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https://issues.apache.org/jira/browse/IGNITE-17263?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Alexander Lapin updated IGNITE-17263:
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Attachment: Screenshot from 2022-07-06 16-48-41.png
> Implement leader to replica safe time propagation
> -------------------------------------------------
>
> Key: IGNITE-17263
> URL: https://issues.apache.org/jira/browse/IGNITE-17263
> Project: Ignite
> Issue Type: Improvement
> Reporter: Alexander Lapin
> Priority: Major
> Labels: ignite-3, transaction3_ro
> Attachments: Screenshot from 2022-07-06 16-48-30.png, Screenshot from
> 2022-07-06 16-48-41.png
>
>
> In order to perform replica reads, it's required either to use read index or
> check the safe time. Let's recall corresponding section from tx design
> document.
> RO transactions can be executed on non-primary replicas. write intent
> resolution doesn’t help because a write intent for a committed transaction
> may not be yet replicated to the replica. To mitigate this issue, it’s enough
> to run readIndex on each mapped partition leader, fetch the commit index and
> wait on a replica until it’s applied. This will guarantee that all required
> write intents are replicated and present locally. After that the normal write
> intern resolution should do the job.
> There is a second option, which doesn’t require the network RTT. We can use a
> special low watermark timestamp (safeTs) per replication group, which
> corresponds to the apply index of a replicated entry, so then an apply index
> is advanced during the replication, then the safeTs is monotonically
> incremented too. The HLC used for safeTs advancing is assigned to a
> replicated entry in an ordered way.
> Special measures are needed to periodically advance the safeTs if no updates
> are happening. It’s enough to use a special replication command for this
> purpose.
> All we need during RO txn is to wait until a safeTs advances past the RO txn
> readTs.
> !Screenshot from 2022-07-06 16-48-30.png!
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