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https://issues.apache.org/jira/browse/OAK-6087?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16198359#comment-16198359
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Marcel Reutegger commented on OAK-6087:
---------------------------------------
Reading entirely from a secondary could be achieved with a custom write
concern. An Oak cluster node would be tied to a nearby secondary and the write
concern would be use a tag set that requires the write to propagate to the
secondary.
> Avoid reads from MongoDB primary
> --------------------------------
>
> Key: OAK-6087
> URL: https://issues.apache.org/jira/browse/OAK-6087
> Project: Jackrabbit Oak
> Issue Type: Improvement
> Components: mongomk
> Reporter: Marcel Reutegger
> Labels: scalability
> Fix For: 1.8
>
>
> With OAK-2106 Oak now attempts to read from a MongoDB secondary when it
> detects the requested data is available on the secondary.
> When multiple Oak cluster nodes are deployed on a MongoDB replica set, many
> reads are still directed to the primary. One of the reasons why this is seen
> in practice, are observers and JCR event listeners that are triggered rather
> soon after a change happens and therefore read recently modified documents.
> This makes it difficult for Oak to direct calls to a nearby secondary,
> because changes may not yet be available there.
> A rather simple solution for the observers may be to delay processing of
> changes until they are available on the near secondary.
> A more sophisticated solution discussed offline could hide the replica set
> entirely and always read from the nearest secondary. Writes would obviously
> still go to the primary, but only return when the write is available also on
> the nearest secondary. This guarantees that any subsequent read is able to
> see the preceding write.
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