Tomek Rękawek created OAK-3865:
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Summary: 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
Fix For: 1.4
*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*
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.
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