Chinmay Kulkarni created PHOENIX-6066:
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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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