ngsg opened a new pull request, #5955:
URL: https://github.com/apache/hive/pull/5955

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   ### What changes were proposed in this pull request?
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   This patch aims to support configurable IMetaStoreClient.
   
   This patch has been submitted three times previously. Austin originally 
proposed HIVE-12679 and submitted the initial patch in 2016. @moomindani later 
resubmitted it in https://github.com/apache/hive/pull/1402, and then @okumin 
took it over in https://github.com/apache/hive/pull/4444.
   
   ### Why are the changes needed?
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   In some scenarios, users may want to run Hive as a general-purpose execution 
engine on top of a third-party data catalog. For instance, [AWS Glue Data 
Catalog](https://github.com/awslabs/aws-glue-data-catalog-client-for-apache-hive-metastore)
 is an example of such usage, and there is also an open ticket to implement an 
Iceberg REST catalog (HIVE-28658). As noted in the previous PR 
(https://github.com/apache/hive/pull/4444), more than 40 people are watching 
HIVE-12679, which I believe reflects strong demand for using Hive in this more 
flexible architecture. Therefore, I believe it is worth completing this patch 
to support external data catalogs in Hive.
   
   ### Does this PR introduce _any_ user-facing change?
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   No, users can use the existing HMS client without any changes in their 
configuration files. 
   For the documentation fix, I will update hive-site once this patch is 
accepted and merged.
   
   ### How was this patch tested?
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   This patch includes two simple tests loading MetaStoreClient via 
`HiveMetaStoreClientFactory`.
   
   ---
   
   Note for those familiar with the earlier patch:
   
   - Regarding the design decision about MetaStoreClientFactory:
   This patch does not support a customizable MSC factory as the original patch 
did; instead, it supports a customizable MSC. In other words, this patch 
introduces `metastore.client.class`, not `metastore.client.factory.class`.
   I made this change not because I preferred this approach, but because I 
followed the majority opinion in the discussion [here]( 
https://github.com/apache/hive/pull/4444#pullrequestreview-1492711587). 
Personally, I think both approaches are valid and have their merits, and I do 
not have a strong opinion among them. I would appreciate it if you could share 
your thoughts on this decision.
   
   - Regarding RetryingMetaStoreClient and other classes that instantiate 
MetaStoreClient 
([link](https://github.com/apache/hive/pull/4444#pullrequestreview-1687155221)):
   After resolving HIVE-27473, Hive now creates ThriftHiveMetaStoreClient only 
in `Hive.java` and `HiveMetaStoreClient`. Therefore, I believe it is sufficient 
to replace these two call sites with the new `HiveMetaStoreClientFactory`.
   


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