[jira] [Commented] (ARTEMIS-3430) Activation Sequence Auto-Repair

["Gary Tully (Jira)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/ARTEMIS-3430?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17407362#comment-17407362 ] 

Gary Tully commented on ARTEMIS-3430:
-------------------------------------

the CLAIM already introduces the notion of exclusive access so I don't think there is any drawback here. An outstanding claim can be satisfied by any broker with in-sync data. then once it activates un-replicated other data is stale in the normal way.

> Activation Sequence Auto-Repair
> -------------------------------
>
>                 Key: ARTEMIS-3430
>                 URL: https://issues.apache.org/jira/browse/ARTEMIS-3430
>             Project: ActiveMQ Artemis
>          Issue Type: Bug
>            Reporter: Francesco Nigro
>            Assignee: Francesco Nigro
>            Priority: Major
>
> This can be seen both as a bug or an improvement over the existing self-heal behaviour of activation sequence introduced by https://issues.apache.org/jira/browse/ARTEMIS-3340.
> In short, the existing protocol to increase activation sequence while un-replicated is:
> # remote i -> -(i + 1) ie remote CLAIM 
> # local i -> (i + 1) ie local commit
> # remote -(i + 1) -> (i + 1) ie remote COMMIT
> This protocol has been designed to allow witness brokers to acknowledge if their data is no longer up-to-date and to save them to throw it away if still valuable, during a partial failure while increasing activation sequence.
> In the current version, self-repairing is allowed just if live broker has performed 2. but not 3. ie local activation sequence is updated, but coordinated one isn't committed yet.
> If the failing broker is restarted it can "fix" the coordinated sequence and move on to become live again, but if 2. fail (or just never happen), the coordinated activation sequence cannot be fixed if not with some admin intervention, after inspecting *all* local activation sequences.
> The reason why other brokers cannot "fix" the sequence is because the local sequence of the failed broker is unknown and just roll-backing the claimed one (to the previous or to the right committed value) can makes the failed broker to believe to have up-to-date data too, causing journal misalignments.
> The solution to this can be to fix the claimed sequence moving it to the right commit value while forbidding any broker to run un-replicated using it.
> This is achieved by further increasing it *after* repaired: it would prematurely age other in-sync brokers (including the failed one), but allowing auto-repair without admin intervention.
> The sole drawback of this strategy is that a further fail of the repairing broker while increasing sequence will give to it an exclusive responsibility to auto-repair (again, on restart) because no other brokers can have an high-enough local sequence.



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