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https://issues.apache.org/jira/browse/IGNITE-18879?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17825157#comment-17825157
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yexiaowei commented on IGNITE-18879:
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[~Denis Chudov] I would like to ask a question about Metastorage. I noticed
that Metastorage relies on a Raft group. However, in many cases when reading
Metastorage data, it doesn't read from the leader or use raft ReadIndex. Would
this lead to issues with reading outdated data? For example, directly reading
lease information PlacementDriver#currentLease from PD.
> Leaseholder candidates balancing
> --------------------------------
>
> Key: IGNITE-18879
> URL: https://issues.apache.org/jira/browse/IGNITE-18879
> Project: Ignite
> Issue Type: Improvement
> Reporter: Denis Chudov
> Priority: Major
> Labels: ignite-3
>
> *Motivation*
> Primary replicas (leaseholders) should be evenly distributed over cluster to
> balance the transactional load between nodes. As the placement driver assigns
> primary replicas, balancing the primary replicas is also it's responsibility.
> Naive implementation of balancing should choose a node as leaseholder
> candidate in a way to save even lease distribution over all nodes. In real
> cluster, it may take into account slow nodes, hot table records, etc. If
> lease candidate declines LeaseGrantMessage from placement driver, the
> balancer should make decision to choose another candidate for given primary
> replica or enforce the previously chosen. So the balancing algorith should be
> pluggable, so that we could have ability to improve/replace/compare it with
> others.
> *Definition of done*
> Introduced interface for lease candidates balancer, and a simple
> implementation sustaining even lease distribution, which is used by placement
> driver by default. No public or internal configuration needed on this stage.
> *Implementation notes*
> Lease candidates balancer should have at least 2 methods:
> - {_}get(group, ignoredNodes){_}: returns candidate for the given group, a
> node from ignoredNodes set can't be chosen as a candidate
> - {_}considerRedirectProposal(group, candidate, proposedCandidate){_}:
> processes redirect proposal for given group provided by given candidate
> (previously chosen using _get_ method), proposedCandidate is the alternative
> candidate. Returns candidate that should be enforced by placement driver.
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