C} installed, or 
>> else it would not receive the unblock(). Therefore, any messages from 
>> A or B would NOT get dropped. Third, even if that was the case (it 
>> isn't), any dropped message from A or B would get retransmitted when 
>> A or B sends the next message to C, or before the next view gets 
>> installed. Again, only with FLUSH though !
> There is a possibility that a message could get dropped for a joining 
> member C. Flush has a latch that is turned on once a joining member 
> receives a first stop flush after it receives the first view. If a 
> message happens to go up the stack before first stop flush - we drop 
> it. See allowMessagesToPassUp field of FLUSH.
>
> If we did not have this switch then we could have reverse order of 
> view, unblock and message, that is messages could go up the stack 
> before unblock.

Oh, I see ! Question: why are we dropping that message rather than just 
blocking ? Concerns that the incoming thread pools get exhausted ?

Ah, the message gets delivered by NAKACK, so we don't retransmit, but if 
FLUSH discards it, we're screwed and never deliver that message to the app.

Your last statement confuses me a bit: I don't think a VIEW would get 
sent up the stack *before* an unblock because they're tagged by seqnos 
aren't they ? So, in other words, do we really need to discard incoming 
mesages ?

-- 
Bela Ban
Lead JGroups / Clustering Team
JBoss - a division of Red Hat