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From Sivaramakrishnan Narayanan <tarb...@gmail.com>
Subject Caching metastore objects
Date Tue, 26 May 2015 03:19:53 GMT
Apologies if this has been discussed in the past - my searches did not pull
up any relevant threads. If there are better solutions available out of the
box, please let me know!

Problem statement
--------------------------

We have a setup where a single metastoredb is used by Hive, Presto and
SparkSQL. In addition, there are 1000s of hive queries submitted in batch
form from multiple machines. Oftentimes, the metastoredb ends up being
remote (in a different region in AWS etc) and round-trip latency is high.
We've seen single thrift calls getting translated into lots of small SQL
calls by datanucleus and the roundtrip latency ends up killing performance.
Furthermore, any of these systems may create / modify a hive table and this
should be reflected in the other system. Example, I may create a table in
hive and query it using Presto or vice versa. In our setup, there may be
multiple thrift metastore servers pointing to the same metastore db.

Investigation
-------------------

Basically, we've been looking at caching to solve this problem (will come
to invalidation in a bit). I looked briefly at DN's support for caching -
these two parameters seem to be switched off by default.

    METASTORE_CACHE_LEVEL2("datanucleus.cache.level2", false),
    METASTORE_CACHE_LEVEL2_TYPE("datanucleus.cache.level2.type", "none"),

Furthermore, my reading of
http://www.datanucleus.org/products/datanucleus/jdo/cache.html suggests
that there is no sophistication in invalidation - seems like only
time-based invalidation is supported and it can't work across multiple PMFs
(therefore, multiple thrift metastore servers)

Solution Outline
-----------------------

   - Every table / partition will have an additional property called
   'version'
   - Any call that modifies table or partition will bump up version of the
   table / partition
   - Guava based cache of thrift objects that come from metastore calls
   - We fire a single SQL matching versions before returning from cache
   - It is conceivable to have a mode wherein invalidation based on version
   happens in a background thread (for higher performance, lower fidelity)
   - Not proposing any locking (not shooting for world peace here :) )
   - We could extend HiveMetaStore class or create a new server altogether

Is this something that would be interesting to the community? Is this
problem already solved and should I spend my time watching GoT instead?

Thanks
Siva

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