Currently in Hive code there are several global and ThreadLocal variables that need to be cleaned.
Specifically, the following classes are involved:
1. HiveConf: contains hive configurations (and a classloader)
2. Hive class: contains a static member Hive db. Hive class contains a member HiveConf conf, as well as a ThreadLocal storage of IMetaStoreClient.
3. SessionState: contains a static ThreadLocal storage of SessionState. SessionState class contains a Hive db, a HiveConf conf, a history logger, and a bunch of standard input/output streams
4. CliSessionState: SessionState plus some command options and the command file name.
5. All classes that try to get Hive db or HiveConf from global static Hive db, or SessionState.
There are several problems with the current design. To name a few:
1. SessionState instances are ThreadLocal, but SessionState contains Hive db which also contains ThreadLocal storage. Not sure a db can be shared across different threads or not? What is the global static Hive db?
2. We pass HiveConf and Hive db in two ways to classes like Task: Sometimes through initialize(), sometimes through SessionState. This complicates the code a lot. It's hard to know which HiveConf and which db we should use.
We need to think about a better way to do it.