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From Tomek Rękawek (JIRA) <j...@apache.org>
Subject [jira] [Commented] (OAK-6087) Avoid reads from MongoDB primary
Date Mon, 30 Apr 2018 09:20:00 GMT

    [ https://issues.apache.org/jira/browse/OAK-6087?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16458422#comment-16458422

Tomek Rękawek commented on OAK-6087:

With regards to OAK-3865: big +1, if MongoDB now provides this kind of consistency we should
use it.

> 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
>            Assignee: Marcel Reutegger
>            Priority: Major
>              Labels: scalability
>         Attachments: OAK-6087.patch
> 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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