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https://issues.apache.org/jira/browse/KUDU-3383?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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shenxingwuying updated KUDU-3383:
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Attachment: image-2022-07-20-23-17-40-718.png
> About strong consistency read from leader
> -----------------------------------------
>
> Key: KUDU-3383
> URL: https://issues.apache.org/jira/browse/KUDU-3383
> Project: Kudu
> Issue Type: Improvement
> Reporter: shenxingwuying
> Priority: Major
> Attachments: image-2022-07-20-23-14-34-519.png,
> image-2022-07-20-23-17-40-718.png
>
>
> As describe as https://issues.apache.org/jira/browse/KUDU-3382.
>
>
> h1. Background && Motivation
> Linearizability read is a very friendly for developers, kudu can support it.
> h1. Issue of linearizability read from leader
> We need talk about the issue.
> Kudu's raft implements is a strong leader, leader's state machine is not
> older than followers, and followers heartbeat timeout or receives leader
> election request(leader transfer) can elect leader and switch leader.
> If kudu need linearizability read, read leader is not enough, because double
> leader may be exist at a very small period time.
> I provide two scenarios. The first one:
>
> {{L1(1)----F2(1) L(1) F2(2)| /
> | ---> /F3(1) L3(2)}}
> # A raft group has 3 replicas, L1, F2, F3. Their states is steady during
> term 1.
> # If network parition, F2 and F3 loss leader's heartbeat, F3 start election,
> F2 vote it.
> # F3 become Leader, we can call it L3. At this moment, there are 2 leaders:
> L1(1) and L3(2).
> # The state will be continued until the network partition recover. The time
> may be short or long.
> During double leader, it's not liearizability read. So kudu should avoid
> double leader at any time, pay the corresponding cost is no leader at a small
> period time. Kudu should make a choice. For user usally need linearizability,
> so I think kudu should support it. During a very small time no leader's
> unavailability can avoid by client's fault tolerance.
> Whether read leader is linearizability read, someone can make sure it or I
> can do a experiment.
>
> {{L(1)----F(1) L(1) F(2)| /
> | ---> /F(1) L(2)}}
> h1. Solution
> kudu should avoid double leaders at a very small period time and network
> fault happens . I review the codes, and think now the problem is exist.
> To avoid the double leader's trouble，leader should be keep alive. If a leader
> receives no enough heartbeats in a period of time, it shoud be leader down
> and and then start another election just like follower does. Leader's timeout
> should be less than follower's election.
> Another scheme: Read should send heartbeat to two follow to make sure it is
> valid leader.
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