Locks ensure data isolation as described in the ACID principle. Hive has supported concurrency access and locking mechanisms since v0.7.0 and updated to a new lock manager in v0.13.0. There are two types of lock provided as follows:

• Shared lock: Also called S lock, it allows being shared concurrently. This is acquired when a table/partition is read. 
• Exclusive lock: Also called X lock. This is acquired for all other operations that modify the table/partition.

To enable locking, make sure the two properties are set in a Hive session or hive-site.xml (refer Transaction, section above):

• hive.support.concurrency = true
• hive.txn.manager = org.apache.hadoop.hive.ql.lockmgr.DbTxnManager

Any query must acquire proper locks before being allowed to perform corresponding lock-permitted operations. When the query is SELECT, it will get an S lock. Concurrent SELECT statements on the same table will get multiple S locks and run in parallel. When the query is INSERT, it will get an X lock. Concurrent INSERT statements will only get one X lock, so an INSERT has to wait until the lock is released by the other INSERT. In addition, a table can only have one X lock. When trying to get an X lock, there should no other locks on the table, or else the operation that requires an X lock, such as INSERT, ALTER, has to wait and retry (the hive.lock.sleep.between.retries property controls the retry time). 

By using the new lock manager, DbTxnManager, locks can only be acquired/released from a query implicitly. To see the locks on the table, use the SHOW LOCKS/SHOW LOCKS table_name statement:

-- Show all locks when running merge into above
> SHOW LOCKS;
+--------+----------+-----------------+------------+------------+
| lockid | database | table           | lock_state | lock_type  |
+--------+----------+-----------------+------------+------------+
| 19.1   | default  | employee_update | ACQUIRED   | SHARED_READ|
| 19.2   | default  | employee_trans  | ACQUIRED   |SHARED_WRITE|
+--------+----------+-----------------+-------------------------+
3 rows selected (0.059 seconds)