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Partial state transfer removed in 3.0

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Partial state transfer removed in 3.0

Bela Ban
FYI,

below's an email I sent to the Infinispan mailing list today.

This is mainly regarding Infinspan's and JBoss AS's use of partial state
transfer, but of course it applied to anyone currently using partial
state transfer. In a nutshell, if you use partial state transfer without
FLUSH, you may end up with incorrect state !

Read on if this caught your interest... :-)


////// email snippet //////

I looked into adding partial state transfer back into JGroups, but found
out that partial state transfer is fundamentally flawed, something I've
always suspected ! (Regular state transfer is correct, and has always
been correct).

- Say we have node A and B. B requests the state from A
- There are partial states X and Y
- Message M1 modifies X, M2 modifies Y

Here's what happens:

T1: A multicasts M1
T2: A delivers M1, and changes X
T3: B sends a GET_STATE("Y") request to A    // partial state request
for state Y
T4: A multicasts M2
T5: A delivers M2, changing Y
T6: A receives the GET_STATE request, sends a SET_STATE response back
including Y and the digest (including M1 and M2's seqnos)
T7: B receives the SET_STATE response, sets its digest (includes now M1
and M2) and state Y *BUT NOT* state X !
T8: *** B receives M1, discards it because it is already in its digest ***
T9: B receives M2, and also discards it


At time T8, M1 (which would have changed state X) is discarded, because
it is already in the digest sent with the SET_STATE response. Therefore
state X is now incorrect, as M1 was never applied !

As a summary, if we get a number of updates to partial states, and don't
receive all of them before requesting the partial state, the last update
includes in the digest wins...

I'm a real idiot, as I've written this down before, in 2006: see [1] for
details.

In a nutshell, [1] shows that partial state transfer doesn't work,
unless virtual synchrony (FLUSH) is used.

So I propose Infinispan and JBoss AS look into how they can replace
their use of partial state transfer. I suggest Infinispan uses the same
approach already used for state transfer with mode=distribution.

Opinions ?

[1]
https://github.com/belaban/JGroups/blob/master/doc/design/PartialStateTransfer.txt

--
Bela Ban
Lead JGroups / Clustering Team
JBoss

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