[jira] [Resolved] (OAK-3865) New strategy to optimize secondary reads

["Tomek Rękawek (JIRA)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/OAK-3865?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Tomek Rękawek resolved OAK-3865.
--------------------------------
    Resolution: Fixed

> New strategy to optimize secondary reads
> ----------------------------------------
>
>                 Key: OAK-3865
>                 URL: https://issues.apache.org/jira/browse/OAK-3865
>             Project: Jackrabbit Oak
>          Issue Type: Improvement
>          Components: mongomk
>            Reporter: Tomek Rękawek
>            Assignee: Tomek Rękawek
>              Labels: performance
>             Fix For: 1.6, 1.5.4
>
>         Attachments: OAK-3865.patch, ReadDeepTreeNoCacheTest.patch, clustered-oak-setup-improvements.pdf, diagram.png
>
>
> *Introduction*
> In the current trunk we'll only read document _D_ from the secondary instance if:
> (1) we have the parent _P_ of document _D_ cached and
> (2) the parent hasn't been modified in 6 hours.
> The OAK-2106 tried to optimise (2) by estimating lag using MongoDB replica stats. It was unreliable, so the second approach was to read the last revisions directly from each Mongo instance. If the modification date of _P_ is before last revisions on all secondary Mongos, then secondary can be used.
> The main problem with this approach is that we still need to have the _P_ to be in cache. I think we need another way to optimise the secondary reading, as right now only about 3% of requests connects to the secondary, which is bad especially for the global-clustering case (Mongo and Oak instances across the globe). The optimisation provided in OAK-2106 doesn't make the things much better and may introduce some consistency issues.
> *Proposal - tldr version*
> Oak will remember the last revision it has ever seen. In the same time, it'll query each secondary Mongo instance, asking what's the available stored root revision. If all secondary instances have a root revision >= last revision seen by a given Oak instance, it's safe to use the secondary read preference.
> *Proposal*
> I had following constraints in mind preparing this:
> 1. Let's assume we have a sequence of commits with revisions _R1_, _R2_ and _R3_ modifying nodes _N1_, _N2_ and _N3_. If we already read the _N1_ from revision _R2_ then reading from a secondary shouldn't result in getting older revision (eg. _R1_).
> 2. If an Oak instance modifies a document, then reading from a secondary shouldn't result in getting the old version (before modification).
> So, let's have two maps:
> * _M1_ the most recent document revision read from the Mongo for each cluster id,
> * _M2_ the oldest last rev value for root document for each cluster id read from all the secondary instances.
> Maintaining _M1_:
> For every read from the Mongo we'll check if the lastRev for some cluster id is newer than _M1_ entry. If so, we'll update _M1_. For all writes we'll add the saved revision id with the current cluster id in _M1_.
> Maintaining _M2_:
> It should be periodically updated. Such mechanism is already prepared in the OAK-2106 patch.
> The method deciding whether we can read from the secondary instance should compare two maps. If all entries in _M2_ are newer than _M1_ it means that the secondary instances contains at least as new repository state as we already accessed and therefore it's safe to read from secondary.
> Regarding the documents modified by the local Oak instance, we should remember all the locally-modified paths and their revisions and use primary Mongo to access them as long as the changes are not replicated to all the secondaries. When the secondaries are up to date with the modification, we can remove it from the local-changes collections.
> Attached image diagram.png presents the idea.



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