Concurrency is the ability for multiple transactions to access table resources simultaneously. An integral consideration of data consistency is ensuring that changes made by one user do not negatively affect other users.
Locking is the method used to address both data consistency and concurrency requirements. A trade-off exists between database consistency and concurrency. It is necessary for the locking scheme to strike some sort of balance between consistency and concurrency.
The XDB Server controls concurrency by implementing locking schemes with choices of isolation levels. Each user may select the desired level of data locking.