About Concurrency and Scalability
The better your application handles concurrency, the more scalable it is.
Concurrency is the simultaneous execution of multiple transactions. Statements within concurrent transactions can update the same data. Concurrent transactions must produce meaningful and consistent results. Therefore, a multiuser database must provide the following:
Data concurrency , which ensures that users can access data at the same time.
Data consistency, which ensures that each user sees a consistent view of the data, including visible changes from his or her own transactions and committed transactions of other users