[jira] [Commented] (OAK-6087) Avoid reads from MongoDB primary

["Marcel Reutegger (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/OAK-6087?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16447775#comment-16447775 ] 

Marcel Reutegger commented on OAK-6087:
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[~tomek.rekawek], [~chetanm], using client sessions with causal consistency on MongoDB 3.6 looks very promising and would allow us to remove existing code you two wrote. More specifically, OAK-3865 and OAK-1645.

What do you think about removing code introduced with OAK-1645 and OAK-3865 after this improvement is implemented? It would reduce the code base and complexity of the MongoDocumentStore implementation quite a bit. On the other hand, MongoDB 3.6 would be required to route reads to a secondary. Earlier versions of MongoDB will still work, but reads would all go to the primary.

> 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
>
> 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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