[jira] [Commented] (OAK-1453) MongoMK failover support for replica sets (esp. shards)

["Stefan Egli (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/OAK-1453?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13950790#comment-13950790 ] 

Stefan Egli commented on OAK-1453:
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OAK-1641 reports a failure in the google cache when a primary mongo crashes

> MongoMK failover support for replica sets (esp. shards)
> -------------------------------------------------------
>
>                 Key: OAK-1453
>                 URL: https://issues.apache.org/jira/browse/OAK-1453
>             Project: Jackrabbit Oak
>          Issue Type: Improvement
>          Components: mongomk
>            Reporter: Michael Marth
>            Assignee: Thomas Mueller
>            Priority: Critical
>              Labels: production, resilience
>             Fix For: 0.20
>
>
> With OAK-759 we have introduced replica support in MongoMK. I think we still need to address the resilience for failover from primary to secoandary:
> Consider a case where Oak writes to the primary. Replication to secondary is ongoing. During that period the primary goes down and the secondary becomes primary. There could be some "half-replicated" MVCC revisions, which need to be either discarded or be ignored after the failover.
> This might not be an issue if there is only one shard, as the commit root is written last (and replicated last)
> But with 2 shards the the replication state of these 2 shards could be inconsistent. Oak needs to handle such a situation without falling over.
> If we can detect a Mongo failover we could query Mongo which revisions are fully replicated to the new primary and discard the potentially half-replicated revisions.



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