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https://issues.apache.org/jira/browse/IGNITE-15434?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17420884#comment-17420884
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Konstantin Orlov commented on IGNITE-15434:
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[~alapin], another bunch of comments is waiting for you :)
> Implement cursor for partitions
> -------------------------------
>
> Key: IGNITE-15434
> URL: https://issues.apache.org/jira/browse/IGNITE-15434
> Project: Ignite
> Issue Type: Improvement
> Reporter: Alexander Lapin
> Assignee: Alexander Lapin
> Priority: Major
> Labels: ignite-3
> Time Spent: 2h 50m
> Remaining Estimate: 0h
>
> In order to unblock sql activities in required to have partition scanning
> logic. Simple approach would be similar to meta storage range command:
> cursors on the client side with corresponding raft commands that will manage
> server side storage cursors. However given approach has some disadvantages,
> for example, cursor's hasNext(), next(), close() etc are blocking by design
> so it might lead (and as it's seen with Meta storage range will lead) to a
> deadlock if response retrieval logic is blocked by synchronous get on hasNext
> or similar. In order to overcome given issue we might use unblocking cursors
> based on reactive flow model. It actually means that client side range/scan
> should provide Publisher instance*.*
> *Implementation details:*
> Within InternalTable class new method was introduced:
> {code:java}
> /**
> * Scans given partition, providing {@link Publisher<BinaryRow>} that
> reflectively notifies about partition rows.
> * @param p The partition.
> * @param tx The transaction.
> * @return {@link Publisher<BinaryRow>} t{@link Publisher<BinaryRow>} that
> reflectively notifies about partition rows.
> */
> @NotNull Publisher<BinaryRow> scan(int p, @Nullable Transaction tx);
> {code}
> Core publisher logic is implemented inside:
> {code:java}
> InternalTableImpl.PartitionScanPublisher.PartitionScanSubscription#PartitionScanSubscription
> {code}
> Please pay attention that inner batching logic is not implemented.
> PartitionScanSubscription will request same amount of entries from Storage as
> it was specified within subscription.request(). Seems that it doesn't worth
> to implement it right now cause event reactive model wasn't approved yet.
> Three new partition raft commands and corresponding handlers were added:
> * ScanInitCommand
> * ScanRetrieveBatchCommand
> * ScanCloseCommand
> All given commands are write ones, so it'll lead to full scan replication
> over raft nodes. On the one hand such approach leads to transparent remapping
> logic that eases sql quey processing logic in case of raft leadership
> changes, on the other hand it requires more time for replication and more
> space within raft log for such commands. In future such strong restrictions
> might be be weakened.
>
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