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From "Cao Manh Dat (JIRA)" <j...@apache.org>
Subject [jira] [Updated] (SOLR-12011) Consistence problem when in-sync replicas are DOWN
Date Thu, 01 Mar 2018 04:44:00 GMT

     [ https://issues.apache.org/jira/browse/SOLR-12011?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Cao Manh Dat updated SOLR-12011:
--------------------------------
    Attachment: SOLR-12011.patch

> Consistence problem when in-sync replicas are DOWN
> --------------------------------------------------
>
>                 Key: SOLR-12011
>                 URL: https://issues.apache.org/jira/browse/SOLR-12011
>             Project: Solr
>          Issue Type: Bug
>      Security Level: Public(Default Security Level. Issues are Public) 
>          Components: SolrCloud
>            Reporter: Cao Manh Dat
>            Assignee: Cao Manh Dat
>            Priority: Major
>         Attachments: SOLR-12011.patch, SOLR-12011.patch, SOLR-12011.patch
>
>
> Currently, we will meet consistency problem when in-sync replicas are DOWN. For example:
>  1. A collection with 1 shard with 1 leader and 2 replicas
>  2. Nodes contain 2 replicas go down
>  3. The leader receives an update A, success
>  4. The node contains the leader goes down
>  5. 2 replicas come back
>  6. One of them become leader --> But they shouldn't become leader since they missed
the update A
> A solution to this issue :
>  * The idea here is using term value of each replica (SOLR-11702) will be enough to tell
that a replica received the latest updates or not. Therefore only replicas with the highest
term can become the leader.
>  * There are a couple of things need to be done on this issue
>  ** When leader receives the first updates, its term should be changed from 0 -> 1,
so further replicas added to the same shard won't be able to become leader (their term =
0) until they finish recovery
>  ** For DOWN replicas, the leader should also need to check (in DUP.finish()) that those
replicas have term less than leader before return results to users
>  ** Just by looking at term value of replica, it is not enough to tell us that replica
is in-sync with leader or not. Because that replica might not finish the recovery process.
We need to introduce another flag (stored on shard term node on ZK) to tell us that replica
finished recovery or not. It will look like this.
>  *** {"code_node1" : 1, "core_node2" : 0} — (when core_node2 start recovery) --->
>  *** {"core_node1" : 1, "core_node2" : 1, "core_node2_recovering" : 1} — (when core_node2
finish recovery) --->
>  *** {"core_node1" : 1, "core_node2" : 1}



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