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Cloudera blog on HS2: http://blog.cloudera.com/blog/2013/07/how-hiveserver2-brings-security-and-concurrency-to-apache-hive/
Source Code Description
Hopefully this is kind of a manual for a new starter to locate various components in source code.
Thrift IDL file for TCLIService: <BaseDir>/service-rpc/if/TCLIService.thrift (<BaseDir> is typically: <YourWorkspace>/hive/)
Implementation of TCLIService.Iface: org.apache.hive.service.cli.thrift.ThriftCLIService class (subclassed by org.apache.hive.service.cli.thrift.ThriftBinaryCLIService and org.apache.hive.service.cli.thrift.ThriftHttpCLIService)
Subclass of ThriftCLIService class: