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https://issues.apache.org/jira/browse/IGNITE-21084?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17797268#comment-17797268
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Roman Puchkovskiy commented on IGNITE-21084:
--------------------------------------------
Thanks!
> Account for differences of logical part of now() on different nodes when
> waiting after a DDL
> --------------------------------------------------------------------------------------------
>
> Key: IGNITE-21084
> URL: https://issues.apache.org/jira/browse/IGNITE-21084
> Project: Ignite
> Issue Type: Improvement
> Reporter: Roman Puchkovskiy
> Assignee: Roman Puchkovskiy
> Priority: Major
> Labels: ignite-3
> Fix For: 3.0.0-beta2
>
> Time Spent: 20m
> Remaining Estimate: 0h
>
> Client must see the results of a DDL they just executed, no matter through
> which node of the cluster a subsequent request is made. To maintain this, we
> must make sure that ActivationTimestamp(DDL) becomes non-future on all nodes
> of the cluster and only then can we complete the DDL future. As nodes'
> physical clocks might have some skew relatively to each other, we add
> MaxClockSkew to the timestamp till which we wait to compensate for this
> difference.
> Analogously to HLC.now() being different because its physical part differs on
> different nodes, it might be different because *logical* parts are different
> on different nodes.
> Let's assume we don't have any physical clock skew, and MaxClockSkew is 0.
> ActivationTimestamp(DDL) is (100, 5); (100 is physical part, 5 is logical
> part). We wait on node 1 (through which the DDL was executed) till its
> HLC.now() reaches (100, 5), then we complete the DDL future. The user goes to
> node 2 at which HLC.now() is (100, 2). The user executes a query at 'now',
> and the query sees the state before the DDL was executed, which breaks our
> requirement.
> We can fix this by 'rounding' the timestamp-to-wait-for up in the following
> way:
> # If logical part is not 0, increment the physical part and set the logical
> part to 0
> # If the logical part is 0, leave the timestamp as is
> As a result, for (100, 0) we will wait for (100, 0), and at node 1 HLC is at
> least (100, 0), so it cannot see the previous schema. For (100, 5) we would
> wait till (101, 0), which would also guarantee that a query executed after
> waiting sees the new schema version.
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