[jira] [Commented] (PHOENIX-6066) MetaDataEndpointImpl.doGetTable should acquire a readLock instead of an exclusive writeLock on the table header row

["Jacob Isaac (Jira)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/PHOENIX-6066?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17719045#comment-17719045 ] 

Jacob Isaac commented on PHOENIX-6066:
--------------------------------------

FYI [~rushabh.shah] [~tkhurana] 

> 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: 5.0.0, 4.15.0
>            Reporter: Chinmay Kulkarni
>            Assignee: Lokesh Khurana
>            Priority: Major
>              Labels: quality-improvement
>             Fix For: 4.17.0, 4.16.2
>
>
> 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.
> Note that this is exacerbated in cases where we do server-server RPCs while holding rowLocks for example ([this|https://github.com/apache/phoenix/blob/1d844950bb4ec8221873ecd2b094c20f427cd984/phoenix-core/src/main/java/org/apache/phoenix/coprocessor/MetaDataEndpointImpl.java#L2459-L2461] and [this|https://github.com/apache/phoenix/blob/1d844950bb4ec8221873ecd2b094c20f427cd984/phoenix-core/src/main/java/org/apache/phoenix/coprocessor/MetaDataEndpointImpl.java#L2479-L2484]) which is another issue altogether.
> 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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