Introduction to Data Concurrency and Consistency in a Multiuser Environment (308)

In a single-user database, the user can modify data in the database without concern for
other users modifying the same data at the same time. However, in a multiuser
database, the statements within multiple simultaneous transactions can update the
same data. Transactions executing at the same time need to produce meaningful and
consistent results. Therefore, control of data concurrency and data consistency is vital
in a multiuser database.

■ Data concurrency means that many users can access data at the same time.
■ Data consistency means that each user sees a consistent view of the data,
including visible changes made by the user’s own transactions and transactions of
other users.

To describe consistent transaction behavior when transactions run at the same time,
database researchers have defined a transaction isolation model called serializability.
The serializable mode of transaction behavior tries to ensure that transactions run in
such a way that they appear to be executed one at a time, or serially, rather than
concurrently.

While this degree of isolation between transactions is generally desirable, running
many applications in this mode can seriously compromise application throughput.
Complete isolation of concurrently running transactions could mean that one
transaction cannot perform an insert into a table being queried by another transaction.
In short, real-world considerations usually require a compromise between perfect
transaction isolation and performance.

Oracle offers two isolation levels, providing application developers with operational
modes that preserve consistency and provide high performance.

多用户环境下的数据并发和一致性
1. 数据并发访问是指多用户同时访问相同的数据.
2. 数据一致性是指系统中每个用户都能够查看数据的一致性,包括自己或其他用户所提交的事务对数据的修改
3. 串行化处理 :　认为同一时间只有一个事务在运行　,而非并发的

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