[infinispan-dev] Partial state transfer

[Bela Ban]

Correct. Time frame would ideally be 5.0, but realistically it will 
probably be 5.1. Is that feasible from a roadmap point of view ?

On 6/16/11 6:47 PM, Vladimir Blagojevic wrote:
> In another words the essential problem is that digest and channel state
> are per channel abstractions and they do not fit nicely with higher
> level abstractions like substates? We use partial state transfer in
> Infinispan and we need to address this.
>
> What is the time frame here? 5.0 final release?
>
> Vladimir
>
> On 11-06-15 11:39 AM, Bela Ban wrote:
>> 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