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From "Chinmay Kulkarni (Jira)" <j...@apache.org>
Subject [jira] [Created] (PHOENIX-6066) MetaDataEndpointImpl.doGetTable should acquire a readLock instead of an exclusive writeLock on the table header row
Date Mon, 10 Aug 2020 17:21:00 GMT
Chinmay Kulkarni created PHOENIX-6066:
-----------------------------------------

             Summary: MetaDataEndpointImpl.doGetTable should acquire a readLock instead of
an exclusive writeLock on the table header row
                 Key: PHOENIX-6066
                 URL: https://issues.apache.org/jira/browse/PHOENIX-6066
             Project: Phoenix
          Issue Type: Improvement
    Affects Versions: 4.15.0, 5.0.0
            Reporter: Chinmay Kulkarni
             Fix For: 5.1.0, 4.16.0


Throughout MetaDataEndpointImpl, wherever we need to acquire a row lock we call [MetaDataEndpointImpl.acquireLock|https://github.com/apache/phoenix/blob/bba7d59f81f2b91342fa5a7ee213170739573d6a/phoenix-core/src/main/java/org/apache/phoenix/coprocessor/MetaDataEndpointImpl.java#L2377-L2386]
which gets an exclusive writeLock on the specified row [by default|https://github.com/apache/phoenix/blob/bba7d59f81f2b91342fa5a7ee213170739573d6a/phoenix-core/src/main/java/org/apache/phoenix/coprocessor/MetaDataEndpointImpl.java#L2378].


Thus, even operations like doGetTable/getSchema/getFunctions which are not modifying the row
will acquire a writeLock on these metadata rows when a readLock should be sufficient (see
[doGetTable locking|https://github.com/apache/phoenix/blob/bba7d59f81f2b91342fa5a7ee213170739573d6a/phoenix-core/src/main/java/org/apache/phoenix/coprocessor/MetaDataEndpointImpl.java#L2932]
as an example). The problem with this is, even a simple UPSERT/DELETE or SELECT query triggers
a doGetTable (if the schema is not cached) and can potentially block other DDLs and more importantly
other queries since these queries will wait until they can get a rowLock for the table header
row. Even seemingly unrelated operations like a CREATE VIEW AS SELECT * FROM T can block a
SELECT/UPSERT/DELETE on table T since the create view code needs to fetch the schema of the
parent table.

This Jira is to discuss the possibility of acquiring a readLock in these "read metadata" paths
to avoid blocking other "read metadata" requests stemming from concurrent queries. The current
behavior is potentially a perf issue for clients that disable update-cache-frequency.



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